Breeding ecology of the Common Eider (Somateria mollissima) in Breiðafjörður, West Iceland

Thordur Örn Kristjánsson

Faculty of Life and Environmental Sciences University of Iceland

2016

Breeding ecology of the Common Eider (Somateria mollissima) in Breiðafjörður, West Iceland

Thordur Örn Kristjánsson

Dissertation submitted in partial fulfillment of a Philosophiae Doctor degree in Biology

Advisor Dr. Jón Einar Jónsson

PhD Committee Professor Jörundur Svavarsson Professor Snæbjörn Pálsson Dr. Tómas Grétar Gunnarsson

Opponents Dr. Sveinn Are Hanssen Dr. Jonathan Green

Faculty of Life and Environmental Sciences School of Engineering and Natural Sciences University of Iceland Reykjavík, September 2016

Breeding ecology of the Common Eider (Somateria mollissima) in Breiðafjörður, West Iceland Dissertation submitted in partial fulfillment of a Philosophiae Doctor degree in Biology Copyright © 2016 Thordur Örn Kristjánsson All rights reserved

Faculty of Life and Environmental Sciences School of Engineering and Natural Sciences University of Iceland Askja, Sturlugata 7 101, Reykjavík Iceland

Telephone: 525 4000

Bibliographic information: Thordur Örn Kristjánsson, 2016, Breeding ecology of the Common Eider (Somateria mollissima) in Breiðafjörður, West Iceland, PhD dissertation, Faculty of Life and Environmental Sciences, University of Iceland, 87 bls.

ISBN 978-9935-9307-1-2

Printing: Háskólaprent ehf. Reykjavík, September 2016

iv Ágrip

Breiðafjörður er mikilvægasta búsvæði æðarfugls (Somateria mollissima) á Íslandi, þar halda æðarfuglar sig árið um kring og hefur æðardúnn verið nýttur þar öldum saman af æðarbændum. Markmið doktorsverkefnisins miðaði að bættum skilningi á varpvistfræði æðarfugla í Breiðafirði. Varphegðun kollna í þéttu varpi í Rifi (1.7 hreiður/m2) var skoðuð til að greina hvort æðarkollur aðstoði hverja aðra við álegu og sinni því fleiri en einu hreiðri á álegutíma. Metið var hvort dúntekja hefði áhrif á orkubúskap álegufuglanna með því að athuga mun á léttingu álegukollna á dúnhreiðrum og á heyhreiðrum. Samtímis var rannsakað hvort kollur sem lágu á stórum urptum (≥7 egg) eyddu meiri orku en kollur með venjulega urptarstærð (≤6 egg). Ytri sníkjudýrum var safnað úr æðarhreiðrum og fjöldi dúnflóa (Cerotophyllus garei) borinn saman á milli æðarvarpa við Rif og í Hvallátrum. Að lokum var athugað hvort fæða fuglanna á varptíma (maí-júlí) væri breytileg yfir tíma og á milli kynja. Í ofurþéttu varpi á Rifi staðfesti atferli litmerktra kollna að þær sinna hreiðrum hverrar annarrar auk sinna eigin. Ástæðan er sennilega sú að kollurnar ruglast vegna hins óeðlilega mikla þéttleika, sterk sjónræn örvun vegna fjölda óvarinna hreiðra nálægt þeirra eigin ýtir svo undir þessa hegðun. Einnig gætu kollurnar átt egg í fleiri en einni urpt í varpinu. Hár fjöldi flóa í þétta varpinu gæti verið ein ástæða þess að kollurnar fara oftar af hreiðri til að snyrta sig en hafa þá staðgöngukollur til að annast hreiðrin á meðan. Æðarkollur í strjálu æðarvarpi í Hvallátrum geta hinsvegar skipt um hreiður milli ára ef mikið er um flær en ekki hefur orðið vart sömu áleguhegðunar og í Rifi. Fæðurannsóknin leiddi í ljós að aðalfæða fuglanna að vor/sumarlagi (maí-júlí) reyndist vera flekkunökkvi (Tonicella marmorea), sem hefur ekki verið talinn mikilvæg fæða æðarfugla. Niðurstöður verkefnisins sýna að dúntekja og urptarstærð (2-9 egg) hafa lítil áhrif á afkomu æðarkollna á Íslandi, í það minnsta í venjulegu tíðarfari. Kollur sem hafa byggt upp góðan fituforða fyrir áleguna geta þannig tekist á við bæði dúntekju og auka egg í urptina án þess að það komi niður á líkamsástandi þeirra eða varpárangri. Dúntekja á Íslandi getur því talist vistvæn, hvað kollurnar sjálfar varðar, þar sem áhrif hennar á álegufugla eru óveruleg.

iii

Abstract

Breiðafjörður Bay, West Iceland, is an important breeding, molting and wintering area for the Common Eider (Somateria mollissima) (hereafter Eider). Eider farmers have collected nest down for centuries in the area. The aims of the doctoral thesis centered on feeding and breeding ecology of Eiders in Breiðafjörður. The incubation behavior in a very dense colony (Rif, 1.7 nests/m2) was studied to evaluate whether the incubating females helped each other through the incubation by attending more than one nest. Incubation cost was indexed with mass loss of the females during the incubation period and was compared between nests with and without down removal, and between females with enlarged clutches (≥7 eggs) and those incubating normal clutches (≤6 egg). Ectoparasites were collected from the Eider nests and abundances of the flea (Cerotophyllus garei) was compared between two colonies at Rif and Hvallátur. The spring/early summer (May-July) diet of the Eiders was investigated between years, months and sexes. Cooperative incubation behavior was confirmed with marked Eiders in the superdense colony at Rif. The females attended each other‘s nests, which may be a behavioral response to lack of new nesting sites and visual stimulus as they see many unguarded nests close to their own. Eiders at Rif might own some eggs in more than one nest in the colony. The ectoparasitic load at Rif is very high and the cooperative behavior gives the incubating females more time to leave their nests to preen while other Eiders attend their nests. At the more sparsely-nested colony at Hvallátur cooperative incubation has never been observed but there Eiders can switch nesting bowls between years if ectoparasitic loads become high. The key food item in spring/early summer feeding was the mottled red chiton (Tonicella marmorea). Chitons have not been considered an important food item for Eiders until now. Down collection was not found to have effect on the incubating Eiders, during these average weather conditions. Likewise, the enlarged clutches did not affect energy expenditure of incubating females or their hatching success. Down collection can therefore be considered to be environment-friendly as it does not harm the incubating Eiders.

v

Dedication: To my loving family.

vii

List of publications:

This doctoral thesis is based on the following papers, which will be referred to by their Roman numerals:

I. Thordur Örn Kristjánsson and Jón Einar Jónsson. 2015. Cooperative incubation behavior in a super dense Common Eider Somateria mollissima colony. Bird Study 62: 146–149. http://dx.doi.org/10.1080/ 00063657.2014.993591 II. Thordur Örn Kristjánsson, Jón Einar Jónsson and Tómas Grétar Gunnarsson. 2016. Do population-specific adaptations explain differing incubation costs? Manuscript submitted. III. Thordur Örn Kristjánsson, Jón Einar Jónsson and Jörundur Svavarsson. 2016. Variation in nest composition and abundances of ectoparasites between nests in colonially breeding Common Eiders (Somateria mollissima). Bird Study, in press. http://dx.doi.org/ 10.1080/ 00063657. 2016.1182965 IV. Thordur Örn Kristjánsson, Jón Einar Jónsson and Jörundur Svavarsson. 2013. Spring diet of Common Eiders (Somateria mollissima) in Breiðafjörður, West Iceland indicates non-bivalve preferences. Polar Biology 36: 51–59.http://dx.doi.org:10.1007/s 00300-012-1238-8 All published papers were reproduced with permission from the journals concerned.

Other publications not included in the doctoral thesis:

I. Jón Einar Jónsson, Þórður Örn Kristjánsson, Árni Ásgeirsson and Tómas G. Gunnarsson. 2015. Breytingar á fjölda æðarhreiðra á Íslandi. Náttúrufræðingurinn 85 (3–4): 141–152. [In Icelandic with an English Summary] II. Thordur Örn Kristjánsson and Jón Einar Jónsson. 2011. Effects of down collection on incubation temperature, nesting behaviour and hatching success of Common Eiders (Somateria mollissima) in west Iceland. Polar Biology 34: 985–994. http://dx.doi.org:10.1007/s00300-010-0956-z

ix

Acknowledgements

I would like to express my gratitude to all the people who have helped me during the years of this Doctoral thesis. In particular I would like to thank my supervisor Dr. Jón Einar Jónsson for all the time that we have spent together in the field, the laboratory and at home. Needless to say we have sent each other an enormous amount of emails and he has written countless memos so we could communicate on a shared level. Thank you Jón for broadening my understanding on research work and not letting my hearing disability stand in our way to achieve our goals. I wish to thank my co-supervisor Dr. Jörundur Svavarsson for a great help with identifying all sorts of invertebrates from Eiders, both on the outside and the inside. Thank you, Jörundur for always being positive and interested in our work. I thank my co-supervisors, Dr. Tómas Grétar Gunnarsson for important comments on our manuscripts and good friendship during the years, and Snæbjörn Pálsson for help with statistical work and in the laboratory at Askja. Smári Lúðvíksson and his wife Auður Alexandersdóttir at Rif are thanked for letting me use their Eider colony for the study and Smári for all his help in the data collection. Thank you, Auður for all the soup, milk and wonderful time in your kitchen at Rif. Owners at Hvallátur for using their Eider colony for the study, it is the most wonderful place on earth. Fishermen at Stykkishólmur for collecting drowned Eiders from fishing nets for the study. I thank Dr. Karl Gunnarssson at Hafrannsóknastofnun for his constructive com- ments on manuscripts. Svanhildur Egilsdóttir, also at Hafrannsóknastofnun, for taking pictures of parasites and food items for the study. Christine Chicoine at the University of Rimouski for help in the data collection at Rif in 2009 and Terry D. Galloway at the University of Manitoba for confirming the identification of fleas. My father Kristján Þórðarson and mother Guðrún G. Þórarinsdóttir I thank for help and comments on everything that we have done during the years. This would not have been done without you. Special thanks to my lovely wife Vala Gísladóttir for patience and more patience during these years. My two sons Fróði and Óðinn for being the best co-workers there are!

xi This work was funded by Íslandsbanki, Icelandic Eider Farmers association, traveling fund of University of Iceland, Educational fund Sigríðar Jónsdóttur and the Icelandic Association of the Deaf.

xii Table of contents

Ágrip ...... iii Abstract ...... v List of publications: ...... ix Acknowledgements ...... xi List of Figures: ...... xv List of Tables: ...... xvi 1 Introduction ...... 1 1.1 The Common Eider ...... 1 1.2 Incubation behavior of the Common Eider ...... 3 1.3 Energy expenditure of incubating Common Eiders ...... 5 1.4 Ectoparasitic loads in the nests of Common Eiders ...... 6 1.5 Diet of the Common Eiders ...... 7 1.6 The study area: Breiðafjörður, West Iceland ...... 7

2 Aims ...... 9 3 Materials and methods ...... 11 3.1 Study sites ...... 11 3.2 Rif colony ...... 11 3.3 Hvallátur colony...... 12 3.4 Incubation behavior of Common Eiders ...... 12 3.5 Energy expenditure of incubating Common Eiders ...... 13 3.6 Ectoparasitic loads in the nests of Common Eiders ...... 14 3.7 Diet of Common Eiders ...... 15

4 Results and discussion ...... 17 4.1 Incubation behavior of Common Eiders in a super-dense Eider colony (paper I)...... 17 4.2 Energy expenditure of incubating Common Eiders (paper II) ...... 18 4.3 Ectoparasitic loads in the nests of Common Eiders (paper III) ...... 22 4.4 Diet of Common Eiders in Breiðafjörður (paper IV) ...... 24

5 Concluding remarks ...... 27 References ...... 29

xiii Paper I ...... 41 Paper II ...... 47 Paper III ...... 65 Paper IV ...... 75

xiv List of Figures:

Figure 1 A map showing Breiðafjörður, West Iceland, and the study sites at Rif and Hvallátur...... 8 Figure 2 The observation site of the Rif colony, Rock island (“Gamli Hólmi”) to the left and Grass island (“Nýi Hólmi”) to the right in the pond. Photo: Thordur Örn Kristjánsson...... 12 Figure 3 A nasal-marked female Eider incubating her nest at Rif in summer of 2011. Photo: Thordur Örn Kristjánsson...... 13 Figure 4 Body mass at the beginning of incubation of 41 Eiders and relative loss mass during the incubation period of 24 days (SE = 73, P<0.001). Red dots indicate enlarged clutches (≥7 eggs)...... 20 Figure 5 Clutch sizes of 41 incubating Eiders from the years 2009 (squares) and 2010 (diamonds), plotted against year of first capture and marking by eider farmer (years of incubating experience, P=0.20). Red dots indicate enlarged clutches (≥7 eggs)...... 20 Figure 6 The flea C. garei, from an Eider nest in Hvallátur. Photo: Svanhildur Egilsdóttir...... 22 Figure 7 The tick I. uriae, from an Eider nest in Hvallátur. Photo: Svanhildur Egilsdóttir...... 23 Figure 8 Chitons (Tonicella spp.) from Eider stomach sample from Breiðafjörður. The mottled red citon (T. marmorea) to the left and the northern red chiton (T. rubens) to the right. Photo: Svanhildur Egilsdóttir...... 24 Figure 9 Annual differences in the percent occurrence (%) of the main diet species (Tonicella marmorea, Lacuna vincta, Buccinum undatum and Hyas sp.) of Eiders during spring and early summer the years 2007–2010 in Breiðafjörður, West Iceland...... 25 Figure 10 Annual differences in percent occurrence (%) of the most common invertebrate classes in the diet of Eiders (Somateria mollissima) during spring and early summer the years 2007–2010 in Breiðafjörður, West Iceland...... 26

xv List of Tables:

Table 1 Behavior of 26 marked incubating female Eiders during the 2012 nesting season (31 May, 6, 8, 11 and 16 June, with total numbers of incubation behavior in bold) at Rif, Snæfellsnes, Iceland...... 17 Table 2 Comparison of Eider studies on energy cost of incubation. Body mass at the start and end of incubation, mass loss/day, relative mass loss and clutch size reported for Eider populations in North America and Europe ...... 21

xvi Introduction

1 Introduction

Birds, as other sexually reproducing animals, have to mate and have offspring to carry their genes on to the next generations. The energy costs of breeding are high and the breeding is often unsuccessful because the animal cannot meet the energy demands for successful reproduction. Most females must accumulate enough energy for production of embryos prior to breeding and a range of behaviors have evolved during the breeding period to increase the energy expenditure of both genders (Pough et al. 2002). There are many ways to overcome these obstacles on the way to a successful breeding which may vary between species, locations and years. Animals show a diverse rearing behavior; some rely on help from other individuals while others raise their offspring on their own. Birds are ideal vertebrates for behavior studies as they are diverse (about 9100 species), widespread and mostly diurnal and relatively easy to follow during the breeding season (Konishi et al. 1989).

1.1 The Common Eider The order Anseriformes includes over 150 different bird species around the globe with 20 different species of seaducks (Merginae) (Livezey 1995). The Common Eider, Somateria mollissima (Linnaeus, 1758) (hereafter Eider), is the largest duck in the northern hemisphere and is widely distributed around the mid- to high latitude coasts throughout the Holarctic region, both in Eurasia and North America. The Eider is one of few Anatidae that occupy the marine environment all year round and thus shares many characteristics with seabirds, such as Alcidae and Laridae, than with other ducks as they feed on similar items. Many of these species can dive at great depths and they have salt glands to excrete salt when drinking from the sea (Cramp and Simmons 1977, Waltho and Coulson 2015). The Eider is divided into different sub-species. S. m. v-nigrum is found in the Pacific while in North America S. m. dresseri is found in coastal Labrador to Maine and S. m. sedentaria is found in Hudson Bay, Canada. The nominal subspecies S. m. mollissima is found in Europe except in Iceland, where S. m. borealis occurs along with Eiders from North Canada, Greenland, Svalbard and Franz Josephs Island (Cramp and Simmons 1977, Livezey 1995, Scott and Rose 1996). Some sources even classify the Icelandic Eider as a special subspecies, S. m. islandica, along with Eiders from South Greenland, Svalbard and North

1 PhD theses Thordur Örn Kristjánsson 2016

Norway (Palmer 1976, Goudie et al. 2000, Baldassarre 2015). The Icelandic Eider population has been estimated at around 970 thousand birds in the winter and about 250 thousand breeding pairs during the summer (Skarphéðinsson 1994, Garðarsson 2009) and it is one of the largest bird populations in Iceland. Most Eiders pair during the fall or early winter but pairing occurs from September/October until spring (Spurr and Milne 1976). Hatching success of females which pair early in the fall or winter can be higher than those that pair late in the winter, because the male protects the female from other males so she has more time to feed and built up energy reserves for egg laying and incubation (Spurr and Milne 1976, Ashcroft 1976). Eiders prey on a large variety of benthic invertebrate species and often specialize on one or more species depending on location, season and food availability (Bustnes and Erikstad 1988, Merkel et al. 2007, Krasno et al. 2009). Access to energy rich food items and extra time for the female to forage during the winter is an important factor for preparation of the next breeding season. The blue mussel (Mytilus edulis) has been considered to be a key food item during the whole year for Eiders in most areas (Guillemette et al. 1992, Madsen 1954, Bustnes and Erikstad 1988, Bustnes 1998, Skírnisson and Jónsson 1996, Skírnisson et al. 2000). Eiders feed heavily 4-6 weeks before the start of egg laying and increase their body mass about 20% above winter levels (Gorman and Milne 1971). Incubation is an energy intensive process for the females but leaving the nest to forage increases the probability of nest predation (Swennen et al. 1993, Bottita et al. 2003, Bolduc et al. 2005) and demands extra energy to restore heat to cold eggs when the female returns from incubation recesses (Haftorn and Reinertsen 1985, Tøien 1989, Brummermann and Reinertsen 1991, Criscuolo et al. 2000). Therefore, Eiders and some other species have developed fasting endurance, high incubation constancy and use endogenous reserves accumulated before nesting, for daily energy expenditure (Afton and Paulus 1992). Bird species that feed little and lose weight during incubation but sustain themselves on endogenous reserves are called capital breeders (Ankney and MacInnes 1978, Ankney and Afton 1988, Parker and Holm 1990). Conversely, species or populations that can forage during incubation recesses are known as income breeders (Meijer and Drent 1999). Among Eiders, the mean clutch size in a nest is 3–6 eggs (Gross 1938, Milne 1972, Coulson 1984, van Dijk 1986, Skarphéðinsson 1993, Snæbjörnsson 1998, Christensen and Falk 2001, D´Alba 2007, Bédard et al. 2008, Kristjánsson 2008, Jónsson et al. 2009, Kristjánsson and Jónsson 2011) and they, as other capital breeders, have a very high incubation constancy as the females stays at the nest for 90–97% of the incubation period (Bolduc and Guillemette 2003,

2 Introduction

Kristjánsson and Jónsson 2011). This high incubation constancy requires energy expenditure from stored body fat, which causes the Eiders to lose 25– 45% of their pre-incubation body mass during the 24–27 day incubation (Korschgen 1977, Parker and Holm 1990, Skarphéðinsson 1993, Harðardóttir et al. 1997, Kristjánsson and Jónsson 2011, Jaatinen et al. 2012). Clutch size of Eiders along with the high incubation constancies is more similar of nesting behavior of geese than other ducks (Thompson and Raveling 1987). This is likely because the Eider is one of the largest ducks and is larger than some of the small geese, such as Brant Goose (Branta bernicla), Red-breasted Goose (Branta ruficollis) and Barnacle Goose (Branta leucopsis) (Waltho and Coulson 2015) and can therefore accumulate more endogenous energy reserves than smaller duck species (Thompson and Raveling 1987). Eiders use nest down to insulate their nests and maintain desirable temperature and humidity for the eggs during incubation (Kristjánsson and Jónsson 2011). Nest down is also used to conceal the eggs while the nest is unattended during egg laying and incubation recesses (Afton and Paulus 1992, Hilton et al. 2004). Nest down has been collected in Iceland for centuries and is traditionally replaced with dry hay, a practice considered not to compromise nest temperature nor hatching success of the birds (Kristjánsson and Jónsson 2011). Nowadays, eiderdown is collected, cleaned and used to make valuable products such as duvets, sleeping bags and clothes (Bédard et al. 2008, Fuller 2015). The high and stable nest temperature is also favored by nest parasites (fleas, lice and ticks) and their abundances increase with a more stable microclimate (Sinclair and Chown 2006). Nest parasites have a substantial negative effect on the incubating bird due to blood loss and increased possibility of infections (Möller 1993, Oppliger et al. 1994, Bush et al. 2001, Lesna et al. 2009, Clayton et al. 2010, Mainwaring et al. 2014, Stenkewitz et al. 2015, Wetherbee 2016).

1.2 Incubation behavior of the Common Eider Sociability and help from related individuals are important ways to pass genetic material to the next generation, when individual parents are not capable of raising offspring all by themselves (Hatchwell 2010, Tiedemann et al. 2011). Many animals assist in rearing related young and thereby take care of their own genetic pool (Hatchwell 2010). In some birds, parasitic behavior occurs where individuals lay their own eggs into nests of an unrelated parent which in turn raises the young like their own (Yom-Tov 2001). If the cost of rearing this offspring is lower than the risk of losing their own offspring the parent accepts the foster offspring (Hatchwell 2010). It is hard to distinguish

3 PhD theses Thordur Örn Kristjánsson 2016 between these helpful or parasitic behaviors, unless by inferring the relatedness of the offsprings by means of genetic analysis. Parasitic nesting within a species (intraspecific brood parasitism, egg dumping) has been documented in 234 bird species. It is widespread among the Anseriformes and the Passeriformes (Yom-Tov 2001). Parasitic nesting is a common nesting strategy among Eiders (Robertson et al. 1992, Waldeck and Andersson 2006, Waldeck et al. 2008). In a study at Hudson Bay, Canada, one third of nests, with a mean clutch size of ≤6 eggs, had at least one parasitic egg in the clutch (Waldeck and Andersson 2006, Andersson and Waldeck 2007). An enlarged clutch of ≥7 eggs is therefore likely to be a parasitic nest where one or more Eiders have laid eggs in the nest of another female (Waldeck and Andersson 2006, Bédard et al. 2008). Many hypotheses have been brought forward to explain this parasitic behavior among Eiders, such as shortage of nest sites and attempts to have offspring when the mother is not fit to raise them herself (Öst et al. 2002, Waldeck and Andersson 2006). It could also be the case that accepting parasitic eggs in a clutch lowers predation risk of the Eiders own eggs (Eadie et al. 1988) or provides inclusive fitness benefits if the host and parasite are related to one another (Andersson and Waldeck 2007, Tiedemann et al. 2011). Parasitic nests are more common on smaller islands with higher nest densities than on larger islands with sparser nest densities (Hines and Mitchell 1984, Rohwer and Freeman 1989, Sayler 1992, Robertson et al. 1992). This behavior is therefore probably closely related to nesting density and number of optimal nesting sites in the colony. Lack of nest cover for the Eiders has most likely influenced this parasitic nesting behavior because good cover can stabilize temperature fluctuations experienced by the females which in turn is energy saving for the incubating bird (D´Alba 2007, Öst et al. 2008). Nest density in Iceland is highly variable among colonies. At Rif, West Iceland, the density amounts to 1.7 nests/m2 (Snæbjörnsson 2001, Kristjánsson and Jónsson 2015) in the more crowded island but about 5500-8500 nests/ha for the whole colony each year. This is among the highest nest densities observed for Eiders to date. At Hvallátur, Breiðafjörður, West Iceland, the nest density was recorded 12–400 nests/ha (Kristjánsson 2008) which is a common density in Eider colonies in Iceland. Nest densities in Eiders generally range from solitary nesting up to 300 nests/ha (Milne 1972, Christensen and Falk 2001, Waltho and Coulson 2015) but at a few areas extreme nest densities have been recorded such as at St. Lawrence estuary at Canada (825 nests/ha), (Munro and Bédard 1977). At Hudson Bay, the nest success of parasitic nests was similar to single-female nests (where all eggs within the clutch had the same mother) when the mean

4 Introduction clutch size was ≤6 eggs. This indicates that energy costs to hatch a parasitic egg are minimal when clutches are of normal size (Waldeck and Andersson 2006). Nevertheless, nest success of larger parasitic nests can be lower than in single- female nests if there are costs of incubating enlarged clutches in Eiders (Kilpi and Lindström 1997, Hanssen et al. 2003). Unusually big clutches, of ≥7 eggs, can increase the probability that the Eider abandons the nest, does not complete incubation, or that eggs develop at different rates and do not hatch simultaneously (Sayler 1992). Sometimes eggs break or fall out of the nest due to lack of space (Sayler 1992) or get buried at the bottom of the nest bowl (Kristjánsson pers. obs.). Different results and clutch sizes between studies calls for some investigations on this phenomenon in Iceland and the impact of these enlarged clutches on the incubating Eider should be determined.

1.3 Energy expenditure of incubating Common Eiders At least three hypotheses have been suggested to explain the relationship between clutch size (eggs that the female lays herself) and body condition in waterfowl. First, Ankney et al. (1991) stated the egg formation hypothesis, i.e. that body reserves available to the laying females determine the optimal clutch size. Females in better physical conditions produce larger clutches than females with small body reserves; generally small clutch sizes indicate food shortage (Parker and Holm 1990, Erikstad et al. 1993, Kilpi and Lindström 1997, Coulson 1999, Hanssen et al. 2005). The second hypothesis, the expected fitness hypothesis, posits that the females adjust their post-laying condition according to the anticipated fitness benefits of the clutch (large clutch = higher prospective fitness benefits; Mehlum 2012). Therefore, Eiders in good condition lay eggs earlier, have larger clutches and a better fledging success than lighter ones due to better nest attendance (Criscuolo et al. 2002, Gorman and Nager 2003) and they also suffer less brood abandonment (Kilpi et al. 2001, Öst et al. 2003). The third hypothesis, the condition dependent model, states that large individual variation in endogenous reserves at the onset of incubation mostly is driven by factors others than clutch size and laying date in Arctic-nesting Eiders (Rowe et al. 1994, Betý et al. 2003, Sénechal et al. 2010). The reproductive effort, (the females own clutch size) is often determined by the extent the birds can build up endogenous reserves (Parker and Holm 1990, Ankney et al. 1991, Erikstad et al. 1993). However, parasitic nesting behavior of Eiders can result in large clutches even though the incubating female has only produced a few eggs of her own (Erikstad and Tveraa 1995). Older and more experienced Eiders are often in a better body condition than young ones

5 PhD theses Thordur Örn Kristjánsson 2016 and might be more capable of accepting these extra eggs in their clutch (Andersson and Waldeck 2007, Tiedemann et al. 2011). Here, we used banding age of the Eiders to investigate if older females were more likely to have parasitic clutches. The regulation of clutch size and its impact on the incubating bird has long been a central issue in ornithology and still remains controversial (Ankney et al. 1991, Arnold and Rohwer 1991). Waterfowl (Anatidae) are unique among birds in shedding down from their bellies and placing it as insulation in the nest. Eider farmers collect the nest down and replace it with dry hay. Nevertheless, females maintain optimal temperatures in nests and have similar hatching success as females incubating in down nests (Kristjánsson and Jónsson 2011). The impact of down collection on energy expenditure during incubation of Eiders has not been estimated even though eiderdown is sold as an environmental friendly product.

1.4 Ectoparasitic loads in the nests of Common Eiders Bird fleas are predominantly found in nests rather than on the birds themselves, and all life-cycle stages (egg, larvae, pupae and adults) are completed within the nest. Fleas overwinter in the bird nests as pupae, which hatch the following spring when a bird returns to the nest bowl (Harriman et al. 2008, Wetherbee 2016). Ectoparasite abundances in bird nests have been observed to increase with more favorable thermal microclimate, due to better insulation depending on bottom material (Sinclair and Chown 2006). Ceratophyllus flea species overwinter in nests of many waterfowl species, where all life cycle stages are completed with no dispersal of fleas until the following year, when new adults emerge from cocoons and infect the incubating females (Harriman et al. 2008, Wetherbee 2016). Densities of fleas can be positively related to the age of the nesting bowl (Harriman et al. 2008). Adult fleas living in the nest suck blood from the incubating bird and defecate blood on eggs in the nest which flea larvae use as nutrition. Adult flea abundances in a nest have been positively related to blood cover on the eggs in Snow Geese (Chen caerulescens) (Harriman et al. 2008) and Barnacle Geese (Branta leucopsis) (Wetherbee 2016). This study was the first to examine the extent of blood covered eggs in Eider nests. The high incubation constancy of Eiders, and other capital breeders such as Snow Geese and Barnacle Geese, helps keep the thermal microclimate stable but also is favorable for the parasites in the nest. The body condition of the Eiders during this extended period may affect their ability to withstand

6 Introduction disease or parasite loads during the incubation (Hanssen et al. 2003). Parasites in Eider nests have not been studied before in Iceland.

1.5 Diet of the Common Eiders Eiders feed on a large variety of benthic invertebrate species and often specialize on one or more species depending on location, season and food availability (Merkel et al. 2007). They often exploit intertidal and subtidal mollusks but also crustaceans and echinoderms (Guillemette et al. 1992, Guillemette et al. 1995, Leopold et. al. 2001). While wintering in the Gulf of St. Lawrence, Canada, Eiders fed mainly on a mixed diet of sea urchins and small mussels (Mytilus edulis) (length 1–25 mm with a mode 7–8 mm) (Guillemette et al. 1992), whereas in Northern Norway, in April and May, and at different locations around Denmark medium sized mussels (length 20–40 mm) were the main food item (Madsen 1954, Bustnes and Erikstad 1988, Bustnes 1998). In Northwest Iceland, adult Eiders ate mostly blue mussels during July and August but while taking care of the ducklings they ate also gastropods and amphipods (Gammarus spp.) in large quantities (Garðarsson et al. 1980). The ducklings ate mainly Gammarus spp. (Garðarsson et al. 1980). Changes in food selection of Eiders during the year was investigated in Southwest Iceland (Skírnisson and Jónsson 1996, Skírnisson et al. 2000) in February, May, end of June and November of 1993. Mussels were the most preferred food item in all months investigated, but gastropods peaked after incubation in June for the females. Crustaceans were mostly eaten during the winter in November and February by both sexes. The diet of Eiders in the large Breiðafjörður Bay, where the bulk of the Icelandic population resides, has not been studied but the area is highly valuable for eider farmers. It is important to investigate the diet of Eiders in this area during the incubation period to track down the key food items that are of high importance to both pre-nesting and post breeding birds.

1.6 The study area: Breiðafjörður, West Iceland Breiðafjörður, West Iceland, is a 50 km wide and 125 km long bay of 6000 km2 (Fig. 1). The bay is relatively shallow, characterized by numerous islands and skerries. A somewhat deep canyon penetrates the fjord from southwest to northeast, called “Kolluáll” in Icelandic. It is deepest in the southwestern part of the bay, about 200 m, becomes gradually shallower and is about 100 m in the middle of the bay, then it levels off and most of the inner part of the bay is less than 30 m deep. Along the shores of the bay numerous shallow fjords (often <10 meters depth) occur, with extensive tidal flats. The mean tidal amplitude during spring tide is 4.4 m, but can extend up to 6 m. Tidal currents can be very strong. The numerous small islands and the jagged coastline are generally surrounded by

7 PhD theses Thordur Örn Kristjánsson 2016 rocky shores and it has been calculated that more than half the surface area of rocky shores of Iceland occur in Breiðafjörður (Ingólfsson 2006). The shores at Breiðafjörður are well sheltered because the islands and skerries at the outer part of the bay provide shelter from the open ocean currents (Hauksson 1977). Breiðafjörður is an important breeding site for Eiders and supports at least 25% of the large Icelandic Eider population (Grimmett and Jones 1989, Jónsson et al. 2009). Down collection by eider farmers is commonly practiced in the area (Jónsson et al. 2015).

Figure 1 A map showing Breiðafjörður, West Iceland, and the study sites at Rif and Hvallátur.

.

8 Aims

2 Aims

The general aims of this thesis was to investigate if super-dense nesting shapes incubation behavior of Eiders and how this behavior influences energy expenditure of the birds during the breeding period. High numbers of Eiders feed at Breiðafjörður and accumulate energy reserves during the pre-breeding period. Therefore, an investigation on the key food items for Eiders in the bay also was performed. The aims of paper I were to determine whether individually marked Eiders at a super-dense colony attended more than one nest during incubation and if other unmarked Eiders tended the marked Eider’s nests during their incubation recesses. Incubation is very costly for capital breeders and behavior during this fasting period plays an important role in the energy expenditure of the bird. The hypothesis in my investigation is that Eiders at Rif show shared nest attendance during incubation. The aims of paper II were to investigate the energy cost of incubation in Eiders, indexed by relative mass loss. In particular, my interests were to: 1) Determine if the energy cost was related to clutch size or body condition of the female at the start of the incubation, particularly with respect to enlarged clutches; 2) Compare nests with and without down collection; and 3) Determine if clutch size was related to body condition of the female at the start of incubation or banding age (incubation experience) of the females. Finally, the findings were compared with those from previous studies of Eiders to assess differences between sub-species and populations of Eiders regarding weight and weight loss. The hypothesis are that 1) body weight of the female at the start of incubation is positively correlated to clutch size and 2) that enlarged clutch size and down collection increases energy costs for the females. The aims of paper III were twofold: 1) To investigate if differences in nest densities and nest ecology in the two Eider colonies are reflected in different parasite loads in the nests at the same time of the year (June); and 2) To compare parasitic loads between months (June/July), where the months represented early-nesting females (June) and late-nesting females (July). At Rif, many of the nests are used by two females within a season (two clutches per nest, i.e. early and late nesters). Conversely, the nests in Hvallátur are only used by a single female (single clutch) within a season. Fleas are irritating for the Eiders and might increase energy expenditure of the bird if the fleas are

9 PhD theses Thordur Örn Kristjánsson 2016 present in high numbers, by forcing them off the nest to preen more often. The hypothesis investigated is that abundances of parasites in the Eider nest increased with: 1) higher nest density of the colony; and 2) re-using of the nest bowls within a season. The aims of paper IV were to investigate the main food items in Breiðafjörður taken by Eiders during late spring and early summer (May-July) when the birds are accumulating energy for breeding and to explore annual variation and sex differences in their diet. The hypothesis is that the key food items at Breiðafjörður were the same as in other studies, the blue mussel, for both sexes and all years investigated.

10 Materials and methods

3 Materials and methods

3.1 Study sites The offshore colony at Hvallátur (65°25’79N, 22°46’05W) is located within the Vestureyjar archipelago, in the middle of Breiðafjörður Bay, Western Iceland. The coastal site at Rif (64°55’24N, 23°49’36W) is located in the southern part of the bay at the western tip of the Snæfellsnes peninsula (Fig. 1). In both colonies, Eider farmers collect the nest down during incubation but the nests studied were left intact during the study period and only subjected to study treatments. The two colonies in the present study differ in size and in nest density. Hvallátur is a large colony (the part Heimalönd is about 200 ha) with a normal nest density (12–400 nests/ha) and the nests are spread over a small archipelago. Each nest is only used once per nesting season. The data from Hvallátur were collected on two small, lowland islands (around 0.2–0.5 ha with 12–58 nests/ha, Kristjánsson and Jónsson 2011). Rif holds a small colony (2.34 ha pond) with a high nest density (up to 1.7 nests/m2) on two small islands (120 m2 and 600 m2), where most of the nests are used at least twice in the nesting season and the females attend each other’s nests and nest parasitism is common (Jónsson and Lúðvíksson 2013, Kristjánsson and Jónsson 2015). Such second use of nest bowls and cooperating nest behavior has never been observed at Hvallátur. The offshore site Hvallátur is exposed to oceanic wind and salt spray from all directions, while Rif is sheltered from wind from the south and east.

3.2 Rif colony The studies presented in papers I, II, and III were carried out in the Eider colony at Rif (Fig. 1). The colony was established in 1972 when farmers dragged large stones on ice during the winter to the middle of the pond (Snæbjörnsson 2001, Jónsson and Lúðvíksson 2013, Kristjánsson and Jónsson 2015). When thawed in the spring, sand and seaweed was spread on the stones, and over the years a small Rock Island (“Gamli Hólmi“) was made and in 1990 the Grass Island (“Nýi Hólmi“) was established by digging a canal separating a small area from the mainland (Fig. 2). Both of these islands are now fully colonized with high and stable nest numbers each year. The Eider is the only species nesting on the islands. Despite the islands being originally man-made, the studied population, presently at 500–600 nests, is wild and free ranging like other Eider populations (Jónsson et al. 2016).

11 PhD theses Thordur Örn Kristjánsson 2016

Figure 2 The observation site of the Rif colony, Rock island (“Gamli Hólmi”) to the left and Grass island (“Nýi Hólmi”) to the right in the pond. Photo: Thordur Örn Kristjánsson.

3.3 Hvallátur colony The study presented in paper III was conducted at both Rif and the Hvallátur archipelago (Fig. 1). Hvallátur is a large colony, with a very high diversity of marine invertebrates in the surrounding waters (Hauksson 1977). Here each nest bowl is only used once per nesting season. The islands studied are small and the distance from an Eider nest to the intertidal zone never exceeds 20 m. The predation risk for Eiders is considered low, and only avian predators have been seen around the islands, which are too far away from the mainland (17 km) for Arctic Fox (Vulpes alopex) to colonize them (Björnsson and Hersteinsson 1991). The American Mink (Neovison vison) has been observed on some islands at Breiðafjörður but not at Hvallátur. Graylag Geese (Anser anser), Puffins (Fratercula arctica), Arctic Terns (Sterna paradisae) and other species of birds have been reported nesting at the study islands.

3.4 Incubation behavior of Common Eiders In paper I, a total of 26 female Eiders incubating at Rif were caught on 25–26 May and 1–2 June 2011 with a noose-pole and marked with nose markers (Juno Minnesota, USA) of different color and shape for identification when incubating on the nest (Fig. 3). The Eiders also were individually marked with a red plastic leg-ring with a white engraving (Pro-Touch, Canada). The nests were marked with plastic poles bearing a nest number.

12 Materials and methods

Figure 3 A nasal-marked female Eider incubating her nest at Rif in summer of 2011. Photo: Thordur Örn Kristjánsson.

Behavior of the birds was observed with a LEICA Televid telescope (20–60×) (Fig. 2). Behavior of the 26 incubating females was observed for 2–3 hours for 5 days (31 May; 6, 8, 11 and 16 June 2011). It was documented if the marked Eider attended to more than one nest, another Eider (both sexes) attended to a marked nest and if marked Eiders attended to other nests while on an incubation recess.

3.5 Energy expenditure of incubating Common Eiders In paper II, a total of 41 incubating Eider females were captured twice at Rif, in the years 2009 and 2010, with a noose pole with a nylon snare (15 and 26 Eiders each year, respectively) in order to weigh them during incubation and again at hatching. Each female was individually marked with a metal leg band. Eiders have been marked in this colony since 1993 so many leg-banded Eiders were already present (Jónsson 2012, Jónsson and Lúðvíksson 2013). Female Eiders generally breed for the first time at 2–3 years of age but first breeding of males happens at 4 years and older (Baillie and Milne 1982, Hario and Rintala 2009). We assumed that this “banding age” is correlated with biological age (Öst et al. 2011). After leg marking, the females were weighed using a Pesola spring scale (with 2.5 g accuracy). The clutch size in each nest was recorded and the eggs were candled by using the sunlight to see through the eggshell to estimate the incubation stage (Weller 1956, Gíslason 1984). These females were re-captured and weighed for the second time right before

13 PhD theses Thordur Örn Kristjánsson 2016 hatching (0–2 days). In the present study, clutch sizes of ≥7 eggs were considered enlarged, as clutches of ≥7 eggs are rarely observed in Breiðafjörður outside the Rif colony (Kristjánsson and Jónsson 2015) whereas clutches of ≤6 eggs were considered to be of normal clutch size. Twelve of these females were caught on control nests (where nest down was not removed) and 29 females were caught on nests where nest down was collected and replaced with hay to use as nest lining. The cost of incubation was calculated as relative mass loss (% of body mass loss throughout the incubation period of 24 days). The effects of nest material (down vs. hay), clutch size (2–9 eggs) and body condition at the start of incubation was investigated. Banding age was used to investigate if older females were more likely to have parasitic clutches. Finally, the results were compared to other studies on the subjects. Statistical analyses were performed with SigmaStat 12.3. A t-test was used when data passed a normality test (Shapiro-Wilkes) but otherwise a Mann- Whitney U test when: 1) comparing female body mass at the start of incubation and at hatching between years; and 2) comparing the daily weight loss during incubation between hay and down nests and comparing clutch size and incubation experience. Linear regression assessed relationships between: 1) clutch size and body condition (body mass at start of incubation); and 2) relative mass loss through the incubation and clutch size. An effect was considered significant at P < 0.05 (Sokal and Rohlf 2001).

3.6 Ectoparasitic loads in the nests of Common Eiders In paper III, a total of 28 nests were collected after incubation was completed and the females had left, 14 nests from Hvallátur and 14 nests from Rif on the 7th and the 5th of June 2012, respectively. At Rif, the 12 nests that were used by a second female, after the first female had led out her brood, were collected on the 17th of July 2012. All collected nests had clutch sizes of four eggs. In all cases the whole nest was collected, i.e. nest bottom materials and all nest down that the females had shed into the nests. Nests were classified as containing blood-covered eggs or not and if nest bowls were old or new. The incubation stage of the eggs was estimated by floating the eggs in water (Liebezeit et al. 2007) and they were also candled for comparison (Weller 1956, Gíslason 1984). The 24 day incubation period of Eiders (Kristjánsson 2008) was divided into three incubation stages (early, mid- and late incubation), were each stage consisted of 8 days of incubation. After

14 Materials and methods collections, the nests were double-bagged separately and individually marked for identification and frozen. In the laboratory, bags were thawed twice to kill the invertebrates living in the nests (Harriman et al. 2008). Invertebrates were separated from the nest down in the laboratory by shaking and sieving. The invertebrates were then placed in 70% ethanol for later identification in a laboratory. The invertebrates from each nest were identified to species when possible and number of individuals observed counted under a dissection microscope at different magnifications, depending on their sizes. The dominant nest bottom material was identified to vegetation species level. Flea abundances (adult and larvae) were estimated in June at Rif and Hvallátur and in July for Rif. The occurrence of blood-covered eggs, nest bowl ages, nest bottom material and the incubation stage of eggs in the nest was also related to the flea abundances. A t-test was used with the SigmaPlot 12.3 program when comparing winter and summer temperature at Hvallátur and Rif (Sokal and Rohlf 2001). Other statistical analyses were performed with the SAS program. A generalized linear model (PROC GENMOD, SAS Institute, Inc., Cary, NC) was used with colony and timing of nest collection (Rif in June, Rif in July and Hvallátur in June) as the class variable and incubation stage, blood cover, nest material and age of nesting bowl as the co-variants to compare abundances of adult fleas and larvae. A Poisson distribution was also used with a logit link for the ectoparasite count data. An effect was considered significant at P < 0.05 (Sokal and Rohlf 2001).

3.7 Diet of Common Eiders In paper IV, total of 192 adult Eiders were collected by fishermen as by-catch in lumpsucker (Cyclopterus lumpus) gillnets for a study of the spring/early summer diet of Eiders in Breiðafjörður. Most of the birds (155 individuals) were caught near Flatey Island in the northern part of Breiðafjörður. Thirty- seven birds were collected from southern part about 2–10 km from the village of Stykkishólmur (Fig. 1). Both of the collection sites are typical spawning habitats for lumpsucker and feeding habitats for Eiders (7–15 meter depth, hard-bottom with kelp beds). The Eiders were collected from the fishermen from the end of May to July during 2007–2010. All birds were bagged, tagged, and frozen on the day of collection. During subsequent dissections the stomach (esophagus and proventriculus) and the gizzard were removed and contents of these organs identified to species level whenever possible. The numbers of individuals of each prey species were counted, and shell length of polyplacophorans, gastropods, bivalves, echinoderms, and crustaceans were

15 PhD theses Thordur Örn Kristjánsson 2016 measured by Vernier caliper to the nearest 0.1 mm, using a dissecting microscope when necessary. Occurrence of key food items was compared between years, months and sexes. A Welch two sample t-test was used for statistical comparisons when the data passed a normality test. A one-way ANOVA test was also used or a Kruskal- Wallis one-way analysis of variance on ranks when the normality test (Shapiro-Wilkes) failed (Sokal and Rolf 2001). When testing for difference in the size of chitons between sexes a t-test was used but an ANOVA when looking at the size difference of chitons between months. A Kruskal-Wallis test was used when testing for the size difference of chitons between years. Analyses were done using the Sigma Stat 12.3 program. An effect was considered significant at P < 0.05 (Sokal and Rohlf 2001).

16 Results and discussion

4 Results and discussion

4.1 Incubation behavior of Common Eiders in a super-dense Eider colony (paper I) A total of 39 incubation recesses were documented among the 26 marked females. In 10 of 39 incubation recesses (26%), the marked females themselves attended to other nests on their way to and from their own nests. When a marked female Eider left her own nest, she stopped for a few moments and covered other clutches with nest down during 8% of recesses (3 of 39 occasions). Marked Eiders were also seen sitting on other nests during 15% of recesses (6 of 39 occasions). In 3% of recesses (1 occasion), marked Eiders were seen both covering other nests and sitting on other nests. During recesses, marked Eiders were also seen drinking water from the pond, bathing and preening. In 31 of 39 incubation recesses (79%), other Eiders attended the focal marked nests. Unmarked Eiders covered a focal nest during 26% of recesses (10 of 39 occasions) and sat on focal nests during 44% of recesses (17 of 39 occasions). In 10% of recesses (4 of 39 occasions) unmarked Eiders were seen both covering and sitting on a focal nest (Table 1). Included here is one occasion where a male was seen sitting on a focal nest and three occasions where males covered a focal nest. Table 1 Behavior of 26 marked incubating female Eiders during the 2012 nesting season (31 May, 6, 8, 11 and 16 June, with total numbers of incubation behavior in bold) at Rif, Snæfellsnes, Iceland.

BEHAVIOR 31 May 6 June 8 June 11 June 16 June Total %

Marked birds incubation recess 2 9 2 22 4 39

Marked bird covers another nest 0 2 0 1 0 3 8%

Marked birds incubates another nest 1 2 1 1 1 6 15%

Marked bird both incubates and covers another nest 1 0 0 0 0 1 3%

Unmarked bird covers a identified nest 0 2 0 6 2 10 26%

Unmarked bird incubates a identified nest 0 5 0 12 0 17 44%

Unmarked bird both incubates and covers a identified nest 0 0 2 2 0 4 10%

17 PhD theses Thordur Örn Kristjánsson 2016

Why Eiders show this incubation behavior toward other nests is not known and has not been reported before. Nevertheless, our findings may be applicable to other Eider colonies with high nest densities. There are at least four possible explanations for our observations that merit further investigation: At Rif, clutch size was high in most years and nests with seven eggs or more were common. We saw that Eiders incubated unmarked nests with as many as 18 eggs. Eiders were also seen hatching and fledging up to 10 young from a single nest at Rif. Nest parasitism in Eiders is often related to poor body condition of the parasitizing female which then does not help the incubating Eider hatch the young (Öst et al. 2005). If the parasitic female’s own clutch is predated early in the egg laying period, a salvage strategy could be to lay the rest of her eggs in another female’s nest. Eiders at Rif are probably closely related to one another and might help related kin in the passing of genetic material to the next generation, when individual parents are not capable of incurring the costs of raising offspring all by themselves (Waldeck and Andersson 2006). An Eider returning from an incubation recess in a colony with extremely high nest densities might become confused and mistake other female nests as their own. Every Eider passes many other unguarded nests during the incubation recess, so there is a strong visual stimulus, possibly a form of supernormal stimuli (Tinbergen 1951). The endocrine system of birds does play an important role in incubation behavior and formation of the brood patch (Lea and Klandorf 2002). High secretion of the hormone prolactin during this period, along with a high visual stimulus of unguarded nests, probably shapes this shared nest attendance of the Eiders. Incubation is costly for the Eiders so the females are on a tight energy budget. Sitting on other nests, with warm eggs, might be a response to minimize heat loss from the brood patch of the bird (Schmidt et al. 2006). This cooperative behavior seen at Rif might have evolved from more than one of these suggestions mentioned. It would greatly help to perform a study were relatedness of the Eiders in the colony and eggs in their nests were investigated on a genetic basis. This would help to determine if females incubating the same clutch are indeed related and whether or not they own some eggs in the clutch themselves.

4.2 Energy expenditure of incubating Common Eiders (paper II) Total mass loss of Eiders was positively related to body mass at the beginning of incubation, because heavier females lost proportionally more mass during incub- ation (Fig. 4). This is in agreement with Erikstad and Tveraa (1995) and Hanssen et

18 Results and discussion al. (2005). Conversely, no such relationship was found in Northern Norway (Hanssen et al. 2005), Northern Iceland (Hardardóttir et al. 1997) nor in Eiders from Denmark (Bolduc and Guillemette 2003). This discrepancy between studies could be explained by annual or local variation in breeding strategies of Eiders, different fat accumulation between years or difference in sample size and weighting between studies. It could be that heavy females in good condition at the start of incubation have higher incubation constancy than lighter females and leave their nests less frequently, which in turn increases their mass and water loss during this period. Lighter females might take longer and more frequent recesses to preen and drink water but still hatch their clutch with less mass and water loss than heavier females, because other females attend their nest during recesses as was seen in paper I. We have then two different nesting strategies at Rif for heavy and light females which both result in the same incubation time and success. Mass loss of Eiders was not related to nesting material as Eiders lost the same amount of mass whether they incubated on down or hay nests. No relationships were observed between: 1) clutch size and body mass at start of incubation; or 2) between clutch size and banding age (an index of incubation experience, Fig. 5). Our findings indicate that down collecting does not influence energy expenditure in incubating Eiders and neither do enlarged clutches of ≥7 eggs. Females with enlarged clutches were most common in the mid-ranges (7–15 years old) of both relative mass loss (27–37%) and banding age. Middle-aged females have more incubation experience and are perhaps more tolerant towards parasitic females than younger ones. This is in agreement with results from north Iceland (Tiedemann et al. 2011) where Eiders with parasitic clutches were average 9.1 years old but Eiders with non-parasitic clutches were average 6.8 years old. The females own clutch size tends to lower with age (Dow and Fredge 1984) at the same time accepting parasitic eggs increases (Tiedemann et al. 2011). At Rif females, 15 years and older, may therefore be incubating normal clutch size even though the clutch has some parasitic eggs from related females (Tiedemann et al. 2011, Andersson et al. 2015). These findings are consistent with the condition dependent model (Rowe et al. 1994, Betý et al. 2003, Sénechal et al. 2010), especially as there seems to be no extra cost for incubating ≥7 eggs in our study. This suggests that the body store differences at incubation onset are mostly caused by environmental conditions encountered by laying females and that body reserves prior to incubation do not control clutch size or laying date (Sénechal et al. 2010, Descamps et al. 2011). Mass of the Eiders at the beginning of incubation are thus not necessarily a good indicator of female quality, as both light and heavy females have the same energy threshold to initiate egg production and complete subsequent breeding phases (Sénechal et al. 2010).

19 PhD theses Thordur Örn Kristjánsson 2016

Figure 4 Body mass at the beginning of incubation of 41 Eiders and relative loss mass during the incubation period of 24 days (SE = 73, P<0.001). Red dots indicate enlarged clutches (≥7 eggs).

Figure 5 Clutch sizes of 41 incubating Eiders from the years 2009 (squares) and 2010 (diamonds), plotted against year of first capture and marking by eider farmer (years of incubating experience, P=0.20). Red dots indicate enlarged clutches (≥7 eggs).

The average body mass of the Eider females at the start of incubation in the present study (1935 g), body mass at hatching (1322 g), the average energy cost of incubation (relative mass loss) (32%) and average rate of weight loss/day (26 g) are all within the range of results for Eiders from other studies

20 Results and discussion

(Table 2). There is a great overlap in both body mass at the start of incubation and at hatching within sub-species of Eiders when comparing different studies (Table 2). At the beginning of incubation, differing fattening capabilities of these sub-species or populations, or the respective ecological conditions they face during pre-breeding fattening might explain these differences. Table 2 Comparison of Eider studies on energy cost of incubation. Body mass at the start and end of incubation, mass loss/day, relative mass loss and clutch size reported for Eider populations in North America and Europe

mass

-

body body

-

loss %

relationship?

species of Eiders

-

Body Body mass at start of incubation (g) Body mass at hatching (g) Mass loss/day (g) Relative mass Clutch size range Evidence a clutchfor size condition Evidence a conditionbody for loss relationship? Reference Sub

Korschgen 1977 dresseri 1830 1290 x 32 x No x

Hardardottir et al. 1997 borealis 1799 1308 19 27 x x No

Gabrielsen et al. 1991 borealis 2106 1357 30 36 4-5 x x

Parker and Holm 1990 borealis 1773 1368 19 22.8 4-6 Yes x

Erikstad and Tveraa 1995 mollissima 1852 1483 24,7 20 3-6 No Yes

Hanssen et al. 2003 mollissima 1865 1496 24,7 20 x No x

Hario 1981 mollissima 2100 1520 x 28 x x x

Bolduc and Guillemette 2003 mollissima 2346 1371 37,5 41.6 5 x No

Hanssen et al. 2002 mollissima 1878 x x x 4-5 No Yes

D´Alba et al. 2009, sheltered nests borealis x x x 30 1-6 No x

D´Alba et al. 2009, exposed nests borealis x x x 33 1-6 No x

Present study borealis 1935 1322 26 32 2-9 No Yes

21 PhD theses Thordur Örn Kristjánsson 2016

4.3 Ectoparasitic loads in the nests of Common Eiders (paper III) The invertebrate composition observed in the Eider nests in Breiðafjördur consisted primarily of two ectoparasite species that could negatively affect the birds if present in high quantities, i.e. the flea Cerotophyllus garei (Fig. 6) and the tick Ixodes uriae (Fig. 7), which was observed in very low quantities. In the North Atlantic, six species of fleas have been reported from seabird colonies (Mehl 1992) and of these species only Cerotophyllus vagabundus and C. garei have been observed on seabirds in Iceland (Brinck-Lindroth and Smith 1971). C. garei was well established at both study sites and was observed at all life-cycle stages in all nests investigated, often in high densities.

Figure 6 The flea C. garei, from an Eider nest in Hvallátur. Photo: Svanhildur Egilsdóttir.

The abundance of C. garei in June was higher at the offshore Hvallátur than at coastal Rif. However, there was greater variation in numbers of individuals between nests at Rif, which might be influenced by a low number of nests in the study. At Rif, the flea abundance increased in July when the incubation of the second brood was almost complete. The high nest density and short distances between nests at Rif might facilitate a relatively fast spread of fleas between the nests.

22 Results and discussion

Figure 7 The tick I. uriae, from an Eider nest in Hvallátur. Photo: Svanhildur Egilsdóttir.

A positive relationship was also observed between C. garei abundance and the bottom material of the nests, the blood cover of eggs and the incubation stage. This might indicate increased favorable microclimate for the C. garei with increased incubation time. The blood-cover in our study was generally low (≤5% cover of the egg shells), while in a study on ectoparasites in nests of Snow Geese, eggs had up to 50% blood cover (Harriman et al. 2008). It is possible that different flea species defecate different amounts of blood in the nests and C. vagabundus may defecate more blood than C. garei. The high numbers of C. garei in some nests compared to studies of Eider from Svalbard (Coulson et al. 2009) and Snow Geese in Canada (Harriman et al. 2008) could be due to more dense vegetation in Iceland, which results in better shelter and more stable environment (as understood by D´Alba et al. 2009) for the fleas in the nests. The seemingly cooperative incubation behavior at Rif (paper I) might be a response to the high C. garei abundances in the colony, as shared nest attendance of females gives the incubating bird some extra time to preen while other females attend their nests.

23 PhD theses Thordur Örn Kristjánsson 2016

4.4 Diet of Common Eiders in Breiðafjörður (paper IV) The most frequent food item of the Eider in Breiðafjörður was the mottled red chiton Tonicella marmorea (Fig. 8) which was consumed by over half of all birds investigated. The second most important species was the whelk Buccinum undatum, then the spider crab Hyas araneus and the gastropod Lacuna vincta (Fig. 9 and Fig. 10). These results are different from what has been observed in other studies from Arctic, sub-Arctic and temperate sites, where bivalves, mostly the blue mussel Mytilus edulis, have often been reported as the dominant food item (Bagge et al. 1973, Garðarsson et al. 1980, Goudie and Ankney 1986, Bustnes and Erikstad 1988, Skírnisson and Jónsson 1996, Larsen and Guillemette 2000, Skírnisson et al. 2000, Nehls and Ketzenberg 2002).

Figure 8 Chitons (Tonicella spp.) from Eider stomach sample from Breiðafjörður. The mottled red chiton (T. marmorea) to the left and the northern red chiton (T. rubens) to the right. Photo: Svanhildur Egilsdóttir.

Here, both the chitons and mussels consumed were of small size, with average sizes of 11.7 and 13.4 mm, respectively. Eiders are known for prey selection of small size mussels (10–20 mm) to maximize flesh and energy intake without increasing the ingestion of shell material (Varennes et al. 2015a). The Eiders might have depleted the mussels to some degree at our study site and thus switched to chitons of similar size as the main prey item, as shown by previous studies where Eiders utilized a new food resource when key food items

24 Results and discussion disappeared from the feeding areas (Beukema 1993, Leopold 1993, Ens et al. 2006). Alternatively, it could also be the case that Eiders have diverse food availability at Breiðafjörður because of the extremely high biomass of the area. Productivity of the fjord gives the Eiders opportunity to feed on diverse food items close to the breeding grounds so they do not have to spend energy foraging at greater depths and distances. There were some differences in dominant prey classes between years as gastropods were the main prey class 2008, 2009 and 2010 but polyplacophorans (chitons) the year 2007 (Fig. 10). Female Eiders fed more on the gastropod Lacuna vincta in May than the males but when all months were pooled the diet was similar between sexes. Our results may support both the following two hypotheses concerning optimal prey selection (Bustnes and Erikstad 1990). The shell-mass minimization hypothesis indicates that predators select food that minimizes shell ingestion rather than maximizing energy intake per prey item (Varennes et al. 2015b) and the risk-averse foraging hypothesis which indicates that large prey is not preferred because of high shell content, low energy value, or some other factors.

80

2007 2008 2009 60 2010

40

Specie found in % of total birds total of % in Speciefound 20

0 T. marmorea L. vincta B. undatum Hyas sp.

Figure 9 Annual differences in the percent occurrence (%) of the main diet species (Tonicella marmorea, Lacuna vincta, Buccinum undatum and Hyas sp.) of Eiders during spring and early summer the years 2007–2010 in Breiðafjörður, West Iceland.

25 PhD theses Thordur Örn Kristjánsson 2016

100

Year 2007 80 Year 2008 Year 2009 Year 2010

60

40

20

Prey class found in % of total birds total of % class found in Prey

0 Gastropoda Polyplacophora Crustacea Bivalvia Echnoidea

Figure 10 Annual differences in percent occurrence (%) of the most common invertebrate classes in the diet of Eiders (Somateria mollissima) during spring and early summer the years 2007–2010 in Breiðafjörður, West Iceland.

26 Concluding remarks

5 Concluding remarks

At Rif, the Eiders must overcome a great flea abundance, which probably forces the females to leave their nest to preen frequently, a behavior which costs the birds extra energy. Nest sites availability at Rif is limited and enlarged clutches are abundant in the colony. As a response it seems that Eiders at Rif have a more complex breeding behavior than previously thought as they help each other attending the nests. This shared nest attendance lowers energy cost of the incubating birds as they do not have to reheat the eggs when returning from an incubation recess. The Eiders in our study feed prior to breeding at Breiðafjörður, and again immediately afterwards. Eiders do not seem to be food limited in the bay and exploit a novel dietary item in the chitons. An exceptional incubation behavior of Eiders was observed when nesting in a super-dense colony. Therefore the hypothesis for paper I passed as Eiders at Rif do show a shared nest attendance. Never before has this kind of shared nest attendance been documented for Eiders during incubation nor documented using marked individuals. This study shows that Eiders have a complicated incubation behavior, especially in colonies where they nest very close to each other and where nesting grounds are limited. Both hypotheses for paper II were not supported as 1) clutch size was not related to mass of the Eiders at the beginning of incubation nor did 2) clutch size and nest material increase energy expenditure of the birds. It seems that neither down collecting (removing all the down at the first visit to the nest) nor clutch size is an important factor in energy expenditure. The body condition of the Eider prior to breeding, because of optimal feeding grounds (paper IV), is more relevant for the incubation than nest material or clutch size. This is good news for Eider farmers at Breiðafjörður as the market for environmentally friendly products has been expanding in recent years (Fuller 2015). The present study constitutes the first comprehensive comparison of ectoparasites in nests of Eiders in two Icelandic Eider colonies that have very different nest densities. Both hypotheses for paper III were supported as it seems that flea abundance increases with higher nest densities of the colonies, increased incubation time and re-using of the nest bowl by another Eider in the same incubation period. The Eider breeding season means a warm season for the fleas, as opposed to a dormant season for the rest of the year. For nest bowls that are used by more than one female, this warm season is twice as long. Such nest

27 PhD theses Thordur Örn Kristjánsson 2016 hotspots are ideal for fleas from the flea’s individual perspective (twice the growing season, and hence, twice the food abundance). The present study is also the first that we know of to report blood cover on Eider eggs and flea abundances in Eider nests. Occasionally, shared nest attendance of the Eider (paper I) might allow the bird extra time to preen which is probably a response to high flea abundance in the super-dense colony at Rif. As a part of this thesis the diet of Eiders in Breiðafjörður was revealed for the first time in 2013. The hypothesis for paper IV failed as the blue mussel was not a key food item for the Eiders in our study based on sexes, months or years investigated. A successful feeding Eider must evaluate 3 questions about their feeding habitat (Waltho and Coulson 2015): Does this food meet its current energy demands (Energy content)? Is there enough food at a particular site to meet its immediate demands (Biomass)? Is there enough food at a particular site to meet its continuing demands (Productivity/Carrying capacity)? At Breiðafjörður the answer is yes to all these questions. The Eider can therefore feed with minimal energy expenditure around the islands and does not have to travel further and dive deeper to meet his energy demands. Therefore, they have a good probability to pay the high energy price of the upcoming breeding season due to good feeding grounds and at Rif also with a cooperative incubation behavior (paper I). Eiders seem to have a flexible breeding strategy in the colonies. Hatching success is high when incubating at extremely high nest densities at Rif as well as in sparser nesting colonies at Breiðafjörður. For the first time a polyplacophoran, the mottled red chiton, has been observed as a key food item for a diving duck. It is likely that the abundance of the chitons on rocky bottoms in shallow water in Breiðafjörður is one of the most important factors for the large population of Eiders in the fjord.

28 References

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Skarphéðinsson, K.H. 1993. Ravens in Iceland: Population ecology, egg predation in Eider colonies, and experiments with conditioned taste aversion. Master degree thesis, University of Wisconsin-Madison.

Skarphéðinsson K.H. 1994. Tjón af völdum arna í æðarvörpum. Skýrsla unnin af Náttúrufræðistofnun Íslands fyrir Umhverfisráðuneytið (In Icelandic) 120 pages.

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38 References

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Wetherbee, R. 2016. Ectoparasite (Ceratophyllus vagabundus vagabundus) infestation reduce hatching success of preciocial birds (Branta leucopsis) nesting in the High Arctic. Master degree thesis, Norwegian University of Life Sciences. 51 pages.

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Öst, M., Ydenberg, R., Lindstrom, K., and Kilpi, M. 2003. Body condition and the grouping behavior of brood-caring female Common Eiders (Somateria mollissima). Behavioral Ecology and Sociobiology. 54: 451– 457.

Öst, M., Vitikainen, E., Waldeck, P., Sundström, L., Lindström, K., Hollmén, T., Franson, J.C., and Kilpi, M. 2005. Eider females form non-kin brood- rearing coalitions. Molecular Ecology 14: 3903–3908.

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Bird Study (2015) 62, 146–149 http://dx.doi.org/10.1080/00063657.2014.993591

SHORT REPORT

Cooperative incubation behaviour in a super dense Common Eider Somateria mollissima colony

THORDUR ORN KRISTJANSSON1,2* and JÓN EINAR JÓNSSON2 1Life and Environmental Science Department, University of Iceland, Sæmundargata 2, 101 Reykjavík, Iceland; 2University of Iceland’s Research Centre at Snæfellsnes, Hafnargata 3, 340 Stykkishólmur, Iceland

Capsule Common Eiders at Rif in west Iceland commonly show joint nest attendance, which may be an exaggerated behavioural response to the visual stimulus of many nests so close to their own nest. This represents a new insight into incubation behaviour in colonies with extremely high nest densities.

Most bird nests are attended by the incubating parents, report a longer average recess duration of 45 min either in pairs or as individuals (Cockburn 2006). In (Kristjánsson & Jónsson 2011). Incubating eiders take waterfowl (Anatidae), females incubate alone and 0–5 incubation recesses per 24 hours (Swennen et al. males are either present and do not incubate (swans 1993, Kristjánsson & Jónsson 2011). In West Europe, and geese) or are absent (ducks). In the Anatidae, recesses occur during the night and eiders use these exceptions that have biparental incubation are the recesses to preen, bathe and drink freshwater Magpie Goose Anseranas semipalmata, Black Swan (Swennen et al. 1993, Bolduc & Guillemette 2003). In Cygnus atratus and the eight species of whistling ducks Iceland, eiders are equally likely to take recesses during (Dendrocygninae) (Afton & Paulus 1992). The female daytime and night-time; this difference is probably Common Eider Somateria mollissima (hereafter eider) is explained by relatively longer daylight time in Iceland generally considered a typical duck, i.e. the female during the summer, allowing avian predators to be incubates her eggs (Ashcroft 1976). In Iceland, the active for 24 hours (Kristjánsson 2008). average clutch size is between three and five eggs, The behaviour of incubating eiders is well studied whereas six eggs or more is rare (Skarphéðinsson 1993, across its range (Scotland: Milne 1972, Greenland: Snæbjörnsson 1998,D’Alba 2007, Kristjánsson 2008) Christensen & Falk 2001, Labrador: Chaulk et al. 2005 and clutches of seven eggs or larger are probably due to and Iceland: Kristjánsson 2008) where density in parasitic nesting behaviour (Robertson et al. 1992, Öst colonies is typically 0.8–250 nests/ha. The eider colony et al. 2005). at Rif in west Iceland has one of the highest reported The eider has one of the highest incubation nest densities in the world, with over 2000 nests/ha constancies reported among birds, staying on the nest (Jónsson & Lúðvíksson 2013). Such dense nesting may for 90–99% of the incubation period (Bolduc & facilitate social cooperation in the incubating birds. Guillemette 2003, Kristjánsson & Jónsson 2011). Such During our previous research at Rif, we noted that incubation constancy requires energy expenditure from female Common Eiders apparently tended other nests stored body fat, causing eiders to lose 25–45% of their while the owner went on an incubation recess. Here, body weight during the 24–27-day incubation period we assessed whether individually marked eiders (Korschgen 1977, Parker & Holm 1990, attended more than one nest during incubation and if Skarphéðinsson 1993, Harðardóttir et al. 1997, other, unmarked eiders tended the marked eider’s nests Jaatinen et al. 2012). Generally, studies have shown during their recesses. that incubation recesses last for 4–17 min (Mehlum We captured 26 female eiders incubating at Rif on 25– 1991, Criscuolo et al. 2000, Bolduc & Guillemette 26 May and 1–2 June 2011 with a noose-pole and 2003, Bottitta et al. 2003) but studies from Iceland individually marked them with a red plastic leg-ring with a white engraving (Pro-Touch, Canada). The *Correspondence author. Email: [email protected] eiders were also marked with nose markers (Juno

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Incubation behaviour of eiders 147

Minnesota, USA) of different colour and shape for In 3% of recesses (1 occasion), marked eiders were identification when incubating on the nest. Nose seen both covering other nests and sitting on other markers had the shapes: round, box, triangle, star and nests. During recesses, marked eiders were also seen oval, and the colours: yellow, red, white, purple, brown drinking water from the pond, bathing and preening. and pink. Eiders were leg-marked and nose marked In 31 of 39 incubation recesses (79%), other eiders with two markers, one on the left and other one on attended the focal marked nests. Unmarked eiders the right. Each nose marker had two holes which we covered a focal nest during 26% of recesses (10 of 39 used to tie the markers together with a nylon seam occasions) and sat on focal nests during 44% of (Biosyn) through the nostrils of the bird. We added a recesses (17 of 39 occasions). In 10% of recesses (4 of drop of super glue to strengthen the knot on the nasal 39 occasions) unmarked eiders were seen both marker. The nylon seam was UV light sensitive and covering and sitting on a focal nest (Table 1). ruptured within a few weeks, so the eiders lost the Included here are one occasion where a male was seen nasal markers after incubation. No females were sitting on a focal nest and three occasions where males observed with nasal markers upon return to the colony covered a focal nest. in 2012. When a marked eider returned back to its nest after a Nests were identified by a plastic pole with a nest recess, the unmarked eider on the nest usually fled or was number next to the nests of marked eiders. Behaviour already gone. In five recesses, the unmarked bird was of the birds was observed with a LEICA telescope (20– aggressively chased away by the owner, which then 60×). We investigated behaviour of the 26 incubating resumed incubating her clutch. Sometimes a returning females for 2–3 hours for 5 days (31 May; 6, 8, 11 and marked female sat on another unguarded nest for a 16 June 2012). Nesting success of these 26 marked short period or covered the eggs there before returning eiders was high, 81% of the birds hatched at least 1 to her own nest. duckling. We: (1) documented all incubation recesses In most eider colonies, males leave at the start of and noted if the marked birds attended to nests other incubation but at Rif, males stay among the incubating than their own on their way to or from the pond and females until mid-June, when males leave the colony (2) documented all approaches and activities by other to moult at sea. We saw male eiders both covering eiders, female or male, towards a marked female’s unguarded nests and sitting on nests while females identified nest. took a recess. The males were even seen rotating the A total of 39 incubation recesses were documented eggs while incubating for a short period. In a study in among the 26 marked females. In 10 of 39 incubation Önundarfjörður, male eiders stayed with the female recesses (26%), the marked females themselves during the whole incubation in only 4–7% of nests attended to other nests on their way to and from and were never seen sitting on the nests incubation recesses. When a marked eider left her own (Skarphéðinsson 1993). nest, she stopped for a few moments and covered other Why eiders show this incubation behaviour towards clutches with nest down during 8% of recesses (3 of 39 other nests is not known and has not been reported occasions). Marked eiders were also seen sitting on before. Nevertheless, our findings may be applicable to other nests during 15% of recesses (6 of 39 occasions). other eider colonies with similar extremely high nest

Table 1. Behaviour of 26 marked incubating female Common Eider S. mollissima during the 2011 nesting season (31 May; 6, 8, 11 and 16 June 2011, with total numbers in bold) at Rif, Snæfellsnes, Iceland, during incubation recesses and (1) the frequency which they attended to nests other than their own and (2) the frequency that other birds attended at the marked focal nests.

31 May 6 June 8 June 11 June 16 June Total %

Marked birds incubation recess 2 9 2 22 4 39 Marked bird covers another nest 0 2 0 1 0 3 8 Marked birds sits on another nest 1 2 1 1 1 6 15 Marked bird does both 1 0 0 0 0 1 3 Unmarked bird covers a focal nest 0 2 0 6 2 10 26 Unmarked bird sits on a focal nest 0 5 0 12 0 17 44 Unmarked bird does both 0 0 2 2 0 4 10

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44 PhD theses Thordur Örn Kristjánsson 2016 Paper I

148 T.O. Kristjansson and J.E. Jónsson

densities. There are at least four possible explanations for hatch the young (Öst et al. 2005). It is hard, however, our observations that merit further investigation: to distinguish between egg dumping into random nests Eiders may be confused in this super dense colony and and egg dumping into the nests of relatives (a kin ‘mistake’ other female nests as their own when returning selection strategy), because relatedness of the offspring from recesses. There is probably a strong visual stimulus is unclear without using molecular methods for for an incubating female to attend an unguarded nest as determining kinship relations (Waldeck & Andersson she walks by. The density at Rif is so high that every 2006). eider passes many other nests during incubation Eiders at Rif are probably closely related to one recesses. In most eider colonies in Iceland, the nest another and might help related kin in the passing of density is closer to 20–250 nests/ha and there is more genetic material to the next generation, when vegetation available for shelter than at Rif (2000 nests/ individual parents are not capable of incurring the ha), so females do not pass other nests as often. costs of raising offspring all by themselves (Waldeck Eiders are on a tight energy budget during incubation and Andersson 2006). Eiders are able to recognize kin and it could be that sitting on an unguarded clutch when but may not choose to associate with kin over they have been disturbed from their own nest (i.e. by the unrelated individuals, or be relatively more likely to eiderdown collectors) is a strategy to minimize heat loss associate with kin in some years (Tiedemann et al. from the brood patch on the bird (Schmidt et al. 2006). 2011, Jaatinen et al. 2012). At Rif, more than one Eiders may egg dump, so incubation in other nests may female could possibly attend the same nest and all may reflect differential investment in care of their own be incubating at least some of their own eggs within offspring. Parasitic nesting within species has been the shared nest. This would then be cooperative documented in 234 bird species and is widespread incubation rather than wholly parasitic behaviour. among Anseriformes and Passeriformes (Yom-Tov However, relatedness may not be required for nest 2001). Large clutches indicative of egg dumping were sharing: females cooperate when clutches hatch present in our study colony. At Rif, clutch size was simultaneously and these are not necessarily related high in most years and nests with seven eggs or more (Öst et al. 2005, Tiedemann et al. 2011, Jaatinen et al. were common. We do not have clutch size data for 2012). Future research should investigate whether the 2011 but in 2012, parasitic clutches (7 eggs or more) females helping to temporarily incubate the nests have were 22% of all nests (Fig. 1), compared to 5.7% indeed laid some eggs in those nests themselves. A reported by Coulson (1999) at Coquet Island, study of relatedness of incubating birds within the England. We saw incubated unmarked nests with as colony and eggs in the nests is needed to assess this many as 18 eggs. Eiders were also seen hatching and possibility and should be a future research topic. fledging up to 9 young from a single nest at Rif. Nest parasitism is known among eiders (Robertson et al. 1992, Waldeck & Andersson 2006) but is often ACKNOWLEDGEMENTS related to poor body condition of the parasitizing We thank Tómas G. Gunnarsson, Guðrún G. Þórarinsdóttir female which then does not help the incubating eider and two anonymous reviewers for their comments to improve earlier versions of this manuscript. We sincerely thank Smári J. Lúðvíksson for his help in the eider colony.

REFERENCES

Afton, A.D. & Paulus, S.L. 1992. Incubation and brood care. In Batt, B. D.J., Afton, A.D., Anderson, M.G., Ankney, C.D., Johnson, D.H., Kadlec, J.A. & Krapu, G.L. (eds.) Ecology and Management of Breeding Waterfowl, Vol. 1: 62–108. University of Minnesota Press, Minneapolis. Ashcroft, R.E. 1976. A function of the pair bond in the Common Eider. Wildfowl 27: 101–105. Bolduc, F.& Guillemette, M. 2003. Incubation constancy and mass loss in the Common Eider Somateria mollissima. Ibis 145: 329–332. Bottitta, G.E., Noel, E. & Gilchrist, H.G. 2003. Effects of experimental manipulation of incubation length on behaviour and body mass of Figure 1. Eider clutch size at Rif, west Iceland in the year 2012. Common Eiders in the Canadian arctic. Waterbirds 26: 100–107.

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Chaulk, K., Robertson, F.J., Montevecchi, W.A. & Ryan, P.C. 2005. Mehlum, F. 1991. Egg predation in a breeding colony of the Common Aspects of Common Eider nesting ecology in Labrador. Arctic 58: Eider Somateria mollissima in Kongsfjorden, Svalbard. Norsk 10–15. Polarinstitutt Skrifter 195: 37–45. Christensen, K.D. & Falk, K. 2001. Status of the common eider Milne, M. 1972. Breeding numbers and reproductive rate of eiders at breeding in the municipality of Avanersuaq (Thule), North-West Sands and Forvie National Nature Reserve, Scotland. Ibis 116: Greenland. Polar Res. 20: 109–114. 135–154. Cockburn, A. 2006. Prevalence of different modes of parental care in Öst, M., Vitikainen, E., Waldeck, P.,Sundström, L., Lindström, K., birds. Proc. Roy. Soc. B 273: 1375–1383. Hollmén, T., Franson, J.C. & Kilpi, M. 2005. Eider females form Coulson, J.C. 1999. Variation in clutch size of the Common Eider: a study non-kin brood-rearing coalitions. Mol. Ecol. 14: 3903–3908. based on 41 breeding seasons on Coquet Island, Northumberland, Parker, H. & Holm, H. 1990. Patterns of nutrient and energy expenditure England. Waterbirds 22: 225–238. in female Common Eider nesting in the high Arctic. Auk 107: 660– Criscuolo, F., Gauthier-Clerc, M., Gabrielsen, G.W. & Maho, Y.L. 668. 2000. Recess behavior of the incubating Common Eider Somateria Robertson, G.J., Watson, M.D. & Cooke, F. 1992. Frequency, timing mollissima. Polar Biol. 23: 571–574. and costs of intraspecific nest parasitism in the Common Eider. D’Alba, L.B. 2007. Micro and macroclimate effects on reproductive Condor 94: 871–879. performance of Common Eiders. PhD thesis, University of Glasgow. Schmidt, A., Alard, F. & Handrich, Y. 2006. Changes in body Harðardóttir, M., Guðmundsson, J. & Petersen, Æ. 1997. temperatures inking penguins at sea: the result of fine adjustments Þyngdartap æðarkolla Somateria mollissima á álegutíma. Bliki 18: in peripheral heat loss? Am. J. Physiol. Regul. Integr. Comp. Physiol. 59–64 (in Icelandic). 291: 608–618. Jaatinen, K., Noreikiene, K., Merila, J. & Öst, M. 2012. Kin Skarphéðinsson, K.H. 1993. Ravens in Iceland: Population ecology, association during brood care in a facultatively social bird: active egg predation in eider colonies, and experiments with conditioned discrimination or by-product of partner choice and demography? taste aversion. Master degree thesis, University of Wisconsin-Madison. Mol. Ecol. 21: 3341–3351. Snæbjörnsson, Á. 1998. Dúnnýting, hreiðurskýli og uppeldi æðarunga. Jónsson, J.E. & Lúðvíksson, S.J. 2013. A choice between two adjacent Rannsóknir á æðarfugli á Bessastöðum á Álftanesi árin 1993–1996. islands: is switching nest sites related to weather or nest density in the Freyr 12: 11–16. (In Icelandic) Common Eider (Somateria mollissima)? Ornis Fenn. 90: 73–85. Swennen, C., Ursem, J.C.H. & Duiven, P. 1993. Determinate laying Korschgen, C.E. 1977. Breeding stress of female eiders in Maine. and egg attendance in Common Eiders. Ornis Scand. 24: 48–52. J. Wildlife Manage. 41: 360–373. Tiedemann, R., Paulus, K.B., Havenstein, K., Thorstensen, S., Kristjánsson, T.K. 2008. Áhrif dúntekju á hita í hreiðri, hegðun og Petersen, A., Lyngs, P. & Milinkovitch, M.C. 2011. Alien eggs varpárangur æðarfugls (Somateria mollissima). Master degree in duck nests: brood parasitism or a help from Grandma? Mol. thesis, University of Iceland (in Icelandic). Ecol. 20: 3237–3250. Kristjánsson, T.K. & Jónsson, J.E. 2011. Effects of down collection on Waldeck, P. & Andersson, M. 2006. Brood parasitism and nest incubation temperature, nesting behaviour and hatching success of takeover in Common Eiders. Ethology 112: 616–624. Common Eiders (Somateria mollissima) in west Iceland. Polar Biol. Yom-Tov, Y. 2001. An updated list and some comments on the occurrence 34: 985–994. of intraspecific nest parasitism in birds. Ibis 143: 133–143.

(MS received 10 September 2014; revised MS accepted 23 November 2014)

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Paper II

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Paper II

Do population-specific adaptations explain differing incubation costs?

Thordur Örn Kristjánsson1,2*, Jón Einar Jónsson2 and Tómas Grétar Gunnasson3

1Department of biology. University of Iceland, Reykjavík, Iceland 2University of Iceland, Snæfellsnes Research Centre, Stykkishólmur, Iceland 3South Iceland Research Centre, University of Iceland, Selfoss/Gunnarsholt, Iceland *Corresponding author; e-mail: [email protected]

Abstract

Incubation is an energy demanding process for birds. The reproductive effort, (the clutch size) is often determined by the amount of endogenous reserves and is considered a good indicator of female quality. Results between studies explaining trade-offs between reproductive investment and use of energy reserves remain equivocal. Thus, we investigated the cost of incubation as relative mass loss (% of body mass loss throughout incubation period of 24 days) in female Common Eiders (Somateria mollissima; here after Eider). Relative mass loss was compared between 1) females with highly variable clutch sizes (2-9 eggs); 2) variable body condition at the start of incubation; 3) different types of nesting material (hay and nest down) and; 4) banding age to investigate if older females were more likely to have enlarged clutches (≥7 eggs). We compared our findings with ten published studies to explain population-specific adaptations in adjusting pre-breeding weight and weight loss during incubation. Average relative mass loss was 32% for the whole incubation period and weight loss per day averaged 26 g and was similar between nesting materials. Heavier females lost proportionally more weight during incubation. There were no relationships between: 1) clutch size and body condition at start of incubation; 2) clutch size and relative mass loss; 3) nest material and relative mass loss. Our findings indicate that nest material or enlarged clutches do not influence energy expenditure during incubation. Different findings between studies on incubation costs could suggest population specific adaptations. Our findings support the condition dependent model, which suggests that the body store differences at incubation onset are mostly caused by environmental conditions encountered by laying females and does not control clutch size.

Key words: Common Eider, Somateria mollissima, relative mass loss, incubation cost, nest down collecting, clutch size, capital breeding, banding age

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49 PhD theses Thordur Örn Kristjánsson 2016

Introduction In many waterfowl species, only the females incubate clutches and raise young and thus, must balance their own daily energy needs against costs of egg laying and incubation [1]. Leaving the nest increases the probability of predation [2, 3, 4] and demands extra energy to rewarm the eggs [5, 6, 7, 8, 9]. Therefore, many birds have developed fasting endurance and use endogenous reserves accumulated before nesting, for daily energy expenditure [10]. The reproductive effort, measured as the clutch size, is often determined by the extent individuals can build up endogenous reserves [11, 12, 13]. Whether clutch size is regulated by body size and how clutch size variation impact incubating birds have long been central issues in ornithology and still remains controversial [12, 14]. The Common Eider (Somateria mollissima; hereafter Eider), is a long-lived seaduck with uniparental care where females invest substantially in the reproduction. Females rarely leave the nest during the incubation period of 24-27 days [15, 11, 16, 17, 18] and have an average incubation constancy (proportion of time spent on nest) of 90-99% [19, 17]. Eiders are among waterfowl that feed little and lose much weight during incubation but sustain themselves on endogenous reserves, these are known as capital breeders [20, 21, 11]. Conversely, species or populations that can forage during incubation are termed income breeders [22]. Female Eiders accumulate significant fat reserves by feeding heavily during 4-6 weeks before nesting and increasing their body mass by approx. 20% above winter levels [23]. Subsequently, they lose up to 40% of the pre-breeding body mass during egg laying and incubation [24]. There are three main hypotheses which explain the relationship between clutch size and body condition in waterfowl:

1. The egg formation hypothesis states that body reserves available to the laying females determine the optimal clutch size [12]. Females in better physical conditions produce larger clutches than females with small body reserves; generally small clutch sizes indicate food shortage [11, 13, 25, 26, 27].

2. The expected fitness hypothesis proposes that females adjust their post-laying condition according to the anticipated fitness benefits of the clutch. Females in good condition lay earlier, have larger clutches and a better fledging success than lighter ones due to better nest attendance [28, 29] and are less likely to abandon their broods [30, 31].

3. The condition dependent model states that large individual variation in endogenous reserves at the onset of incubation mostly is driven by factors others than clutch size and laying date in arctic nesting females [32, 33, 34].

Eider females use nest down to insulate their nests and maintain desirable temperature and humidity for the eggs during incubation [17] and also conceals the eggs during incubation recesses [10, 35, 36]. Nest down has been collected from Eider nests in Iceland for centuries, and is traditionally replaced with dry hay, this practice does not seem to decrease nest temperatures or hatching success [17], but the influence of down collection on the body mass dynamics of the incubating female has not been investigated. Older Eider females are usually in better body condition, are the first to arrive and choose nesting sites and lay their eggs earlier than younger females [4, 37, 38]. They generally lay larger clutches [13] and attain a higher nesting success [39]. Clutch size in Eiders commonly ranges between 3-6 eggs [40, 41, 42, 43, 44, 37, 45]. Under favorable

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50 PhD theses Thordur Örn Kristjánsson 2016 Paper II

environmental conditions, female Eiders may adjust their reproductive effort in relation to their pre-breeding body condition so females in good condition can deal with the higher energetic demands required by laying and incubating a larger clutch [46]. However, studies disagree on whether there is a relationship between clutch size and body condition at the beginning of incubation [47, 48, 34]: females incubating larger clutches may or may not lose proportionally more weight during incubation than females incubating smaller clutches. Females with large clutches tend to shorten the incubation period and minimize predation risk of the clutch with higher nest attendance than Eiders incubating small clutches [47]. Clutch size in Eiders has been related to nest density, with increasing clutch size in colonies with higher nest densities [48, 49]. Intraspecific nest parasitism (egg dumping) influences clutch size [50, 49] as laying eggs in other females nests is a common breeding strategy among Eiders, especially in colonies with higher nest densities [51, 52, 53, 54]. Tiedemann et al. [55] reported that older females in a colony in North Iceland were more likely to accept such extra eggs than younger females and that relatedness between the host and parasitizing female was higher than background relatedness within the Eider colony. This corroborated earlier findings on host-parasite relatedness in Hudson Bay Eiders at La Pérouse Bay, Canada [52]. But still, the owners of the clutch were not always relatives in the study of Tiedemann et al. [55]. Similarly, nest takeover, where two Eiders lay eggs in the same nest bowl and one takes over the whole clutch, does not necessarily involve relatives [56, 50]. Studies have not shown definitively whether such extra eggs affect energy budgets of Eiders, when clutches are of normal size, but this is the first study we know of that includes enlarged clutches of ≥7 eggs The aim of this study was to investigate the energy cost of incubation in Eiders, indexed by relative mass loss. Therefore, we compared relative mass loss between 1) highly variable clutch sizes (2-9 eggs); and 2) individual body condition at the start of incubation; 3) different types of nesting material (hay and nest down); and 4) banding age (new breeders, 2 years breeding experience etc.) to investigate if older females had different body size dynamics or were more likely to have enlarged clutches (≥7 eggs) than younger females. Finally, we made a systematic comparison with ten previous studies of Eiders (S.m. mollissima, S.m. borealis and S.m. dresseri) to explain population-specific adaptations of Eiders in adjusting pre-breeding weight and weight loss during incubation

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Materials and methods

EidersEthics were Statement marked and weighted in this study with the banding permits of Jón Einar Jónsson and Smári J. Lúðvíksson. Eider farming is regulated by Icelandic law no. 84/1989, for the purpose of which Eider is protected by law (currently no. 64/1994) from hunting and egg collection. The Eider has been completely protected from hunting in Iceland since 1849 and from egg collection since 1787 because of the economic importance of down collection [57]. This study complies with the laws of the Republic of Iceland. No Eiders were hurt in this study.

Study site

This study was carried out at Rif, an Eider colony at the Western tip of the Snæfellsnes peninsula, western Iceland (Fig. 1) which is the south coast of the Breiðafjörður bay, in May and June 2009 and 2010. Breiðafjörður is an important breeding site for Eiders and supports at least 25% of the large Iceland Eider population [58, 59] and down collection is commonplace in the area. Nowadays eiderdown is collected, cleaned and used to make products such as duvets, sleeping bags and clothes [60, 61]. The Eider colony at Rif has a record-high nest density of about 2000 nests/ha and nest parasitism is common, as 17% of the clutches are enlarged (7 eggs or greater [53]). The colony is comprised of two islands (120 m2 and 600 m2) in a small lake (2.34 ha in size, 0.5 km from the Atlantic shoreline; 64°55’14’’N; 23°49’23’’W), and was established in 1972 by farmers at Rif [62, 63, 53]. The study population recently has been stable at 500-600 nests and is wild and free-ranging, despite the islands originally being man-made.

Fig. 1. West Iceland, Breiðafjörður and Snæfellsnes peninsula. The study site at Rif is indicated by a circle.

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52 PhD theses Thordur Örn Kristjánsson 2016 Paper II

In 2009 and 2010, 15 and 26 incubating Eider females were captured twice within each breedingData collection season, respectively,and analysis with a noose pole with a nylon snare. The Eiders were first caught during the first half of the incubation period (total 24 days). Each female was individually marked with a metal leg band. The Eider marking is an ongoing project in this colony since 1993 so many marked Eiders are present [63]. The females were weighed using a Pesola spring scale (with 2.5 g accuracy). The clutch size in each nest was recorded and the eggs were candled by using the sunlight to see through the eggshell to estimate the incubation stage [64, 65]. These females were re-captured and weighed for the second time right before hatching (0-2 days). In this study, clutch sizes of ≥7 eggs were considered enlarged, as clutches of ≥7 eggs are rarely observed in Breiðafjörður outside the Rif colony [53, 66] whereas clutches of ≤6 eggs were considered to be normal clutch sizes. It is possible that nests with fewer eggs than 7 are parasitic in the colony, as was reported by Waldeck and Andersson 2006 [50], but we cannot distinguish between the mothers of the eggs except on genetic basis. We do not have a genetic comparison on these clutches so relatedness between the host and eggs in this study is not clear; females can accept eggs from unrelated individuals anyway [55]. Of the 41 females that were caught twice, 10 females (24%) had not been marked before the first catch in this study. The remaining 31 females were recoveries, where 3 females (7%) were marked the year before and 28 females (68%) were marked ≥2 years before this study started. Twelve of these females were caught on control nests (where nest down was not removed) but 29 females were caught on nests where nest down was collected and replaced with hay to use as nest lining (hereafter hay nests). Female Eiders can breed for the first time at 2-3 years old but males 4 years and older [67, 68]. We assumed that marking age is correlated with biological age [69] and that newly marked are generally females breeding for the first or second time in their lifetime. Eider females have been marked annually at Rif since 1993 [63]. The proportion of recaptures each year generally ranges between 50-76% (Smári J. Lúðvíksson, eider farmer at Rif, unpublished data). Females which were marked the year before this study had at least one year breeding experience, females which were marked two years before this study had at least two years breeding experience etc. Incubation was assumed to have started when no new eggs were laid for 2 days [2] in those nests that were monitored from the first day of incubation. The hatch date was estimated as the day the first egg hatched. The mean incubation period in this colony is 24 days [17, 53]. The energy cost was expressed as weight loss during incubation and was defined as weight at day 1 minus the weight at day 24. The mean weight loss/day in g during incubation of 24 days was calculated by dividing the total weight loss during incubation with the 24 days incubation period. All eggs were candled to estimate the incubation stage [64, 65]. If eggs were not found on day one we back calculated the weight of the female at the beginning of the incubation using how much weight the female lost per day between the two weightings. The relative mass loss was calculated as percentage of mass lost in the incubation period of 24 days (weight loss during the incubation / mass in the beginning of incubation). Statistical analysis was performed with SigmaStat 12.3 A t-test was used when data passed a normality test but a Mann-Whitney U test when normality tests failed when: 1) comparing female body mass at the start of incubation and at hatching between years; and 2) comparing the daily weight loss during incubation between hay and down nests and comparing clutch size and banding age. Relationships between clutch size and body condition (body mass at start of incubation) and between relative mass loss through the incubation and clutch size were assessed by linear regression. We surveyed the literature about energy expenditure of incubating Eiders [15, 70, 11, 71, 47, 16, 25, 26, 72, 73, 46, 19, 24, 48, 39, 34, 49] and compare the results in Table 1. In

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Table 1 we included studies that reported relative mass loss and those that answered both of the following questions: 1) Is there evidence for a clutch size-body condition relationship and 2) Is there evidence for a body condition-mass loss relationship? We subsequently compared these findings to look for patterns among groups of populations and among sub-species of Eiders to determine if there are population-specific adaptations in the strategies of incubation and energy expenditure in Eiders.

Table 1: Comparison of Eider studies on energy cost of incubation. Body weight at the start and end of incubation, mass loss/day, relative mass loss and clutch size of the sub-species S. m. mollissima, S. m. dresseri and S. m. borealis.

Evidence for a Body mass Body Evidence for a body at start of mass at Mass Relative Clutch clutch size- condition- Sub-species incubation hatching loss/day mass size body condition weight loss Reference of eiders (g) (g) (g) loss % range relationship? relationship? Korschgen 1977 dresseri 1830 1290 x 32 x No x Hardardottir et al. 1997 borealis 1799 1308 19 27 x x No Gabrielsen et al. 1991 borealis 2106 1357 30 36 4-5 x x Parker and Holm 1990 borealis 1773 1368 19 22.8 4-6 Yes x Erikstad and Tveraa 1995 mollissima 1852 1483 24,7 20 3-6 No Yes Hanssen et al. 2003 mollissima 1865 1496 24,7 20 x No x Hario 1983, mollissima 2100 1520 x 28 x x x Bolduc and Guillemette 2003 mollissima 2346 1371 37,5 41.6 5 x No Hanssen et al. 2002 mollissima 1878 x x x 4-5 No Yes D'Alba 2009, sheltered nests borealis x x x 30 1-6 No x D'Alba 2009, exposed nests borealis x x x 33 1-6 No x Prensent study borealis 1935 1322 26 32 2-9 No Yes

Results

Total mass loss during incubation was positively related to body mass at the beginning of incubation,Body weight suggesting that heavier females lost proportionally more weight during incubation (Fig. 2). Mean female body mass at start of incubation (t=316.5, n(2009)=15 n(2010)=26, P=0.98) and weight at hatching were similar between the two years (t=1.3, df=39, P=0.20). Therefore, data were pooled over the years, yielding average weights of 1935 g (SD=94) and 1322 g (SD=74) at the start of the incubation and at hatching, respectively. The total weight loss during 24 days of incubation was on average 613 g (SD=116 g) and the relative mass loss 32% of the body mass at start of incubation (Fig. 2). The daily weight loss during incubation on hay (25 g/day, SD=5) and down nests (27 g/day, SD=4.5) were similar (t=306.5 n(small)=12 n(big)=29 (P=0.12)) so data were pooled over the treatments, yielding an average weight loss of 26 g per day (SD=5) (Table 1).

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54 PhD theses Thordur Örn Kristjánsson 2016 Paper II

Fig. 2. Body weight at the beginning of incubation for 41 female Eiders, caught nesting at Rif, West Iceland in 2010 and 2011, and relative mass loss during the incubation period of 24 days (SE=73, P<0.001). White dots indicate parasitic nests (≥7eggs).

Fig. 3. Relationship between body mass at the start of incubation and clutch size for 41 female Eiders caught nesting at Rif, West Iceland in 2010 and 2011 (P=0.31). White dots indicate parasitic clutches (≥7 eggs).

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The clutch size at the start of incubation ranged between 2-9 eggs, clutches of 4 eggs were mostClutch common size (34%). Seven nests (17%) were considered enlarged (≥7 eggs). Body weight at the start of incubation was independent of clutch size (t=-1.0, df=38, P=0.31). No relationship was observed between clutch size and female body condition at start of the incubation (Fig. 3), although no females lighter than 1870 g were observed with enlarged clutches. No difference was observed in relative mass loss of Eiders during incubation between enlarged (≥7 eggs) and normal clutches in the study (t=181.0, nsmall=7, nbig=34, P=0.24). No relationship was found between relative mass loss of the females through the incubation period and clutch size (Fig. 4), nor clutch size and banding age (t=0.85, df=39, P=0.20) (Fig. 5). Females with enlarged clutches were most common in the mid-ranges of both relative mass loss (27-37%; Fig. 4) and marking age (females banded between 2000 and 2003; Fig 5).

Fig. 4. Clutch sizes of 41 incubating female Eiders, caught nesting at Rif, West Iceland in 2010 and 2011, plotted against relative mass loss during the incubation (P = 0.24). White dots indicate parasitic clutches.

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56 PhD theses Thordur Örn Kristjánsson 2016 Paper II

Fig. 5. Clutch sizes of 41 incubating female Eiders caught nesting at Rif, West Iceland from the years 2009 (squares) and 2010 (diamonds), plotted against year of first capture/banding by Eider farmers (years of incubating experience, P=0.20). White dots indicate parasitic clutches.

Discussion

We found a positive relationship between the rate of weight loss and body mass at the start of incubation,The cost of as incubationheavier females lost proportionally more weight during incubation than did the lighter females. These findings, which are summarized in table 1, are in agreement with Erikstad and Tveraa 1995 [47], Hanssen et al. 2002 [73] and a pooled dataset from the literature [see 19]. It could be that heavy females in good condition at the start of incubation have a better incubation constancy than lighter ones and leave their nests less frequent which in turn increases their energy expenditure during this period. Lighter females might take longer and more frequent recesses but still hatch their clutch with less energy expenditure than heavy ones because at Rif other females attend their nest during recesses [53]. Conversely, no such relationship, between mass loss and body mass at the beginning of incubation, was found in Northern Norway [27], Northern Iceland [16] nor in Eiders in Denmark [19]. This discrepancy between studies could be explained by annual or local variation in breeding strategies of Eiders, different fat accumulation between years by the Eider or “sampling artefacts” such as different sample sizes or weighting procedures between these studies. Notably, these discrepancies between studies occur within both Norway and now Iceland, which could suggest that the different findings represent annual variation, or even different strategies or trade-offs at the individual level. The average body mass of the Eider females at the start of incubation in this study (1935 g), the average energy cost of incubation (relative mass loss) (32%) and average rate of weight loss/day (26 g) are all within the range of results for Eiders from other studies. The greatest relative mass loss (41.6%) and the fastest rate of weight loss (37.5 g/day) for Eiders were recorded in Denmark, where the females also had the highest pre-incubation body mass

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(2346 g) [19]. The lowest relative mass loss (20%) was recorded in northern Norway [47, 24] but the slowest rate of weight loss (19 g/day) was recorded in Svalbard [11].

We found no relationship between clutch size and female body mass at start of incubation, whichClutch is size in contrast and female with several body massother studiesat start conducted of incubation on Eiders from northern Norway [27], England [26], the southern tip of Finland [25] and Svalbard [11], but in agreement with studies from Canada [34], western Finland [48] and northern Norway [47]. Interestingly, these two groups of studies do not represent any geographical grouping, such as Arctic vs. temperate populations [see 74]. Notably, results differ within Norway and Finland. Our results do not support the egg formation hypothesis or the expected fitness hypothesis that the cost of incubation increases with clutch size and that clutch size is a good indicator of female quality at the start of incubation. Our findings are more in agreement with the condition dependent model, especially as there is no extra cost observed for incubating ≥7 eggs. Large clutches can have negatively affect survival of female Eiders under unfavorable conditions, such as an avian cholera epizootic [75]. Conditions in this study were favorable and no mortality was observed among incubating females during the two years investigated. In this study, the weight loss during incubation was not related to normal clutch size (3-6 eggs), or when the enlarged clutches (≥7 eggs) were included so clutch size in this study was independent of female quality. It is quite likely that many clutches at Rif with 7≤ eggs are formed with parasitic egg laying or nest take-over [50, 52]. The difference between our results and those of Erikstad and Tveraa 1995 [47], which did find that Eiders lost more weight on 6 egg clutches compared to 3 egg clutches, might be explained by different incubation behavior of females between these study colonies. It has been observed that females at Rif attend nests that are not their own when the owner takes an incubation recess [53]. Therefore, the eggs are still warm when the owner arrives back from a recess and no energy cost is incurred for re- heating the clutch. We found no relationship between clutch size and marking age (incubation experience), Clutch sizes of females that were marked in the colony ≥2 years before this study were similar to clutch sizes of females that were marked for the first time in this study. The oldest females in this study did not have clutches of ≥7 eggs but such clutches were most frequent among Eiders in their middle ages (7-15 years old). Older, and more experienced females reject these parasitic females and aggressively chase them away from their nests [54]. Another possible explanation for the lower clutch sizes among older females in this study is that at older age, they reduce their own breeding output and do not lay more than 1-2 eggs themselves but still accept 2-3 parasitic eggs from certain individuals, resulting in clutches of 3-5 eggs. Small clutches of 1-2 eggs can be up to 25% of all clutches and generally produce few ducklings [76]. Individuals laying small clutches may benefit from parasitic eggs, which subsequently allow them hatch a regular-sized brood of 3-5 ducklings because a bigger clutch or brood lowers the predation risk of the Eiders own eggs or broods [77] or that of inclusive fitness benefits if the host and parasite are related [52, 55]. The smallest clutch size in this study, 2 eggs, was observed in 5% of the nests. Clutches of 2-3 eggs might be explained by predation [78] or young females which lay relatively few eggs [13].

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Conclusion

This study indicates that energy costs during incubation is related to body condition of the female at the start of incubation at Rif as heavier females lost proportionally more weight than lighter ones. The energy cost was not related to nesting material as down collection did not affect weight loss of the incubating females. In an earlier investigation down collection did not affect productivity, incubation temperature, recess frequency or recess duration in Eiders in Breiðafjörður [17]. Thus, females in hay nests can compensate for loss of nest down while incubating. It would be an interesting future study to examine temperature in nests and relative/daily weight loss of females during incubation nesting without any nest material, possibly at a colder (more Arctic) study sites than Breiðafjörður. It could test whether or not the nest down is a by-product of the brood patch rather than an important nesting material for the Eiders. Likewise, extra eggs in a clutch did not increase relative mass loss of the Eiders during incubation. Furthermore, there seems to be no additional cost from breeding at higher nest density at Rif. The Eider colony at the study site is very dense, the behavior of the females is unusual, as they attend each other’s nests and egg parasitism is not uncommon [53]. There is some evidence that middle aged females incubate the largest clutches, as older and younger females incubated more often on the normal clutch size. Comparing energy costs of incubating Eiders is problematic because many previous studies did not answer both of the following questions: Is there evidence for a clutch size- body condition relationship? Or is there evidence for a body condition-weight loss relationship? To our knowledge this is only the third study of Eiders to report answers to both these questions, and the first to include enlarged clutches or effects of age of known individuals. The discrepancies between studies, more than once within countries of origin, may well indicate population-specific adaptations or simply annual variation in breeding strategies driven by variable environmental conditions.

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66. Pálsdóttir AE. 2016. Varphættir æðarfugls (Somateria mollissima) og afrán á hreiðrum í Breiðafirði. Ms thesis, department of life and environmental sciences, University of Iceland. (In Icelandic with an English abstract)

67. Baillie SR, Milne H. 1982. The influence of age on the breeding of the eider. Bird Study 29: 55-56.

68. Hario M, Rintala J. 2009. Age of first breeding in the common eider Somateria m. mollissima population in the northern Baltic Sea. Ornis Fennica 86: 81– 88.

69. Öst M, Lehikoinen A, Jaatinen K, Kilpi M. 2011. Causes and consequences of fine-scale breeding dispersal in a female-philopatric species. Oecologia 166: 327-336.

70. Hario M. 1983. Weight loss of incubating female eiders. Suomen Riista 30: 28-33. (In Finnish with an English summary).

71. Gabrielsen GW, Mehlum F, Karlsen HE, Andresen Ö, Parker H. 1991. Energy cost during incubation and thermoregulation in the female common eider Somateria mollissima. Norsk Polarinstitutt Skrifter 195: 51-62.

72. Robertson GJ, Reed A, Gilchrist HG. 2001. Clutch, egg and body size variation among common eiders breeding in Hudson Bay, Canada. Polar Research 20: 85-94.

73. Hanssen SA, Engebretsen H, Erikstad KE. 2002. Incubation start and egg size relation to body reserves in the common eider. Behavioural Ecology and Sociobiology 52: 282-288.

74. Bustnes JO, Moe B, Hanssen SA, Herzke D, Fenstad AA, Nordstad T, Borga K, Gabrielsen GW. 2012. Temporal dynamics of circulating persistent organic pollutants in a fasting seabird under different environmental conditions. Environ. Sci. Technol. 46, 10287– 10294.

75. Descamps S, Gilchrist HG, Béty J, Buttler EI, Forbes MR. 2009. Costs of reproduction in a long-lived bird: large clutch size is associated with low survival in the presence of a highly virulent disease. Biology Letters 5: 278-281.

76. Waltho C, Coulson J. 2015. The common eider. T & AD Poyser, London, United Kingdom.

77. Eadie JM, Kehoe FP, Nudds TD. 1988. Pre-hatch and post-hatch amalgamation in North American Anatidae. A review of hypotheses. Canadian Journal of Zoology 66: 1709—1721.

78. Öst M, Steele BB. 2010. Age-specific nest site preference and success in eiders. Oecologia 162: 59-69.

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ISSN: 0006-3657 (Print) 1944-6705 (Online) Journal homepage: http://www.tandfonline.com/loi/tbis20

Variation in nest composition and abundances of ectoparasites between nests in colonially breeding Common Eiders Somateria mollissima

Thordur Örn Kristjánsson, Jón Einar Jónsson & Jörundur Svavarsson

To cite this article: Thordur Örn Kristjánsson, Jón Einar Jónsson & Jörundur Svavarsson (2016): Variation in nest composition and abundances of ectoparasites between nests in colonially breeding Common Eiders Somateria mollissima , Bird Study, DOI: 10.1080/00063657.2016.1182965

To link to this article: http://dx.doi.org/10.1080/00063657.2016.1182965

Published online: 12 May 2016.

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67 PhD theses Thordur Örn Kristjánsson 2016

BIRD STUDY, 2016 http://dx.doi.org/10.1080/00063657.2016.1182965

Variation in nest composition and abundances of ectoparasites between nests in colonially breeding Common Eiders Somateria mollissima Thordur Örn Kristjánssona,b, Jón Einar Jónssonb and Jörundur Svavarssona aFaculty of Life and Environmental Sciences, University of Iceland, Reykjavík, Iceland; bUniversity of Iceland’s Snæfellsnes Research Centre, Stykkishólmur, Iceland

ABSTRACT ARTICLE HISTORY Capsule: The occurrence of high numbers of ectoparasites in nests of Common Eiders may be Received 6 October 2015 related to nest densities and nesting behaviour. Accepted 9 February 2016 Aims: To estimate abundances of ectoparasites and occurrence of blood-covered eggs, and relate those to nest bowl ages, nest bottom material and the incubation stages of eggs, in nests at two different Common Eider colonies. Methods: Nests were collected at Hvallátur and Rif, two sites at Breiðafjörður, West Iceland, in June and July 2012. The nest bottom material was classified to vegetation species and invertebrates were identified to species when possible. Results: The flea Ceratophyllus garei was the dominant ectoparasite at both sites, with median abundances higher at Hvallátur than at Rif in June. In July, the mean abundance of fleas was higher than observed in June at Rif. There were positive relationships between the flea abundances and the incubation stages of the nests, the blood cover of the eggs and the type of nesting material. No relationship was observed between the age of nesting bowls and adult flea abundances. Conclusion: Disadvantages of large parasite loads on the later nesters (second clutch in each nest) at Rif may be compensated by shared nest attendance and the concurrent added time for preening for females while other females attend their nests.

Nests of Common Eider Somateria mollissima, hereafter probabilities of infections (Möller 1993, Oppliger et al. Eider, are unique among waterfowl by being lined inside 1994, Bush et al. 2001, Lesna et al. 2009, Clayton et al. by an extensive amount of nest down feathers, that are 2010, Mainwaring et al. 2014, Stenkewitz et al. 2015). shed from the brood patch area (Lea & Klandorf 2002, Breiðafjörður, West Iceland, is an important breeding, Kristjánsson & Jónsson 2011). Other nest materials, moulting and wintering area for Eiders and supports at such as seaweed and straw are gathered by the bird least 20–25% of the Icelandic Eider population (Choate 1967). The use of nest down allows Eiders to (Grimmett & Jones 1989, Jónsson et al. 2009), which is

Downloaded by [46.239.198.193] at 09:05 13 May 2016 maintain stable incubation temperatures in the nest at estimated about 850 000 birds in winter (Gardarsson high, cold latitudes, resulting in a high hatching 2009). Within Breiðafjörður, the Eider colonies differ success (Kristjánsson & Jónsson 2011). Furthermore, considerably in their sizes and nest densities; the Eiders have one of the highest incubation constancies smallest colonies often having the highest densities of birds, remaining on the nest for about 90–97% of (Jónsson 2001, Jónsson et al. 2013, Kristjánsson & the time, during the 24–27 days long incubation period Jónsson 2015). In the present study we evaluate (Bolduc & Guillemette 2003, Kristjánsson & Jónsson ectoparasite loads at two Eider colonies (Rif and 2011). However, such high and stable nest Hvallátur) with very different nesting densities and temperatures during incubation, maintained by high different nesting behaviour of the birds (see Methods). incubation constancy, also favour nest-living The aim of this study was to investigate if differences ectoparasites (fleas, lice and ticks), as abundances of in nest densities and nest ecology in the two Eider ectoparasites in the nests have been observed to colonies are reflected in different parasite loads in the increase in a favourable thermal microclimate (Sinclair nests at the same time of the year (June). We evaluate & Chown 2006). Accumulating evidence shows that also differences in parasitic load between months ectoparasites have substantial negative effects on (June/July) for early versus late nesting females at Rif, incubating birds due to loss of blood and increased where many of the nests are used by two females

CONTACT Thordur Örn Kristjánsson [email protected] Faculty of Life and Environmental Sciences, University of Iceland, Sturlugata 7, Reykjavík 101, Iceland © 2016 British Trust for Ornithology

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within a season (two clutches/early and late nesters). months were similar for Rif and Hvallátur: in June Conversely, the nests in Hvallátur are only used by a 2012 the mean temperatures were 9.6°C, sd = 1.7 and single female (single clutch) within a season. This led 9.8°C, sd = 1.9 (t-test; P = 0.610), respectively (Icelandic us to two comparisons, that is Hvallátur in June versus Meteorological Office). Rif in June, but also Rif in June versus Rif in July. Furthermore, we investigated parasite loads in relation Collection of nests and ectoparasite identification to the nests’ bottom material, presence/absence of blood cover, age of the nesting bowls and the After incubation, 14 nests from Hvallátur and 14 nests incubation stages of the eggs. from Rif were collected on 7 and 5 June 2012, respectively. At Rif, the 12 nests that were used by a second female, after the first female had led out her Methods brood, were collected on 17 July 2012. All collected nests had a clutch size of four eggs. In all cases the Study areas whole nest was collected, that is, nest bottom materials The offshore colony Hvallátur (65°25′79N 22°46′05W) is and all nest down that the females had shed into the located in the middle of Breiðafjörður Bay, and the nests. Nests were classified as containing blood-covered coastal site Rif (64°55′24N 23°49′36W) is located in the eggs or not (presence/absence). Age of nest bowl was southern part of the bay (Figure 1). In both colonies, grouped as: (1) old nest bowls, which had been used in Eider farmers collect the nest down during incubation, previous years and contained old eggshells, nesting but the nests studied were left intact during the study material or nest markers set up by Eiderdown period. Nowadays Eiderdown is collected, cleaned and collectors and (2) new nest bowls without these used to make products such as duvets, sleeping bags features. The incubation stage of the eggs was and clothes (Bédard et al. 2008). Nest densities in Eider estimated by floating the eggs in water and they were colonies are generally reported to range from solitary also candled by using the sunlight to see through the nesting up to 300 nests/ha (Waltho & Coulson 2015). eggshell for comparison (Weller 1956). Both the The two colonies in the present study differ in size and floating and candling technique yielded the same in nest density. Hvallátur is a large colony (about 200 results. The 24-day incubation period of Eiders ha) with a low nest density (12–58 nests/ha) and the (Kristjánsson 2008) was divided into three incubation nests are spread over a small archipelago. Each nest is stages (early, mid- and late incubation), and each stage only used once per nesting season. The data from consisted of 8 days of incubation. Hvallátur were collected on two small, lowland islands After collection, the nests were double-bagged (around 0.2–0.5 ha) (Kristjánsson & Jónsson 2011). Rif separately, individually marked for identification and holds a small colony (2.34 ha pond) with a high nest frozen. In the laboratory, bags were thawed twice to density (1.7 nests/m2) on two small islands (120 and kill the invertebrates living in the nests (Harriman 600 m2), where most of the nests are used at least twice et al. 2008). Invertebrates were separated from the nest Downloaded by [46.239.198.193] at 09:05 13 May 2016 within a nesting season, the females attend each others’ down in the laboratory by shaking and sieving. The nests and brood-parasitism is common (Jónsson & invertebrates were then placed in 70% ethanol for later Lúðvíksson 2013, Kristjánsson & Jónsson 2015). identification in a laboratory. The invertebrates from Brood-parasitism with seven or more eggs in a single each nest were identified to species when possible and nest has been observed in 17% of nests in the colony number of individuals observed counted under a (Kristjánsson & Jónsson 2015). Re-use of nest bowls by dissection microscope at different magnifications, two females within a breeding season and cooperative depending on their sizes. The dominating nest bottom nest behaviour of females has never been observed at material was identified to vegetation species level Hvallátur. (Table 1). The offshore site Hvallátur is exposed to oceanic wind and salt spray from all directions, while Rif is sheltered Statistical analysis from wind from the south and east. Hvallátur has a colder nest environment for invertebrate pupae during A t-test was used with the SigmaPlot 12.3 program when the winter, as the mean ambient temperature was comparing winter and summer temperature at Hvallátur higher for Rif during December 2011 to March 2012 and Rif (Sokal & Rohlf 2001). Other statistical analyses (t-test: P = 0.005, mean = 0.73°C, sd = 2.7, mean = were performed with the SAS program. We used a −0.43°C, sd = 2.8, respectively). Conversely, the mean generalized linear model (PROC GENMOD, SAS ambient temperatures during the summer sampling Institute, Inc., Cary, NC) with colony and timing of

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Figure 1. A map showing the study sites Hvallátur and Rif in Breiðafjörður west Iceland.

nest collection (Rif in June, Rif in July and Hvallátur in becomes very fragile when dry and the pupae break June) as the class variable and incubation stage, blood frequently during handling of the down, so the cover, nest material and age of nesting bowl as the abundances of pupae were not analysed further (Table 2). co-variates to compare abundances of adult fleas and The median abundances of both adult and larvae larvae. We used Poisson distribution with a logit link C. garei were higher in Hvallátur in June than at Rif at for the ectoparasite count data. The level of the same time (χ2 = 182.2, df = 1, P < 0.001, se = 0.09 significance for statistical tests was P < 0.05 (Sokal & and χ2 = 705.2, df = 2, P < 0.001, se = 0.08 respectively; Rohlf 2001). Table 2). However, the highest abundance observed was at Rif in nests of late nesting females (July) which Downloaded by [46.239.198.193] at 09:05 13 May 2016 was different from what was observed in nests of Results earlier nesting females (June) at the same site, both for 2 Comparison of invertebrates adults (χ = 513.3, df = 1, P < 0.001, se = 0.11) and larvae (χ2 = 700.2, df = 1, P < 0.001, se = 0.10; Table 2). The invertebrate fauna at both Eider colonies consisted I. uriae occurred in very low abundances at both fl only of two truly ectoparasitic species, the ea colonies and months. They were observed only in a Ceratophyllus garei and the tick Ixodes uriae (Table 2). single nest in Hvallátur (7%) in June, but not at all at fi Various species of unidenti ed non-parasitic soil mites Rif at the same time. In July I. uriae was found in five (Acari: Oribatida) and insects were also found (Table nests at Rif (42%) and the highest number was four 2) but not analysed further for this study as they are specimens in one nest (Table 2). associated with the bottom material in the nest or the soil rather than to the nest material or the incubating birds. Comparison of nests between colonies C. garei was the dominant invertebrate species observed in all samples investigated and the only flea Red Fescue grass Festuca rubra was the most common species. We analysed C. garei as two groups: adult and nest bottom material in Hvallátur (64%), followed by larvae. Few C. garei pupae (0–42 individuals) were Marram grass/Lyme grass Leymus arenaria (28%) and found in the nests. The exoskeleton of the pupae only 7% of the nests were built on dry Brown seaweed

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Table 1. Bottom material, blood-covered eggs, nesting bowl age and incubation stages of eggs in Common Eiders Somateria mollissima nests in Hvallátur and Rif in Breiðafjörður, West Iceland in 2012. Investigation site Hvallátur Rif Rif Number N = 14 N = 14 N = 12 Date 6/7/2012 6/5/2012 7/17/2012 % of nests % of nests % of nests Bottom material Red fescue grass (Festuca rubra) 64 100 100 Marram grass (Leymus arenaria)28 00 Brown seaweed (Ascophyllum nodosum)7 0 0 Blood-covered eggs 36 7 0 Bowl age New 36 0 0 Old 64 100 100 Incubation stage Stage 1 0 50 0 Stage 2 36 43 0 Stage 3 64 7 100

Ascophyllum nodosum (Table 1). Red Fescue grass was 0.1, df = 1, P = 0.846). However, more larvae were the only nest bottom material observed at Rif (Table 1). found in older nest bowls than in new ones (χ2 = 24.0, Age composition of the nesting bowls differed between df = 1, P < 0.001). All nest bowls were old at Rif and sites. All bowls at Rif were old, while 64% of the bowls in therefore no comparison could be made from that Hvallátur were old (Table 1). Blood-covered eggs were colony with respect to age of the nest bowl. observed in 36% of the nests in Hvallátur in June, but The abundances of adult C. garei were positively only in 7% of the nests at Rif at the same time. In July, correlated with the incubation stage of the nest when no blood-covered eggs were observed at Rif (Table 1). all data were pooled (χ2 = 102.5, df = 1, P < 0.001), as The numbers of C. garei adults (χ2 = 56.1, df = 1, P < were the abundances of larvae in the nests and the 0.001) and larvae (χ2 = 16.2, df = 1, P < 0.001) differed incubation stage (χ2 = 471.2, df = 1, P < 0.001). between groups of nest bottom material in Hvallátur. Red Fescue grass was the only type of nest material Discussion found at Rif, besides nest down. The abundances of C. garei were higher in nests with Only two true ectoparasite species were observed in the blood-covered eggs than in nests without blood-covered Eider nests in Breiðafjörður: the flea C. garei and the eggs, when data from Hvallátur and Rif in June were tick I. uriae. We found more fleas in the Eider nests at pooled, both for adults (χ2 = 4.7, df = 1, P = 0.031) and the offshore colony Hvallátur than at the coastal larvae (χ2 =6.1, df=1, P = 0.014). However, all blood- colony Rif. Also, the abundances of the fleas increased covered eggs in the present study had less than 5% of the with an increasing incubation stage. Most fleas in our egg surface covered (see Figure 2 in Harriman et al. 2008). study were found in nests of later nesters (second clutch

Downloaded by [46.239.198.193] at 09:05 13 May 2016 Abundances of C. garei adults were similar in new in each nest) at Rif. The densities of C. garei larvae were (N = 5) and old (N = 9) nest bowls in Hvallátur (χ2 = positively related to the age of the nesting bowl. Both

Table 2. The invertebrate fauna from Common Eider nests in Hvallátur and Rif in Breiðafjördur, West Iceland in 2012. Investigation site Hvallátur Rif Rif Number of nests N = 14 N = 14 N = 12 Position 65°25′79N, 22°46’05W 64°55´24, 23°49´36 Date 7/6/2012 5/6/2012 17/7/2012 Total ind. Average Min Max Med Total ind. Average Min Max Med Total ind. Average Min Max Med Insecta Ceratophyllus garei Adults 688 49 6 156 31 320 23 1 138 6 1284 107 32 308 78 Larvae 1179 84 1 282 13 399 29 1 152 13 970 81 1 318 16 Pupea 100 7 0 42 0 12 1 0 4 0 22 1.8 0 8 0 Arctorthezia cataphracta 148 9.9 0 54 0 8 1 0 4 0 24 2.2 0 18 0 Nebria rufescens 8 0.50 40 0 0 0 00 0 0 0 00 Diptera sp. 8 0.6 0 4 0 0 0 0 0 0 24 2.2 0 8 2 Arachnida Ixodes uriae 2 0.1 0 2 1 0 0 0 0 0 14 1.2 0 4 0 Arachnida sp. 8 0.5 0 6 0 0 0 0 0 0 0 0 0 0 0 Acari: Oribatida Mites sp. 244 17.4 0 100 12 50 4 0 10 4 140 11.7 2 40 8

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adult and larvae C. garei abundances in nests were caused 5% more Brünnich’s Guillemot (Uria lomvia) to positively related to blood cover of Eider eggs; both be infected with ticks (I. uriae) (Descamps 2013). these relationships were previously observed for Snow Thus, the relatively small differences in temperatures geese by Harriman et al.(2008). However, the between the two sites investigated in the present study abundances of the fleas were not related to nest could facilitate an appreciable difference in ectoparasite initiation date in this study. abundance. In the North Atlantic, six species of fleas have been We found a relationship between the adult and larval reported from seabird colonies (Mehl 1992) and of abundances of C. garei and the nest materials. It seems these species only Ceratophyllus vagabundus and that nests built on Marram Grass had a higher parasite C. garei have been observed on seabirds in Iceland load than nests built on Red Fescue grass. In Snow (Brinck-Lindroth & Smith 1971). C. garei was common geese, Harriman et al.(2008) reported the highest flea in both Eider colonies studied here and was observed abundances in nests which had the best insulation due at all life-cycle stages in all nests, often in high to shelter from snow and surrounding material. abundances. The abundances of C. garei were much However, it has been shown that Eider females laying higher (up to 107 adult individuals per nest) than in nests located on Red Fescue Grass and Marram previously reported for fleas in an Eider colony in Grass maintain similar temperatures in the nests Svalbard, where the flea Mioctenopsylla arctica (not during incubation (Kristjánsson & Jónsson 2011). But found in this study) was the only flea species found in still, Marram grass provides more vegetation (biomass) the nests (Coulson et al. 2009). This difference in flea and shelter due to its height and density than the low numbers could be due to more dense vegetation in Red Fescue grass, which might explain the differences Iceland than at Svalbard, which results in better shelter in parasite load between these nest materials. and a more stable environment (sensu D´Alba et al. In the present study, no relationship was observed 2009) for the fleas in the nests. Additionally, snow between new and old nest bowls when looking at adult often covers the nesting grounds at Svalbard during parasite loads. This finding differs from that of the spring, so fleas may not emerge as early at Svalbard as Snow Goose colony at Karrak Lake, where more fleas they can in Iceland. It could also be that different were found in older nest bowls than newer ones abundances of fleas in these two studies were caused (Harriman et al. 2008). The Eider colony in Hvallátur by different sampling methods. has expanded in recent years and new nest bowls are The abundances of adult C. garei were higher in June at detected yearly, while the colony at Rif seems to be offshore Hvallátur than at coastal Rif. However, there was almost fully colonized by nesting Eiders as indicated by greater variation in the number of individuals between high and stable nest numbers from 2008 to 2013 nests at Rif than observed at Hvallátur which might be (Jónsson & Lúðvíksson 2013, Kristjánsson unpubl. data). influenced by a low number of nests in our study. The The establishment of new nest bowls by some Eiders at high nest density and short distances between nests at Hvallátur might be caused by a behavioural strategy, Rif could facilitate a relatively fast spread of fleas used to minimize effects of fleas by avoiding last year’s Downloaded by [46.239.198.193] at 09:05 13 May 2016 between the nests. Occasional shared nest attendance at pupae altogether. At Rif, the Eiders must use every Rif (Kristjánsson & Jónsson 2015) might lower available nest bowl, even though the nest is infested ectoparasite loads of some birds by allowing the with fleas from previous nesting attempts. incubating females increased time for preening while Ceratophyllus fleas suck blood from the incubating other females attend their nests. The higher abundances birds and then defecate on the eggs to provide their of adult fleas in July than in June at Rif suggests that larvae with nutrition. Negative effects on the adult fleas in Iceland are alive in the nests during incubating birds can be extensive judging from the whole incubation period and do not die during frequency of blood-covered eggs (Harriman et al. mid-incubation like fleas in the Snow Goose colonies at 2008). Frequency of blood-covered eggs may, therefore, Karrak Lake (Harriman et al. 2008). be a good indicator of the numbers of fleas in the nest. Hvallátur had lower winter temperatures than Rif Such blood-covered eggs have, to our knowledge, not during the years of the study. A milder winter been documented previously in Eider nests. A positive temperature at Rif is probably favourable for the pupae relationship was observed between the blood-covered and could allow them to emerge as fleas earlier in the eggs and the abundances of the fleas in the nests. spring because of earlier melt of snow and frost from However, the percentages of blood-covered eggs were the nest bowl. Thus, fleas in Eider nests at Rif probably low (36% and 0–7% at Hvallátur and Rif, respectively) had a higher winter survival than those at Hvallátur. compared to what has been observed in another At Svalbard, only a 1°C change in winter temperature colonially nesting waterfowl species, the Snow Goose

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(Harriman et al. 2008). In Snow Goose nests the amount Gunnarsson, Karl Skírnisson and two anonymous reviewers of blood on eggs was also correlated with the number of for their constructive comments which improved the paper fleas in the nest, and fluctuating annually (55–98% eggs substantially. had blood cover; Harriman et al. 2008). Here, the blood cover was generally low (≤5% cover of the egg shells), while Snow Goose eggs had up to 50% blood cover on References individual egg shells (Harriman et al. 2008). It is Bédard, J., Nadeau, A., Giroux, J.-F. & Savard, J.-P.L. 2008. possible that different flea species defecate different Eiderdown: Characteristics and Harvesting Procedures. amounts of blood in the nests and C. vagabundus may Sociéte Dutvetnor Ltée and Canadian Wildlife Service, defecate more blood than C. garei. Environment Canada, Quebec Region, Québec, 48 pp. The tick I. uriae was the only ectoparasite observed in Bolduc, F. & Guillemette, M.M. 2003. Incubation constancy and mass loss in the Common Eider Somateria this study beside the flea C. garei, but was found in low mollissima. Ibis 145: 329–332. abundances and the occurrence was limited to a few Brinck-Lindroth, G. & Smith, F.G.A.M. 1971. The Kenner nests. The abundances of I. uriae can vary greatly collection of Siphonaptera in the Entomological Museum, between nests and even within a colony (McCoy et al. Lund, with a check-list of the fleas of Sweden. Ent. Scand. 2003). The highest abundance was observed at Rif in 2: 269–286. July, compared to the other samples in June. The tick Bush, A.O., Fernandez, J.C., Esch, G.W. & Seed, J.R. 2001. Parasitism: The Diversity and Ecology of Animal Parasites. is often found on the birds, making it easy to spread to Cambridge University Press, New York. new nests, and is common in Iceland, especially in sea Choate, J.S. 1967. Factors influencing nesting success of Eiders bird colonies on Puffins Fratercula arctica, Black in Penobscot Bay. J. Wildlife Manage. 31: 769–777. Guillemots Cepphus grylle and Fulmars Fulmarus Clayton, D.H., Koop, J.A.H., Harbison, C.W., Moyer, B.R. & glacialis (Lindroth et al. 1973, Richter et al. 2013). Bush, S.E. 2010. How birds combat ectoparasites. Open Ornithol. J. 3: 41–71. Ticks are vectors of several pathogenic infectious Coulson, S.J., Moe, B., Monson, F. & Gabrielsen, G.W. 2009. organisms causing a number of diseases of medical and The invertebrate fauna of high Arctic seabird nests: the veterinary importance in cases of high level of microarthropod community inhabiting nests on infestation (Olsen et al. 1993, Gauthier-Clerc et al. 1998). Spitsbergen, Svalbard. Polar Biol. 32: 1041–1046. In conclusion, the Eider colonies at Rif and Hvallátur D’Alba, L.B., Monaghan, P. & Nager, R.G. 2009. Thermal fi differed in nest densities, age of nest bowls, vegetation bene ts of nest shelter for incubating female Eiders. J. Therm. Biol. 34: 93–99. types, use and re-use of nest bowls within the season, Descamps, S. 2013. Winter temperature affects the prevalence the behaviour of the incubating females and in winter of ticks in an arctic seabird. Plos One 8: 1–5. temperatures. Only two ectoparasite species were found Gardarsson, A. 2009. Fjöldi æðarfugls, hávellu, toppandar in the nests and only one of them, C. garei, in such an og stokkandar á grunnsævi að vetri [Numbers of abundance that could be considered a potential stress Common Eider, Long-tailed duck, Red-breasted Merganser, and Mallard, wintering on the coast of factor. The abundance of C. garei was different between Iceland]. Bliki 30: 49–54 (in Icelandic with an English the colonies in June and increased at Rif in July when Summary). Downloaded by [46.239.198.193] at 09:05 13 May 2016 the incubation in the second brood was almost over. A Gauthier-Clerc, M., Clerquin, Y. & Handrich, Y. 1998. positive relationship was also observed between C. garei Hyperinfestation by ticks Ixodes uriae: a possible cause of abundance and the bottom material of the nests, the death in adult King penguins, a long-lived seabird. Colon. blood cover of eggs and the incubation stage. This might Waterbirds 21: 229–233. Grimmett, R.F. & Jones, T.A. 1989. Important Bird Areas in indicate increased favourable microclimate for the Europe, Vol 1. International Council for Bird Preservation, C. garei with increased incubation time. The shared nest Cambridge. attendance at Rif might be a response to the high Harriman, V.B., Alisauskas, R.T. & Wobeser, G.A. 2008. The C. garei abundances in the colony, giving the incubating case of the blood-covered egg: ectoparasite abundance in an bird some extra time to preen while other females arctic goose colony. Can. J. Zool. 86: 959–965. Jónsson, J. 2001. Æðarfugl og æðarrækt á Íslandi [Common attend their nest. Eider and Eider Culture in Iceland]. Mál og mynd, Reykjavík (in Icelandic). Jónsson, J.E., Gardarsson, A., Gill, J., Petersen, Æ. & Acknowledgements Gunnarsson, T.G. 2009. Seasonal weather effects on a subarctic capital breeder: Common Eiders in Iceland over We thank Smári J. Lúðvíksson and Auður Alexandersdóttir at 55 years. Climate Res. 38: 237–248. Rif and the Eider farmers in Hvallátur for their support of this Jónsson, J.E., Gardarsson, A., Gill, J., Petersen, Æ. & research. Thanks are also due to Terry Galloway for Gunnarsson, T.G. 2013. Relationships between long-term confirming parasite identifications, Trausti Jónsson for the demography and weather in a sub-Arctic population of temperature data and Guðrún G. Thórarinsdóttir, Karl Common Eider. PLoS ONE 8: e67093.

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Jónsson, J.E. & Lúðvíksson, S.J. 2013. A choice between two McCoy, K.D., Tirard, C. & Michalakis, Y. 2003. Spatial genetic adjacent islands: is switching nest sites related to weather or structure of the ectoparasite Ixodes uriae within breeding nest density in the Common Eider (Somateria mollissima)? cliffs of the colonial seabird host. Heredity 91: 422–429. Ornis Fenn. 90: 73–85. Mehl, R. 1992. Fleas (Siphonaptera) from seabirds and their Kristjánsson, T.Ö. 2008. Áhrif dúntekju á hita í hreiðri, nests in mainland Norway and Spitzbergen. NIPH Annals hegðun og varpárangur æðarfugls (Somateria mollissima). Oslo 15: 3–15. [The impact of down collection on nest temperature, Möller, A.P. 1993. Ectoparasites increase the cost of behavior and nesting success in Common Eiders]. M.Sc reproduction in their hosts. J. Anim. Ecol. 62: 309–322. Thesis, University of Iceland (in Icelandic). Olsen, B., Duffy, D.C., Jermson, T.G.T., Gylfe, A., Kristjánsson, T.Ö. & Jónsson, J.E. 2011. Effect of down Bonnedahl, J. & Bergström, S. 1993. A lyme borreliosis collection on incubation temperature, nesting behaviour cycle in seabirds and Ixodes uriae tick. Nature 362: 340–342. and hatching success of Common Eiders (Somateria Oppliger, A., Richner, H. & Christe, P. 1994. Effect of an mollissima) in west Iceland. Polar Biol. 34: 985–994. ectoparasite on lay date, nest-site choice, desertion, and Kristjánsson, T.Ö. & Jónsson, J.E. 2015. Cooperative hatching success in the Great tit (Parus major). Behav. incubation behaviour in a super dense Common Eider Ecol. 5: 130–134. colony. Bird Study 62: 146–149. Richter, S.H., Eydal, M., Skírnisson, K. & Ólafsson, E. 2013. Lea, R.B. & Klandorf, H. 2002. The brood patch. In Deeming, Tick species (Ixodida) identified in Iceland. Icel. Agric. Sci. C. (ed.) Avian Eggs and Incubation, 100–118. Oxford 26: 3–10. University Press, Oxford. Sinclair, B.J. & Chown, S.L. 2006. Caterpillars benefit from Lesna, I., Wolfs, P., Faraji, F., Roy, L., Komdeur, J. & thermal ecosystem engineering by wandering albatrosses Sabelis, M.W. 2009. Candidate predators for biological on sub-Antarctic Marion Islands. Biol. Lett. 2: 51–54. control of the poultry Red mite Dermanyssus gallinae. Sokal, R.R. & Rohlf, F.J. 2001. Biometry. W.E. Freeman and Exp. Appl. Acarol. 48: 63–80. Company, New York. Lindroth, C.H., Andersson, H., Bodvarsson, H. & Richter, S. Stenkewitz, U., Nielsen, O.K., Skirnisson, K. & Stefansson, H. 1973. Surtsey, Iceland: The Development of a New Fauna, G. 2015. The relationship between parasites and spleen 1963–1970, Terrestrial Invertebrates, Entomologica and bursa mass in the Icelandic Rock ptarmigan Lagopus Scandinavica, Suppl. 5: 280 p. Munksgaard, International muta. J. Ornithol. 156: 429–440. Booksellers and Publishers Ltd., Copenhagen K. Waltho, C. & Coulson, J. 2015. The Common Eider.T&AD Mainwaring, M.C., Hartley, I.R., Lambrechts, M.M. & Poyser, London. Deeming, D.C. 2014. The design and function of birds’ Weller, M.V. 1956. A simple field candler for waterfowl eggs. nests. Ecol. Evol. 4: 3909–3928. J. Wildlife Manage. 20: 111–113. Downloaded by [46.239.198.193] at 09:05 13 May 2016

74 PhD theses Thordur Örn Kristjánsson 2016 Paper IV

Paper IV

Paper IV

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Polar Biol DOI 10.1007/s00300-012-1238-8

ORIGINAL PAPER

Spring diet of common eiders (Somateria mollissima) in Breiðafjo¨rður, West Iceland, indicates non-bivalve preferences

Thordur O¨ rn Kristja´nsson • Jo´n Einar Jo´nsson • Jo¨rundur Svavarsson

Received: 11 April 2012 / Revised: 24 July 2012 / Accepted: 14 August 2012 Ó Springer-Verlag 2012

Abstract Breiðafjo¨rður is an important molting, breed- common prey species for common eiders is a chiton and ing, and wintering area for about 25 % of the Icelandic not blue mussels as reported elsewhere. common eider (Somateria mollissima) population. How- ever, feeding habits of eiders in this area have not been Keywords Somateria mollissima Common eider investigated until now. Prey selection was analyzed from Spring food Prey selection BreiðafjoÁ ¨rður IcelandÁ Á Á Á 192 stomach samples (esophagus and proventriculus) collected in spring 2007–2010. Thirty-five prey species were identified; the highest percentage occurrences were of Introduction gastropods (79 %), chitons (polyplacophorans) (58 %), crustaceans (43 %), bivalves (26 %), and echinoderms Common eiders (Somateria mollissima, Linnaeus, 1758) (8 %). The most common food species was the mottled red prey on a large variety of benthic invertebrate species and chiton Tonicella marmorea (58 %), followed by the com- often specialize on one or more species depending on mon whelk Buccinum undatum (40 %), the spider crab location, season, and food availability (Merkel et al. 2007). Hyas arenarius (39 %), and the chink shell Lacuna vincta They prey mainly on mollusks but also on crustaceans and (35 %). The majority of the food items was of small size echinoderms in intertidal and subtidal areas (Guillemette and consumed in high quantity. The chitons and mussels et al. 1992, 1995; Leopold et al. 2001). Eiders fed mainly were of similar average sizes (11.7 and 13.4 mm, respec- on mixed diet of sea urchins and small mussels (length tively), which might suggest that prey size could be as 1–25 mm with a mode 7–8 mm) in the Canadian Gulf of important as species in food selection. There were some St. Lawrence (Guillemette et al. 1992). Medium-sized inter-annual differences in dominant prey classes between mussels (length 20–40 mm) were the main prey item in years. For example T. marmorea was found in 60–70 % of Northern Norway and around Denmark (Madsen 1954; birds in the years 2007 and 2009 but only in 30 % of the Bustnes and Erikstad 1988; Bustnes 1998). Adult eiders in birds in the other years investigated. Diets of males and Northwest Iceland preyed mostly on blue mussels (Mytilis females were equally diverse and similar when all months edulis) but while taking care of the ducklings, they also and years were pooled. Prey selection was highly variable preyed on gastropods and amphipods (Gammarus spp.) in but most individuals focused on few or a single species in large quantities. The ducklings ate mainly Gammarus the hours prior to collection. Results indicate that the most (Gardarsson et al. 1980). The main prey of adult eiders in Southwest Iceland was also the blue mussel but other bivalve species, gastropods, and crustacean were consumed T. O¨ . Kristja´nsson (&) J. Svavarsson Department of Life andÁ Environmental Sciences, in smaller quantity (Skı´rnisson and Jo´nsson 1996). University of Iceland, Reykjavı´k, Iceland Breiðafjo¨rður, West Iceland, is the study site for this e-mail: [email protected] paper and one of the most productive fjords in Iceland. This area is a molting, breeding, and wintering area for at T. O¨ . Kristja´nsson J. E. Jo´nsson Snæfellsnes ResearchÁ Centre, University of Iceland, least 25 % of the eider’s population in Iceland (Grimmett Stykkisho´lmur, Iceland and Jones 1989). The Icelandic fall population has been

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Polar Biol estimated about 900,000 birds and is possibly 8–17 % of the world population of this species (Bustnes and Tertitski 2000; Gardarsson 2009). Despite the large number of birds and the extended period of time that eiders spend in Breiðafjo¨rður, their diet in this area has not been studied. Rising sea temperature of 2 °C the last 10 years has been observed in Breiðafjo¨rður and changes in the benthic fauna have been noted simultaneously (Jonasson et al. 2006), which may influence the food availability of eiders in the area. Consequently, it is important to identify the main food species at this important site for further understanding of changes in the local eider population. In this paper, we examine food choice of eiders drowned in lumpsucker (Cyclopterus lumpus) gillnets in spring (late May to middle of July) in the middle of Breiðafjo¨rður during 2007–2010. We compare diets in different years, months, and sexes. The fishing season for lumpsuckers in Breiðafjo¨rður starts on May 20 and in March for other areas of Iceland. The difference in start date of the lumpfish Fig. 1 A map of Breiðafjo¨rður, West Iceland. The sampling site fishing season in Breiðafjo¨rður is meant to protect the (Flatey) is inside the circle eiders prior to their breeding season that normally starts April/May. Eiders are particularly valuable in Breiðaf- jo¨rður because of down collection. Common eiders com- lumpsucker gillnets. Most of the birds (155 individuals) monly feed in hard-bottom areas with kelp patches were caught near Flatey island (65°22.300N, 22°55.400W) (Guillemette and Himmelman 1996). Such kelp patches in the northern part of Breiðafjo¨rður, about 16 km from the also are preferred netting sites by lumpfishermen in mainland. Thirty-seven birds were collected from southern Breiðafjo¨rður, and therefore, we assume the shallow waters part about 48 km from the village Stykkisho´lmur used for gillnetting lumpsucker to be the representative of (65°04.390N, 22°43.360W; Fig. 1). Both the collection sites eider feeding habitats. are typical spawning habitats for lumpsucker and feeding habitats for common eiders (7–15 mm depth, hard-bottom area with kelp beds). No difference in food items from Methods southern and northern part of the bay was observed, so the samples from the two sites were pooled for further analysis. Study area The eiders were collected from the end of May to July during 2007–2010. All birds were bagged, tagged, and Breiðafjo¨rður is a 6,000 km2 bay in western Iceland that is frozen on the day of collection. a 50 km wide and 125 km long (Fig. 1). The bay is rela- During subsequent dissections, we removed the stomach tively shallow and characterized by numerous islands and (esophagus and proventriculus) and the gizzard. We skerries. A relatively deep canyon penetrates the bay from examined the contents of these organs and identified food southwest to northeast. Breiðafjo¨rður is about 200 m deep items to prey species whenever possible. The numbers of in the southwest and about 100 m in the center. It is rela- individuals of each prey species were counted, and shell tively shallow in the southeast and the inner part of the bay length of polyplacophora, gastropods, bivalves, echino- (\30 m depth) where eiders were collected for this study. derms, and crustaceans were measured by Vernier caliper Numerous shallow fjords and extensive tidal flats are found to the nearest 0.1 mm using a stereoscope when necessary. around the margin of the bay. The mean tidal amplitude at Chitons identified in eider samples included both Toni- spring tide is 4.4 m, but can extend up to 6 m. Strong tidal cella marmorea and Tonicella rubens (Table 1). T. mar- currents are common. The numerous small islands and the morea was most common in this group because all birds jagged coastline are rocky (Ingo´lfsson 2006). that consumed T. rubens had also eaten T. marmorea but in much higher quantities. Mytilidae in samples included both Collection and statistical methods Mytilus edulis and Modiolus modiolus. Most of these individuals were very small (\10 mm), and it was difficult A total of 188 adult eiders (106 males and 82 females) and to determine the species. However, larger Mytilus edulis four 1-year-old males were collected by fishermen using was generally identifiable.

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Polar Biol (%) 8 4 83 11 11 11 1 11 24 6 18 5 16 2 73 13 12 3 18 3 58 5 22 4 50 8 144 23 127 14 Item Occurred Total 20) = N (%) ccurrence in all birds) and total 2 10 167 14 1 5 779 19 5 5 2 5 12 5 5 175 14 15 83 1 5 34 10 260 40 19 40 138 38 112 15 3,253 58 199 45 314 9 202 15 462 22 Item Occurred 2) Female ( = (%) N Item Occurred July 19) Male ( = N (%) 6 16 1 5 6 16 5 16 1 5 2 11 8 11 1 5 9 11 15 32 40 5 13 11 10 37 11 26 Item Occurred 46) Female ( = (%) N 2 4 6 7 1 2 1 2 1 2 2 4 6 4 12 1 2 11 11 77 41 29 11 20 15 12 7 26 33 43 26 29 7 283 22 1,562 58 22 50 1,308 80 4,282 35 167 30 Item Occurred June 41) Male ( = N (%) 2 2 3 2 7 2 5 10 7 5 2 5 9 10 6 10 Item Occurred 61) Female ( = (%) N 5 5 8 5 2 3 2 312 5 2 3 5 3 5 22 12 53 21 109 22 12 10 51 3 23 7 22 15 18 17 10 7 16 10 41 7 90 15 64 29 41 36 42 49 1057 3 30 33 22 28 13 82 26 16 20 Item Occurred May Male ( 135 57 33 34 453 26 290 27 406 20 701 39 ) 967 74 616 54 1,132 67 426 53 ) 83 33 ) 2 3 T. ruber ) and M. modiolus and Gammarus duebeni T. marmorea tessulata, virginia sp. ( M. edulis spp. ( Eosophageal-proventricular content of male and female common eiders in May, June, and July in all years investigated (numbers of items, percentage o spp. ( spp. Phylum Mollusca, Class Gastropoda Acmaea Trachydermon albus Phylum Mollusca, Class Polyplacophora Tonicella Boreotrophon truncatus B. undatum G. tumida Helcion pellucidum L. vincta Littorina obtusata Littorina saxatilis Lacuna pallidula Eupagurus pubescens Phylum Arthropoda, Class Crustacea Amphipoda M. groenlandicus H. arenarius Mytilidae ( Phylum Echinodermata, Class Echinoidea Asterias rubens Margarites helicinus N. clausa Idotea baltica Sclerocrangon borealis Mya truncata Nucella lapillus Heteranomia squamula items and occurrence for all months combined Table 1 Unidentified gastropods C. islandica Phylum Mollusca, Class Bivalvia Astare

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Statistical comparisons were made using a Welch two sample t test when the data passed a normality test. We also

(%) used one-way ANOVA test or a Kruskal–Wallis one-way analysis of variance on ranks when the normality test failed 52 11 10 2 10 5 (Sokal and Rolf 2001). When testing for difference in the Total Item Occurred 10,451 size of chitons between sexes, we used a t test but a ANOVA test when looking at the size difference of chitons 20)

= between months. A Kruskal–Wallis test was implied when N (%) testing for the size difference of chitons between years. 25 52 2 5 1 5 5 25 Analyses were done using the Sigma Stat 12.3 program. 1,899 Item Occurred

Results 2) Female ( = (%)

N Overall food selection (pooled over years and sexes) July 28 Item Occurred The 192 eider samples as a whole contained 10,500 food items comprised of 35 species and 27 genera (Table 1). The

19) Male ( taxa commonly found in the samples included gastropods = (79 % of samples), chitons (polyplacophorans; 58 %), crus- N (%) taceans (43 %), bivalves (26 %), and echinoderms (8 %). 1 5 6 50 1 5 The five most common species in terms of frequency of Item Occurred 2,118 occurrence were the chiton T. marmorea (58 %), the whelk Buccinum undatum (40 %), the spider crab Hyas arenari- us(39 %), the chink shell Lacuna vincta (35 %), and the 46) Female (

= gastropod Natica clausa (23 %). All other species were (%)

N found in \20 % of all samples (Table 1). The mean number 2 4 1 2 13 7 of prey species in the eiders was 3.8 (range: 1–10 mm). About June Item Occurred 1,871 half (15 out of 35) of the species observed were found in less than 5 % of the birds (Table 1).

41) Male ( Based on the number of individual prey items counted, the =

N predominant groups were the gastropods (L. vincta, mean (%) length 6.8 mm, SD = 2 mm, range: 2.5–11.5 mm; Gibbula 3 5 8 7 4 5 tumida, mean length 7.2 mm, SD = 1.5 mm, range: 5.3–11.3 Item Occurred

2,051 mm; B. undatum mean length 17.2 mm, SD = 10.9 mm, range: 5–55 mm), chitons (Tonicella spp., mean length

61) Female ( 11.7 mm, SD = 2.6 mm, range: 6.5–20.1 mm), bivalves =

(%) (Mytilidae, mean length 13.4 mm, SD = 7.7 mm, range: N 5–61 mm), and crustaceans (H. arenarius, mean length 12 2 3 1 2 1 2 32 1 2 7 5 Male ( May Item Occurred 29.2 mm, SD = 12.6 mm, range: 10–70 mm; Table 1). 2,552 Variation in food selection by years (pooled over sexes)

A total of 59, 10, 86, and 37 samples were collected during 2007, 2008, 2009, and 2010, respectively. Diversity was highest in 2009 when 32 prey species were identified. Twenty-five prey species were identified in 2007 and 2010 but only 12 in 2008 (Table 2). The most common prey classes varied among years. In 2007, chitons (polyplacophora) were found in 73 % of birds, continued sp. but in 2008, 2009, and 2010, gastropods dominated, in 60, 82, and 93 % of birds, respectively. Crustaceans, bivalves, and Phylum Priapulida, Class Priapulidae Priapulus caudatus Phylum Annelida, Class Polychaeta F. sabella Seaweed Strongylocentrotus droebachiensis Unidentified tissue Rocks Ophiura Total number Table 1 echinoderms followed in decreasing order (Fig. 2).

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Table 2 Esophageal-proventricular content (number of birds and occurrence) of 192 common eiders during summer 2007–2010 on Breiðaf- jo¨rður West Iceland Phylum Mollusca Year 2007 (N = 59) Year 2008 (N = 10) Year 2009 (N = 86) Year 2010 (N = 37) Total no. of Occur Total no. of Occur Total no. of Occur Total no. of Occur birds (%) birds (%) birds (%) birds (%)

Polyplacophora Tonicella spp. (T. marmorea 43 73 3 30 53 62 12 32 and T. ruber) T. albus 4 7 15 10 5 161 1 3 Gastropoda Acmaea spp. (tessulata and, 1 2 16 19 9 24 virginia) B. truncatus 35 45 613 B. undatum 20 34 3 30 43 50 10 27 G. tumida 6 10 24 28 6 16 H. pellucidum 3 5 1 10 13 35 L. pallidula 23 2 223 L. vincta 17 29 3 30 22 26 25 68 L. obtusata 9 15 1 10 12 14 3 8 L. saxatilis 12 5 6 M. groenlandicus 1 2 18 21 8 2 M. helicinus 7 82 5 N. clausa 11 19 2 20 22 26 9 24 N. lapillus 1 2 1 10 13 15 1 3 Unidentified gastropoda 3 5 19 2 6 16 Bivalvia Astare spp. 11 C. islandica 12 H. squamula 11 Mya truncata 47 2 2 Mytilidae (M. edulis 9 15 5 50 20 22 9 24 and M. modiolus) Crustacea Amphipoda spp. (G. duebeni)12 2 238 E. pubescens 1 2 5 6 4 11 H. arenarius 21 35 5 50 37 43 10 27 I. baltica 12 2 225 S. borealis 11 Echinodermata A. rubens 1 2 5 52 Ophiura spp. 1 1 3 5 Strongylocentrotus 33 8 droebachiensis Priapulida P. caudatus 11 Annelida F. sabella 1 12 5 Seaweed 2 3 1 1 6 16 Pebbles 2 3 10 7 8 1 3 Unidentified tissue 1 2 1 1 1

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In 2007 and 2009, T. marmorea was the most common 30 species. Out of 11 species that were observed in [20 % prey species found in 73 and 62 % of the samples, of males and females, seven species were found in samples respectively. However, in 2008 and 2010, it was only from both sexes. However, females fed more on the snails observed in 30 and 32 % of samples. H. arenarius (50 %) Margarites groenlandicus and L. vincta, while males on and Mytilidae (50 %) were most common in 2008 but mytilid bivalves (Mytilus edulis and Modiola modiolus; L. vincta (68 %) was most common in 2010. Both B. und- Table 1). atum and H. arenarius were observed in 27–50 % of all the The number sampled (n = 65) and the ratio of females samples collected (Fig. 3). (71 % males and 29 % females) decreased in June. The diversity of species taken decreased to 27. Six species were Food selection by months and sex: May, June, and July observed in [20 % of males and females and all of them (pooled over years) the same for both sexes. Mytilid bivalves (Mytilus edulis and Modiola modiolus) were observed in 30 % of males The total number of birds and the number of each sex but only in few females (Table 1). sampled varied by month (Table 1). The highest number of In July, few individuals were sampled (n = 22) and samples were taken (n = 102), and the sex ratio was most 90 % of them were females. A total of 20 prey species even (61 % males and 41 % females) during May. In May, were found, but only 5 occurred in [20 % of females. Only the diversity of food items was also highest with a total of two males were sampled during July. L. vincta was observed in one of the samples and the other individual was the only sample in the whole study that did not contain any 100 food items (Table 1).

Year 2007 When all months and all years were combined, the 80 Year 2008 diversity of species found in [20 % of all eiders was Year 2009 Year 2010 similar between the sexes (Table 3; t = 1.868, df = 10,

60 P = 0.091). The most frequently consumed prey species in males (n = 110) was T. marmorea, which occured in 70 % of males and in greatest abundance. 40 The snail L. vincta was the prey species observed in highest percentage of females (52 %) and was also the 20 most abundant species. T. marmorea was the second most

Prey class found in % of total birds frequent prey species in females (44 %) but B. undatum in

0 males (49 %). The third most common prey species in both Gastropoda Polyplacophora Crustacea Bivalvia Echnoidea sexes was H. arenarius observed in 35 and 30 % of males and females, respectively, but the abundance was much Fig. 2 Prey composition in common eiders (S. mollissima) during lower than in the other two dominant species (Table 3). summer 2007–2010 in Breiðafjo¨rður, West Iceland No significant difference (t =-0,448 df = 66, P = 0.66) was observed in the shell length of chitons consumed by the

80 sexes. The mean length of chitons consumed by males was 11.4 mm (SD = 2.6, range: 5–19.1 mm) and 11.7 mm 2007 2008 (SD = 2.4, range: 6.8–16.8 mm) by females. The length of 2009 chitons did not change significantly between the 4 years 60 2010 investigated (H = 0.77, df = 3, P = 0.86). The length of chitons consumed increased a little bit between months but the

40 difference was not significant (SS = 3.01, MS = 0.32, df = 2, P = 0.73). In May, the mean length was 10.7 mm (SD = 1.9, range: 7.5–15.2 mm), in June 11.1 mm (SD =

20 2.4, range: 7.4–16.3 mm), and in July 11.3 mm (SD = 2.3, range: 6.6–14.8 mm). Species found in % of total birds

0 T. marmorea L. vincta B. undatum Hyas sp. Discussion

Fig. 3 The most important prey species (percentage) in common eiders (S. mollissima) during summer 2007–2010 in Breiðafjo¨rður, Our study in Breiðafjo¨rður identified 35 prey species West Iceland consumed by common eiders with a mean number of 3.8 in

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Table 3 Prey species observed in [20 % of esophageal-proventricular content of chitons has not been investigated but the items content of male and female common eiders (S. mollissima) (males; consumed in the present study were small (mean length N = 110, females; N = 82) in Breiðafjo¨rður, West Iceland 11.7 mm; range: 6.5–20.1 mm) compared to average length Species Number of items Number of birds % of total (45 mm) cited in the literature (Poppe and Goto 1991). Males Females Males Females Males Females Many studies have conducted that eiders prefer blue mussels over other type of foods and prefer different size Tonicella 2,099 1,154 77 36 70 44 classes but mostly individuals of small size when available. spp. (T. marmorea Small size classes are considered chosen by the eiders and T. because they have the lowest relative shell content and the ruber) highest energy content (Hamilton 2000). In kelp dominat- L. vincta 711 3,571 23 43 21 52 ing areas in St. Lawrence, eastern Canada, the eiders ate Mytilidae 250 212 35 10 32 12 mainly very small mussels ( 10 mm; Guillemette et al. (M. edulis \ and M. 1993), small mussels (10–15 mm) in northern Norway modiolus) (Bustnes and Erikstad 1990), Greenland (Merkel et al. B. undatum 212 100 54 22 49 27 2007), and the Swedish Skagerak (Nystro¨m et al. 1991) but N. clausa 100 44 30 14 27 17 middle sized (20–40 mm) in the Baltic and the Wadden H. arenarius 67 71 38 25 35 30 Sea (Kallenborn et al. 1994). The eiders in New Brunswick Total 3,439 5,152 in Canada were also size selective but their preferences varied with season and prey availability. During most of the year, they selected relatively smaller mussels (20–30 mm) than those that would have been the most each eider. This is similar to what was observed in eiders energetically profitable on a per prey basis (Hamilton et al. wintering in Southwest Greenland (39 species; 1999). Similar results were observed by Nystro¨m et al. mean = 3.9; Merkel et al. 2007) and within the range (1991) in the Swedish Skagerak, where the eiders selected (11–42 mm) from other investigations (Bustnes and mostly mussels 17–18 mm, smaller than the average of Erikstad 1988; Larsen and Guillemette 2000; Merkel et al. those available. Where the eiders prefer blue mussels of 2007). Most of these prey species are probably of little optimal size, they may locally deplete this size class importance as the high diversity is related to the large through winter and spring as has been observed in Canada sample size. The most frequent prey specie of the eider in (Guillemette et al. 1995; Hamilton 2000) and the Nether- Breiðafjo¨rður was the chiton T. marmorea, which was lands (Leopold et al. 2001). consumed by over half of all birds investigated and always In our study, both the chitons and mussels consumed in high number. The second most important species was the were small sized, with a very similar average size of 11.7 whelk B. undatum, then the spider crab H. arenarius, and and 13.4 mm, respectively. Following Hamilton et al. the gastropod L. vincta. These results are different from (1999), blue mussels in this size range have not the highest what has been observed in other studies from Arctic, sub- energy content. In our study, the eiders might have depleted arctic, and temperate sites, where bivalves, mostly the blue the mussels to some degree and switched to chitons. Some mussel (Mytilus edulis), have often been reported as the investigations have shown that eiders utilize a new food dominant prey species (Bagge et al. 1973; Gardarsson et al. resource when key food items disappear from the feeding 1980; Goudie and Ankney 1986; Bustnes and Erikstad area. In the Dutch Wadden sea during 1988–1990, mussels 1988; Skı´rnisson and Jo´nsson 1996; Larsen and Guille- and cockles, which are usually key food items, were at low mette 2000; Nehls and Ketzenberg 2002). abundance due to fishing and low recruitments. About Eiders feed on large variety of species depending on 20,000 eiders in the area starved to death but many left to location (site and depth), time of the year, and sex/age. The the North sea during this period to feed on another food diet composition can vary considerably among feeding sites resource, Spisula subtruncata (Beukema 1993; Leopold located short distance apart and is often dominated by a few 1993). In 2002, the Razor clam (Ensisdirectus) became benthic species (Bustnes and Erikstad 1988; Merkel et al. numerous in the Dutch Wadden sea (Tulp et al. 2010), and 2007; Krasno et al. 2009). However, chitons have not been 3 years later, it turned out to be a valuable food item that reported before as a common prey species for diving ducks. helped the eiders escape another mass starvation due to low In Southwest Greenland, T. marmorea was observed in only recruitments of mussels and cockles (Ens et al. 2006). 3 % of the eiders investigated (Merkel et al. 2007), and in The present study was conducted in a rocky area, and North America, chitons (Mopalia spp. and Tonicella line- hard-bottom prey species were the main prey. Mytilid ata) were reported in 7 % of Harlequin ducks (Histrionicus bivalves (Mytilus edulis and M. modiolus) occurred in only histrionicus) (Rodway and Cooke 2002). The energy 22 % of all samples and in low numbers. Blue mussels may

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Polar Biol not be common at our study site, although they are con- not been previously studied. In the present study, diversity sidered common in Breiðafjo¨rður in proximity to the changed slightly between years, was highest in 2009 (32 largest eider colonies. There has been observed a decline in food species), lower during 2007 and 2010 (25 species), and the stock of M. modiolus at the study site the last 10 years lowest during 2008 (12 species). During 2007 and 2009, key (Guijarro-Garcı´a 2007) but no investigation has been prey species were the same and T. marmorea was the most carried out on Mytilus edulis. preferred species. During 2008 and 2010, H. arenarius and An increasing sea bottom temperature of 2 °C during the L. vincta were the most abundant prey species. This slight last 10 years has been identified as a critical vector in mass difference might rather be due random sampling error rather mortality of the Iceland scallop (Chlamys islandica) stock than different food supply or selection between years. In in Breiðafjo¨rður (Jonasson et al. 2006). The increase in 2007 and 2009, a relative high number of birds was sampled bottom sea temperature has made the scallops more sus- compared to the other years investigated. ceptible to infections of an apicomplexan parasite causing A slight difference was observed in prey selection mass mortality in the stock (Kristmundsson et al. 2011). In between months in the present study. Diversity was highest all the eiders investigated in the present study, only one in May decreasing in June and lowest in July. This dif- individual of C. islandica was observed. ference is probably more related to random sample error of Wintering eiders in the Gulf of St. Lawrence in Canada differences in sample size or the sex ratio of samples rather fed predominantly on the blue mussel Mytilus edulis and on than different prey availability between months. the echinoid Strongylocentrotus droebachienensis (Guille- The diversity of prey species was the same for both sexes mette et al. 1995), while in the present study, S. droe- when all samples were pooled in agreement with previous bachienensis occurred only in 2 % of birds observed. This findings (Bustnes and Erikstad 1988; Frimer 1997; Merkel et al. echinoid is though common in Breiðafjo¨rður and com- 2007). In May and June, male eiders were caught in greater mercially exploited there. numbers than females but the opposite was observed in July. During the prenesting period in the Gulf of St. Law- May and June are the incubation period for eiders in rence, the females fed mainly on the polychaete Nereis Iceland (Kristja´nsson and Jo´nsson 2011). Eiders maintain virens and on herring eggs. The main prey was Littorina high incubation constancy (Bolduc and Guillemette 2003; spp. and amphipods while accompanying ducklings Kristja´nsson and Jo´nsson 2011). Females rarely eat during (Cantin et al. 1974). In contrast, in our study, only one this period (Swennen et al. 1993) and are therefore not species of polychaete was found (Fabricia sabella) and likely to drown in lumpsucker nets during May or June. As it was observed in only 2 % of all birds sampled while in many other investigations (Gardarsson et al. 1980; Littorina spp. was observed in 16 % of the birds. Skı´rnisson and Jo´nsson 1996; Merkel et al. 2007), the Eggs of lumpsuckers (C. lumpus) have been observed as majority of prey species observed in present study were of an important spring diet for eiders in northern Norway little significance, as about half of the species were found (Bustnes and Erikstad 1988). However, we did not find any in less than 5 % of the birds and in most cases, few indi- eggs in our study 2007–2010 even though they were viduals of each species observed. sampled at the peak spawning time for the lumpsucker in The main prey selection by the eiders in Breiðafjo¨rður is Breiðafjo¨rður and from lumpsucker nets. The male lump- different from what has been previously documented from the sucker protects the eggs, so eiders might prefer prey that North Atlantic because mussels were relatively rare and gas- requires less energy expenditure. tropods and chitons were most common. The low occurrence Eiders feed on different species when feeding at dif- of mussels in the diet of the eiders in Breiðafjo¨rður warrants ferent depths in response to changes in benthic communi- further studies. Direct observations on species and habitat ties and prey densities (Guillemette et al. 1993). In our selection in the study area compared to food intake should be a study, the eiders fed mainly on hard-bottom species of future research project and could give us information on small size at a depth range of 7–15 mm. Our result may whether the eiders select out species or if they just feed on the support both the following two hypothesis concerning species that have the highest density at the current time. optimal prey selection (Bustnes and Erikstad 1990). The Investigation into energy value of the main prey species, shell–mass minimization hypothesis indicates that preda- T. marmorea, would make it possible to compare energy tors select food that minimizes shell ingestion rather than content of the various species consumed by the eiders and if maximizing energy intake per prey item. The risk-averse they are maximizing the energy intake by selection. foraging hypothesis indicates that large prey is not pre- ferred because of high shell content, low energy value, or Acknowledgments We thank all the lumpfishermen around Flatey some other factor. and Stykkisho´lmur, which provided us with eiders and the harbor- master in Stykkisho´lmur, HrannarPe´tursson, for his assistance. As far as we know, variation in diversity and dominate We acknowledge Sma´ri J. Lu´ðvı´ksson and Auður Alexandersdo´ttir prey species of eiders in the same area between years has for their support of this research and the eider farmers at Hvalla´tur.

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