163

Diet and habitat use of scoters Melanitta in the Western Palearctic - a brief overview

A. D. Fox

Department of Wildlife Ecology and Biodiversity, National Environmental Research Institute, Kalø, Grenåvej 12, DK-8410 Rønde, Denmark. E-mail: [email protected]

If patterns of scoter distribution and abundance are to be understood, there is a need to know upon which prey items these birds feed, how they obtain these prey items and the habitats from which these food items are most easily harvested. Dietary studies and descriptions of habitats exploited by Common and Velvet Scoter in the non-breeding season are reviewed. The existing literature strongly suggests that, outside of the breeding season, these species forage mainly upon marine bivalve mol­ luscs (especially those less than 4cm long) that live on the surface or within the upper 3cm of clean, coarse, sandy substrates in waters less than 20m deep. Although there is a large energetic cost to diving, han­ dling and crushing such prey prior to digestion, such sedentary prey items often occur in very high densities, offering a locally abundant and predictable feeding resource. Since single species often dominate the diet, but dominant food items differ between feeding areas, it seem s likely that scoters simply take whatever prey is locally available in suffi­ cient abundance to fulfil nutritional needs. Large differences in docu­ mented prey size frequency distributions suggest that scoters may not select for specific prey size classes below an upper digestive limit. However, in the absence of any precise understanding of how scoters obtain their prey, nor any simultaneous studies of available benthic food abundance and size class distributions in scoter diets, it is not possible to confirm if differences simply reflect differences in profitability between different prey at different sites at different times of the year. There remains a considerable need to study the basic feeding ecology and the behaviour of scoters and their prey if their patterns of distribu­ tion and abundance are to be better understood.

Key Words: feeding ecology, Common Scoter, Velvet Scoter, bivalve molluscs, diving ducks © Wildfowl & Wetlands Trust Wildfowl (2003) 54: 163-182 164 Diet and habitat use of scoters

Of all the factors determining the cient profitability to the predators. In distribution of organisms, the extent this case, sufficient profitability' is and availability of food is assumed to be defined as prey that yields sufficient of central importance. For Common energy/nutrient gain per unit handing M elanitta nigra and Velvet Scoters M. time to ensure profitable foraging fusca, which spend the majority of their (sensu Zwarts et al. 1992). This is espe­ time in shallow marine waters, food cially important am ongst organism s supply is likely to play a major role in that crush the tough outer shells of determining how birds are distributed ingested molluscs in a muscular giz­ in time and space. Although little is zard to obtain nutrients and energy. In known about the potential predators of the Comm on Eider Somateria moltissi­ these birds, it seem s likely that the ma, Nehls (1995) showed that 50-60% response to risk of natural predation of the ingested energy was expended in plays at best a secondary role in deter­ capturing, handling, processing and mining how scoters distribute them­ digesting prey. In birds that dive to feed selves at sea. Certainly, the role of on benthic organisms in the open sea, quasi-predator stimuli (such as human physiological limits on the depths to disturbance) apparently can play a which predators can dive may further major modifying role in the distribution affect the availability and profitability of of scoters locally. In order to under­ prey in deeper water. Energetic models stand why scoters occur where they do, and empirical observations suggest when they do, it is important to under­ that diving ducks show strong prefer­ stand what features of the food supply ences for foraging in the shallowest characterise those areas where scoters waters, but that prey depletion may are especially abundant or remain for force birds to shift to deeper waters as prolonged periods of time. So what pre­ a season progresses (Lovvorn & Jones cisely are the major features that char­ 1991; van Eerden ef at. 1997; de Leeuw acterise moulting and wintering habi­ 1997, 1999; de Leeuw et at. 1999). tats used by scoters in the Western Hence, selection of rich prey resources Palearctic? may change in time and space because This question needs to be of changes in water depth and deple­ addressed in a number of ways. Firstly, tion of the food base. Furthermore, prey there is a need to be able to determine items may be too cryptic, lie too deep in the diet and link the abundance of the sediment or dig themselves too dietary prey species to the habitats that deep to be easily accessible or available support the greatest densities of those to the predators (Piersma 1994; Kube prey items. Secondly, there is a need to 1996). Hence, an apparently suitable understand the way in which the birds rich benthos exhibiting high biomass exploit their prey, since conditions may may effectively lie beyond the reach of restrict the availability of prey of suffi­ the feeding apparatus of foraging scot- Diet and habitat use of scoters 165

ers. Even when abundant and available degree (Schricke 1993; Aulert & to predators, the size of prey (eg too Sylvand 1997). Crustaceans (isopods, small to be profitable or too large to be amphipods and small crabs), annelids, physically ingested) may affect the echinoderms and fish may also figure profitability of predation. Prey biomass in the oesophagus contents (Cramp & alone may thus not reflect the suitabil­ Sim m ons 1977; Stempniewicz 1986; ity of a given area to a given predator. Schricke 1993; Byrkjedal et al. 1997; Indeed, in one of the larger investiga­ Frengen & Thingstad 2002; Zydelis tions of benthos communities and scot­ 2002). Several observers report a more er distributions, Degraer et al. (1999) varied diet amongst Velvet Scoter and found no good correlation between consider that this is because they tend bivalve and scoter densities, perhaps to forage closer to the coastline than because water depth played such a sig­ Common Scoter, resulting in a more nificant role in determining the distri­ diverse diet and more prey items typical bution of the predators. For this reason, of rocky or stony shores (eg Madsen this synthesis draws on unpublished 1954; Cram p & Sim m o ns 1977). grey and published literature to briefly Nevertheless, in both scoter species, consider not merely the favoured prey molluscs comprised more than 95% of species of scoters, based on existing the diet by frequency and 80% by vol­ information, but also their abundance ume, so it is clear that these represent and habitat use, and those factors like­ the dominant prey items. ly to affect the suitability of the avail­ The composition of prey items able prey. varies considerably between studies - for instance in Lithuania, Common Diet Scoter was found to have consumed most frequently Macoma balthica, with Studies show that both Common relatively high proportions of poly- and Velvet Scoter consum e many chaetes and isopods, whereas Velvet bivalve molluscs (Madsen 1954; Cramp Scoter took mainly Mya arenaria & Simmons 1977; Stempniewicz 1986; (Zydelis 2002). In contrast, in the Gulf of Bräger & Nehls 1987; M eissner & Gdansk, Poland, Stempniewicz (1986) Bräger 1990; Durinck et al. 1993; found that Mya arenaria, Macoma baltica Schricke 1993; Bräger et al. 1995; and Cardium lamarcki were the most Leopold et al. 1995; Leopold & van der important items for both scoters. This Land 1996; Hughes et al. 1997; Degraer contrasts with the dominance of Spisula et al. 1999; Zydelis 2002). The most fre­ in North Sea coasts of Denmark and quently reported species are those list­ the Netherlands (Offringa 1991; ed in Tables 1 and 2. In addition to Durinck et al. 1993; Leopold et at. 1995; bivalve molluscs, gastropods (such as Leopold 1996). Analyses of the whelks Nassarius and periwinkles oesophageal content of shot scoters Littorina} are also taken to a lesser 166 Diet and habitat use of scoters

Table 1. Commonly reported mollusc species present in the diet of Common Scoter, giving typical sediment type used by each organism (based on McMillan 1973 and other sources), the origin of the study and the source reference.

Species Substrate Region

Macoma balthica Mud/sand Belgium', Britain7, Danish Baltic4, Sweden5,

Lithuania*, German Baltic12, Danish Baltic13 Cardium edule Sand Danish Wadden Sea3, Britain', Poland1, German

Baltic1', Danish Baltic13

Mytilus edulis Various Danish Baltic4, Britain7, White Sea1", German Baltic1’ Mya arenaria Sand Lithuania'1, Poland", German Baltic1', Danish Baltic11

Spisula subtruncata Sand Belgium', Netherlands2,8, Danish Wadden Sea3

Cyprina islandica Sand Geman Baltic1*, Danish Baltic'3, Danish Wadden Sea13 Donax vittatus Sand Belgium1, Britain7 Tellina tenuis Sand Belgium', Britain7,

Littorina spp. Various Danish Baltic4, Britain7 Venus gallina Sand Danish Baltic4, Britain7 Mactra corallina Sand Netherlands7, Britain7

Pharus legumen Sand Britain7 Spisula elliptica Sand Britain7 Lutraria lutraria Mud/sand Britain

Scrobicularia plana Estuarine mud Britain7

Gibbula cineraria Various Britain7 Cardium echinatum Sand Britain7

Abra alba Mud/sand Belgium'

Tellina fibula Sand Belgium' Barnea candida Wood/peat/day Belgium1 Nassarius reticulatus Various Danish Baltic4

Cardium scarbrum Sand Danish Baltic4

Nucella sp. Various Danish Baltic4 Neretina fluviatilis Brackish Danish Baltic4

Musculus nigra Sand Danish Baltic4 Mya truncata Sand German Baltic'3 Cardium lamarcki Sand Poland"

Ensis sp. Sand Danish Wadden Sea13

Taxonomy follows McMillan (1973]. Source references: 'Degraer ef al. (19991, 'Leopold et al. (19951, ‘Durinck et al. (1993) "Madsen (1954), 5Nilsson (1972), “Zydelis I2002), ’Hughes et al. (1997), “Offinga (1991), 'Bottelier 11938), "'Bianki (1992), "Stemniewicz (1986), 1 Meissner & Bräger (1990), "see text. Diet and habitat use of scoters 167

Table 2. Commonly reported mollusc species present in the diet of Velvet Scoter, giving typ­ ical sediment type used by each organism (based on McMillan 1973 and other sources), the origin of the study and the source reference.

Species Substrate Region

Spisula subtruncata Sand Danish Wadden Sea3, Danish Baltic4 Cardium edule Sand Danish Wadden SeaJ, Danish Baltic4 Macoma balthica Mud/sand Danish Baltic4 Mytilus edulis Various Danish Baltic4 Nassarius reticulatus Various Danish BaLtic4 Littorina littorea Various Danish Baltic4 Venus gallina Sand Danish Baltic4 Bittium reticulatum Various Danish BaLtic4 Gibbuta cineraria Various Danish Baltic4 Buccinum undulatum Various Danish Baltic4 Nucula nitida Sand Danish Baltic4 Macoma calcarea Sand Danish Baltic4 Mya truncata Mud/sand Danish Baltic4 Bela turricula Danish Baltic4 Massarius pygmaea Danish Baltic4 Mu cula nitida Various Danish Baltic4 Cyprina islandica Sand Danish Baltic4 Aporrhais pes-pelicani Danish Baltic4

Taxonomy follows McMillan ( 1973). Source references: 3Durinck et al. 11993], 'Madsen (19541. from Ebeltoft Vig in Eastern Jutland and in the Juvre Dyb [Danish Wadden (Denmark) were dominated by Sea). Finally, indirect evidence for

M u s c u lu s nigra and Mya arenaria in Com m on Scoters relying on bivalve samples from the winters of 1986/1987 species such as Abra alba, Tellina spp. and 1987/1988, but Cardium edule, or Cardium edule comes from observa­ Macoma balthica and Cyprina islandica tions of large flocks situated over areas in 1989/1990 (K. Asferg, in litt.). with very high densities of these poten­ Cardium edule and Cyprina/Ensis spp. tial prey species in Dutch waters (van respectively (K. Asferg, in litt.) dominat­ Steen 1978; Leopold et al. 1995). ed samples taken from Common The results from all these studies Scoter shot off Læsø (Danish Kattegat) share the common dominance of one 168 Diet and habitat use of scoters or another bivalve in the diet of taken when encountered on sandy-bot- Common Scoter examined, although tomed habitats with sufficient frequen­ the specific prey species may vary cy in sufficient abundance to figure in between areas. This local predomi­ the diet. Likewise, Velvet Scoter exhibit nance that is characteristic of most a more varied range of dietary items studies suggests that Common Scoter than Common Scoter. Nevertheless, in take those mollusc species that are terms of frequency and biomass, it is most abundant, albeit that they utilise fair to conclude that the majority of the different species at different locations diet in the animals studied has com ­ depending on the composition of the prised bivalves, and therefore the local marine benthic community. Locally, distribution and abundance of scoters this may therefore even result in some is likely to be strongly influenced by the prey items being taken preferentially. local abundance and availability of For example, Common Scoter in the these benthic organisms. Gulf of Gdansk fed selectively on Mya arenaria and Cardium edule, and avoid­ Substrate type ed Mytilus edilus, which occurs in the diet elsewhere (Stempniewicz 1986). All the accounts listed above su g­ gest that Common Scoter favour feed­ However, as data on diet and benthos ing on areas of sandy bottom (see also availability were not studied simultane­ the habitats normally occupied by prey ously, prey selection in Common Scoter species in Tables 1 and 2). Here, they has still to be demonstrated. One most commonly take immobile prey, extreme case of dietary selectivity can which can be detected and consumed, be found in Bauer & Glutz von on the bottom or in the most superficial Blotzheim (1969), who reported the layer. Almost all of the important prey stranding of a ship laden with beans on items are typical of sand substrates, Heligoland in the German Bight in the although Mytilus edulis is one apparent 1800s, which attracted 1, 000 Common exception, intriguingly, the Comm on Scoters to forage on spilled beans for Scoter does appear to consume large up to one month! amounts of Mytilis edulis at some loca­ It is misleading to leave the impres­ tions studied (eg Schricke 1993; sion that Com m on Scoter eat only Hughes et al. 1997), but Madsen (1954) bivalve molluscs. A number of accounts report gastropod molluscs, and there suggested that they did not take them from hard, submerged rock reefs, as do are consistent reports of birds taking Comm on Eiders. Blue m ussels will significant proportions of polychaetes, attach themselves to any hard surface echinoderms and small crustaceans, with their byssus threads (including to such as isopods (eg Madsen 1954; shells of conspecifics) and are hence Byrkjedal et ai. 1997; Żydelis 2002). It reported from areas of pure sand, mud seem s highly likely that these are often and scattered boulders (Franz & Harris Diet and habitat use of scoters 169

1988; Newell 1989). Seaducks feeding around the Danish coasts. This sandy on blue m ussels must mainly tear the sediment benthic community is also m ussels apart from each other, so the common and widespread throughout sediment type has little effect on the eastern North Sea coasts, and extends birds' ability to forage on this food to the west coast of Britain, including source. It may therefore be that even the North Wales coast and Liverpool this species is derived from sandy sub­ Bay (Mackie 1990) and Carmarthen Bay strates where it occurs and is encoun­ (Lownds 1982; Hughes et al. 1997) tered by scoters. where Common Scoter are also abun­ On this basis, it appears therefore dant. that scoters are to a high degree Madsen (1954) was convinced that selecting for shallow in- or epi-faunal Velvet Scoter foraged closer to the species which live in coarser sub­ coast than Common Scoter, with the strates, especially sands, such as the result that they took more rocky shore bivalve molluscs, dog whelk g a s­ species, such as periwinkles and crabs. tropods, isopods and annelid (especial­ Nevertheless, he concluded that the ly Pectinarian) worms. Quite how the Velvet Scoter was also a sandy sub­ prey are obtained remains unknown. strate specialist, predominantly taking However, it can be assumed that the a very similar array of sand-dwelling upper 4. 5cm horizons [ie the extent to bivalve species to those taken by which a scoter bill can penetrate the Common Scoter (Table 2). sediment based on measurement of bill length, shown in Figure 4.10 on Availability - distribution in substrate page 54 of Kube 1996) are sifted or siphoned in some way to abstract indi­ Although Cardium spp., Spisula spp. vidual prey. Certainly, such sediments and Donax spp. always lie in the top 2cm are easier to sift through for relatively of the substrate, other bivalves bury large bivalves than, for example, finer themselves deeper in the substrate, clays, muds and silts and substrates of where they are unavailable to feeding high organic content. scoters. Assum ing that Com m on Both scoter species are known to Scoter are only able to extract prey feed on Spisula subtruncata, which is items 4.5cm down into the sediment, widely distributed in Danish waters less deeper burying, older and larger speci­ than 20m deep, forming part of the m ens of Mya arenaria (Kube 1996), 'Venus community' that characterises Echinocardium (Byrkejdal ef al. 1997) or sandy substrates as defined by Thorson Ensis spp. may be beyond reach, partic­ (1979). This type characterises the typ­ ularly of young, inexperienced birds ical benthic community in many of the (Aulert & Sylvand 1997). internationally important areas used by It is therefore important in any con­ scoters in moult and winter periods sideration relating to how to charac­ terise the attractiveness of the benthic 170 Diet and habitat use of scoters fauna that the accessibility of the prey armoured to be profitably cracked open within the substrate is adequately in the gizzard. Many diving ducks are known. This requires a sampling proce­ known to take large single items to the dure that can identify the depth within surface before swallowing them (Ball the substrate at which prey organisms 1990; de Leeuw & van Eerden 1992; normally lie, since this can affect both Nehls 1995; Byrkjedal et al. 1997). accessibility and prey size distributions Therefore, below the absolute upper (see Kube 1996). Furthermore, prey can threshold of prey size imposed by abili­ show a range of escape responses that ty to swallow, the handling time could render them inaccessible to involved in processing large prey items potential predators. Spisula elliptica can reduce intake rates or profitability uses its muscular foot to 'leap' 6-8cm (Draulans 1982). The energetic costs of from potential predators (Thorson crushing shells also increases with 1979) while Pharus legumen and Ensis increasing shell length (Piersma et al. directus can rapidly bury themselves 1993a; Nehls 1995), which may impose 15-20cm down in the substrate, making another energetic constraint upon the it hard even for humans to capture taking of large bivalves. Attempting to them (Swennen et al. 1985; Fish & Fish eat very large prey may also prove haz­ 1989; H ughes et al. 1997). This also ardous: large razor clams (Ensis spp.) presents some challenges for design­ as well as very large m ussels or m us­ ing adequate protocols to sample the sels with abundant attached barnacles normal undisturbed distribution of the may even kill birds if they damage the in-fauna within the sediment column. oesophagus or get stuck in the throat Flence, the presence of such members (Swennen 1976; Jukema 1979; of a species- or biomass-rich bivalve Swennen & Duiven 1989). in-fauna may not equate to ideal forag­ Madsen (1954) stated that the pre­ ing conditions for scoter, and any ben­ ferred size of the hard-shelled mollusc thic surveys anticipated to attempt to food items ingested by Common Scoter correlate bird abundance with benthic ranged from 5-10mm up to 20mm, with communities need to take these factors Mytilus consumed up to 30mm long and into consideration. Mya spp. up to 40mm long, but gave no details of frequencies. Fie also conclud­ Availability - size class distribution ed that Velvet Scoter, being slightly larger than Common Scoter, took Prey can only be considered 'avail­ slightly larger prey, potentially swal­ able' if it is ingestible (ie it can be swal­ lowing Buccinium undulatum individuals lowed and digested). Som e prey up to 60mm long. Fie reported this species are too small to be worth species taking soft prey (such as fish ingesting, others too large to be physi­ and worms) of longer length, but con­ cally swallowed (Draulans 1984), whilst ceded the normal upper limit for hard- the shells of others may be too well Diet and habitat use of scoters 171 shelled prey would probably be 10mm in size. In this situation, it may c.50mm. Kube (1996) recorded well have been the ease of extraction of Common Scoter feeding on Mya arenar­ large numbers of very small prey that ia specimens of between 5mm and made such foraging profitable under 30mm length. Stewart (cited in Hughes these circumstances. Under other cir­ et at. 1997) found that the average size cumstances, whilst it may be more of Donax vittatus taken by Common effective to consume larger bivalves, it Scoter in Carmarthen Bay, South would also make no sense to reject Wales, was 28mm. Meissner & Bräger smaller specimens once harvested. (1990) found a mean shell size of The interesting feature is the normal 15.3m m am ongst ingested Cyprina distributions shown by these frequency islandica from a sample of Common distributions that differ markedly in Scoter from the German Baltic, with their means. These data are extremely very few specimens larger than 30mm. hard to interpret, since they draw from In what can be considered a sympatric non-random samples of birds taken study of bivalve size selection, Durinck from different areas in Danish waters et at. (1993) found no differences in size at different times of year. The differ­ class distributions between the two ences could therefore reflect differ­ scoter species off the west coast of ences in selection of size classes by the Denmark amongst either Cardium birds (because of different nutritional edule or Spisula subtruncata (see Figure requirements at different times of year 1). There were significant differences or the difference in profitability of dif­ between the size class distributions of ferent sizes of prey items, for example). the two bivalve species, suggesting that On the other hand, these differences neither predator selected for a specific could merely reflect the random selec­ size class but rather took a 'represen­ tion of birds taking whatever is the tative' subset of what was available to most profitable prey in proportion to it. their availability locally. It is impossible Overall, in the literature there is to distinguish between these two possi­ considerable variation in the size class ble interpretations with our available distributions of bivalve prey taken by knowledge, but, based on the available Common Scoter (Figure 2). The pres­ information, it seem s likely that there is ence of large numbers of very small an upper limit on the size of hard prey prey items in the sam ple from taken, which seem s to be around 30- Helgenæs, east Jutland (Danish Baltic, 40mm. It also seem s likely to be the November 2001 ), suggests that there is case that the wide range of prey sizes not an obvious 'lower' critical threshold within the range taken reflects the local that limits prey size within the range conditions in some way. However, con­ measured here, since none of the siderably more research needs to be abundant prey was found to exceed undertaken before one can be certain 172 Diet and habitat use of scoters

Figure 1. Frequency distribution of shell lengths of Cardium eduie (upper) and Spisula subtruncata (lower] taken by Common and Velvet Scoter [data from Durinck et at. 1993). Diet and habitat use of scoters 173

Figure 2. Frequency distributions of bivalve shell lengths of prey items taken from the oesophagus of Common Scoters killed in Denmark. These are: [il Helgenæs, east Jutland, Danish Baltic, November 2001 ln=46, unpublished data], (ii) south of Læsø, Danish Baltic, August 2001 (n=22, unpublished data], liii] Cardium edule and (iv) Spisula subtruncata taken in the Danish North Sea [latter two datasets from Durinck et al. 1993].

that scoters exploit available profitable consistent with the highest available bivalve populations rather than show a suitable prey biomass. Assessm ents of high level of selection for specific sizes the depths exploited by scoters vary classes. considerably between authors and pre­ sumably reflect local water column Water depth depth and benthic community types. For the Common Scoter, Dewar (1924) All other things being equal, scoters recorded 81% of dives between 2.2 and would be expected to dive in the shal­ 3.7m depth (maximum 6. 7m], yet lowest water necessary to obtain their Madsen (1954) claimed that 10-20m food. Locomotory costs of diving are was the 'norm al' diving depth for second only to flight in terms of energy Common Scoter and 14-30m for Velvet expenditure (c. 3.5 times basal meta­ Scoter. On the breeding areas, the nor­ bolic rate, Woakes & Butler 1983, mal diving depths for Common Scoter 1986], so foraging represents a trade­ are 1-3m [Dementiev & Gladkov 1952; off between the food gain obtained from Cramp & Sim m ons 1977) and 2-5m for a dive to the bottom versus the costs of Velvet Scoter (Cramp & Sim m ons travel and handling. It would therefore 1977). Recent aerial surveys of moult­ be expected that scoters select for the ing and wintering Common and Velvet shallowest foraging areas possible, Scoter in Alborg Bugt, eastern 174 Diet and habitat use of scoters

Denmark (see Petersen et at. 2003] prey abundance (eg Nilsson 1972; show that birds were Localised over a Durinck et at. 1993). Nevertheless, it is mean water depth of 5.93m (+0. 015 extremely difficult from the available S.E., n=16,605 bird observations) in literature to determine prey selectivity July. However, as the winter pro­ of scoters. Very few of the studies sa m ­ gressed, the mean water depth associ­ pled the benthic communities and the ated with localisations fell in a pre­ diet of scoters at the same point in time dictable fashion through each of the and space. In general terms, it would winter months to reach to 9.40m certainly appear that scoters take (+0.019 S.E., n=30,805] in April, and this whatever bivalve mollusc species is pattern was consistent between three most abundant where they feed, and it different years. The results of these is apparent that the prey species vary aerial surveys showed that a mere throughout the non-breeding range of 0.015% of 568, 543 non-flying Common these ducks. Hence in Lithuanian, Scoter were observed at positions with Polish and German Baltic coasts, water depths of >20m (Petersen et at. Common Scoter consumed the locally 2003). abundant Mya arenaria, in Danish Baltic in summary, scoter are not ran­ waters Cardium edule was predomi­ domly distributed at sea: they select for nantly taken where this species is specific benthic communities and the abundant, in Danish and Dutch Wadden energetic constraints on diving versus Sea waters Spisula subtruncata was derived energy and nutrient gain con­ taken, whilst in NW France mainly strains them to select shallow water, Donax vittatus was taken. In this sense, which appears to be less than 20m in the diet appears to simply reflect local most of the western Palearctic. More abundance, the major constituents of data are required on the interplay the benthic communities dominating between prey biomass, prey availability, the diet in most studied situations. prey quality and water depths to under­ There is a need to undertake specific stand foraging dynamics in tidal waters synchronous studies of benthic com ­ and in response to food depletion. munities and diet of scoters in a num­ ber of different situations throughout Discussion their non-breeding range to establish feeding selectivity in terms of species

The widespread distribution and and size class distributions. abundance of diving ducks suggest that Extremely little is known about fac­ they are unlikely to be restricted to any tors affecting site selection, given that particular type of food but readily take ducks show little evidence of strong whatever prey is available in sufficient species or size selection. This is espe­ quantity. Certainly, local dominance of cially interesting, since scoter are con­ species in the diet does vary with local spicuously mobile, with small flocks Diet and habitat use of scoters 175 flying daily between favoured feeding aggregation of Common Scoter. areas, apparently monitoring the quali­ Scoters do seem to prefer to feed in ty of their feeding environment. sandy substrates during moult and Common Scoter wintering off The winter. Not only do most of the most Netherlands' coast will sometime frequently consumed prey items occur switch en m asse to alternative sites in such substrates, but also the sites of tens of kilometres distant (eg after many of the most important concentra­ severe disturbance from shell fisheries tions are characterised by sandy sedi­ or mass mortality of Spisula, M. F. ment types. These include the Danish Leopold, pers, comm.), suggesting Alborg Bugt area south of Læsø [prob­ prior knowledge of these alternative ably the most important moulting and feeding grounds. Nevertheless, the wintering site for the species in the basis for site selection remains world) and the eastern North Sea obscure. In The Netherlands, for exam­ coastline. However, the presence of ple, scoters have been described this community type does not guaran­ spending a whole winter on a Spisula tee the presence of scoters, so clearly bank where prey averaged 10mm shell there is more to habitat selection than length, returning the following year just benthic community and sediment when the Spisula had grown to c.20 type. The information presented here mm, and again the following year when comes mainly from the Baltic and they had reached 25-30mm (Offringa North Sea areas, which represent the 1991; Leopold 1996). Here, prey size core of the non-breeding range of the and prey density changed between scoter species in the Western three consecutive years [although the Palearctic. Although the wintering sites water depth clearly did not) because of coastal Portugal, Spain and North the Spisula was not replenished by good Africa are more peripheral to the spat fall, yet this single age class in one range, it is interesting to speculate area apparently supported 100,000 about the potential differences in feed­ Common Scoter for three consecutive ing profitability of the exploited habitats winters. Studies failed to demonstrate in this region, not least to understand the specific impacts of birds on the the potential benefits that justify the Spisula stocks of such a bank because extra energy expenditure needed to fly other agents (such as fisheries, winter there from the main centres of the win­ mortality and storm damage) had a far tering range. greater impact on their distribution and This review has also highlighted the abundance (Leopold ef al. 1998). In this lack of knowledge relating to seasons context, there appears little evidence other than autumn and winter. Most of for either prey depletion or size class the studies presented here have not frequency selection playing a role in considered potential changes in prey feeding site selection for this large composition in late winter and spring 176 Diet and habitat use of scoters prior to migration to breeding areas. strong water flow generated in the bill. Several diving duck species include fish This is thought to be a common filter­ roe in their diet during pre-breeding ing technique used by many benthic staging (and at other times of the annu­ feeding diving ducks, perhaps including al cycle as well), and this may explain scoter, to sift small prey items (Madsen anomalous shifts in scoter distribu­ 1954; Böhme 1992; de Leeuw & van tions in spring to normally unoccupied Eerden 1992; Kube 1996). If this is the areas in inshore Danish waters. Since case, the relatively 'clean' sandy sub­ these periods in the life cycle may be strates, with little organic or fine mud critical for accumulation of pre-migra- fractions, would enable simple differ­ tion and pre-breeding nutrient and entiation of prey items using this type of energy stores, and hence affect breed­ feeding technique and may contribute ing success, this lack of knowledge to the explanation of why this substrate identifies a priority area for future type is selected. This could also explain investigations. Furthermore, it would how other food items, such as annelids, be extremely profitable if it were possi­ crustaceans (especially isopods and ble to incorporate information on diet amphipods, which inhabit the upper with the characterisation of seasonal sand horizons) and even small fish fig­ use, sediment types and general ben­ ure in the diet of some individuals. thic community types in any compila­ Direct observational studies of feeding tion of a site inventory for non-breeding techniques in captivity would be useful. scoters in the Western Palearctic. Flowever, observations of their natural Researchers still have no idea about feeding techniques (perhaps in the wild how scoter locate, capture and handle using remote underwater video meth­ their prey, but it seem s likely that the ods) would be highly desirable to combination of high prey encounter understand the basic feeding ecology of rate and the ease of probing and these species, establish prey capture siphoning in sand combine to make for­ techniques and perhaps gain insights aging in these substrates highly prof­ into fine-grained habitat selection. itable. Some diving ducks are known to Knowledge of how prey items are probe and dig in soft substrates for sin­ extracted from the substrate would gle food items (eg Suter 1982; Ball also assist in understanding what pro­ 1990) and it seem s likely that scoters portion of the potential benthos bio­ may use this feeding technique, espe­ m ass is actually available to diving, cially for prey items towards the upper scoters. Earlier, it was speculated that end of the size spectrum selected. prey more than 4.5cm below the sur­ Long-tailed Duck Clangula hyemaIis face was totally unavailable to foraging and Tufted Duck Aythya fuligula are scoters, but bivalves may show diurnal thought to strain small (<16mm) prey patterns in their vertical distribution items from sediments by creating a and this again deserves further Diet and habitat use of scoters 177 investigation. Is bivalve biom ass so populations of the two species. great in the upper 2cm of rich sandy Unfortunately, although such a calcula­ substrates that scoters have no need to tion could be interesting, understand­ probe deeper? Even the most rudimen­ ing of the feeding ecology of the tary understanding of how these birds Common and Velvet Scoter remains feed is Lacking. poor. There exists an abundant litera­ The differences in mean prey size ture on the seasonal and annual varia­ from different sites at different times tions in the benthic fauna of intertidal based on different prey make it difficult sediments as these relate to con­ to distinguish between the hypotheses sumers (wading birds, Wanink & that scoter select for specific prey size Zwarts 1993; Zwarts & Wanink 1993). or that they simply take prey in propor­ These have enabled the interpretation tion to their availability. The remarkable of the distribution and abundance of similarity between the size class fre­ the predator (in this case the waders) in quencies of the Cardium and Spisula terms of the abundance, availability taken by Common and Velvet Scoter in and distribution of their prey. This has the Danish North Sea suggests that the been achieved both by measuring feed­ birds are taking prey in proportion to ing intake rates (eg Goss-Custard ef at. their availability. Differences in bill and 1982; Zwarts et al. 1992) and by com ­ gape size and morphology between paring available prey distribution with these two predator species might be wader densities (eg Zwarts ef al. 1990; expected to create differences only in Piersma et al. 1993b). In these studies, maximal prey size between the two it has been shown that shorebirds can scoter species. Prey selection within a remove substantial proportions of the selected site can only be established by m acrozoobenthos biom ass (10- the simultaneous study of the available 20%), a depletion rate which affects benthos and scoter prey. This remains foraging profitability and hence the dis­ a priority area of research for the tribution of predators (Zwarts & Drent future. 1981; Meire 1993). The offshore subm a­ In summary, scoters seem to prefer rine theatre for foraging scoters foraging in clean, sandy substrates that restricts our ability to undertake such support benthic communities rich in studies forseaducks, which are difficult bivalve biomass. Within such sites, prey even to count and map accurately, let species are probably taken in propor­ alone to observe in preferred feeding tion to their abundance. This would habitats. Food choice has only been make the identification of suitable habi­ estimated from stomach content analy­ tat signatures relatively simple sis (see references above) or inferred throughout the Western Palearctic and from the benthos present at preferred enable a crude extrapolation of the sites, and diet selection and prey han­ potential food resource available to the dling studies of diving ducks have 178 Diet and habitat use of scoters remained in the domain of laboratory Acknowledgements experiments (eg Draulans 1982; de Leeuw & van Eerden 1992). My sincere thanks to all the people The challenge is clear: if researchers who supplied me with published and are to be able to make the simplest of unpublished information relating to predictions of where, when and in what scoters, their feeding ecology and diet, numbers moulting, wintering and pre­ especially Karen Asferg for access to breeding scoters occur in relation to her unpublished data and Mardik their food supply, they need to under­ Leopold for sharing his enormous stand more about their basic foraging knowledge of the species in Dutch ecology and behaviour and their prey. It waters. I am especially indebted to is evident that, just as in intertidal sys­ Stefan Pi h L for help, support and advice tems, the fraction of bivalve prey har- throughout and to him and Else-Marie vestable by scoters will depend upon Nielsen for helping to organise the energy density, body weight, biomass, Seaduck Specialist Group Scoter burying depth and behaviour of the prey Workshop at Fuglsøcentret, Jutland, as well the nature of the sediment, the Denmark, in December 2000. With depth of the water column that sits great pleasure, I thank all the partici­ above it and the level of disturbance to pants at that workshop for their enthu­ the site. As in the case of waders har­ siastic and expert contributions to the vesting the intertidal benthos, the con­ meeting, especially Baz Hughes, who sumption rates of the predator scoters was very supportive of my drafting this will deplete prey in a manner that will analysis and, together with Stefan Pihl also affect local profitability of the food and an anonymous referee, gave con­ supply and levels of interference structive criticism on an earlier draft. between foraging individuals and mod­ ify predator densities. The challenge is References to be able to understand the basics of these systems in a hostile and difficult Aulert, C. & Sylvand, B. 1997. Les environment in which to carry out Macreuses noires Melanitta nigra et research. Nevertheless, to be better brunes Melanitta fusca hiverant au Large able to relate the distribution of non­ de Côtes dus Calvados: relation entre le régime alimentaire et les peuplements breeding scoter to their food supply and macrozoobenthiques marins littoraux. begin to make some predictions about Ecologie 28: 107-1 17. the effects of changes in habitat or habitat loss on scoter populations, Bail, J. P. 1990. Active diet selection or pas­ investment must be made in such sive reflection of changing food availabili­ ty: the underwater foraging behaviour of research in the future. Canvasbacks. In: Behavioural mechanisms of food selection, (ed. R. N. Hughes). Springer-Verlag, Berlin-Heidelberg; pp. 95-109. Diet and habitat use of scoters 179

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