ORNIS SVECICA 13: 75Ð84, 2003

Factors affecting different spatial distribution of wintering Tufted fuligula and Goldeneye Bucephala clangula in the western part of the Gulf of Gdaøsk (Poland)

DARIUSZ JAKUBAS

Abstract Tufted Duck and Goldeneye wintering in the western part numerous group during severe winters (with ice-cover in of the Gulf of Gdaøsk (Poland) were counted in January part of the study area). This could be a result of arrival of 1988Ð1999. In all seasons, the concentrations of the two from frozen wintering areas in northern species were observed in different places. Tufted (predominance of males) and of abandonment of the study gathered in areas well protected against waves and winds area by females and young birds, which are less resistant (in harbours, near breakwaters, in the river mouths). to bad conditions. In mild winters, individuals in female Goldeneyes preferred areas abundant in food and with plumage predominated in the majority of the coastal sectors. shallow water. This distribution difference of the species could be affected by their behaviour. Tufted Duck searched Dariusz Jakubas, University of Gdaøsk, Dept. of Vertebrate quiet resting places by day when Goldeneye mainly foraged Ecology and Zoology, al. Legionów 9, 80-441 Gdaøsk, in the same time. During all seasons, adult male ratio of Poland (e-mail: [email protected]). both species was similar. Adult males were the most

Received 31 July 2003, Accepted 17 October 2003, Editor: S. Svensson

Introduction 1985Ð1986/1987 (Michno et al. 1993, Meissner & The Gulf of Gdaøsk is an important basin for winte- Klawikowska 1993) showed that the two species ring waterbirds. Relatively shallow water makes this concentrated in different places. area attractive for Tufted Duck Aythya fuligula and This paper is an attempt to explain the distribution Goldeneye Bucephala clangula. The populations of of Tufted Duck and Goldeneye flocks wintering in both species wintering in the Gulf of Gdaøsk perio- the Gulf of Gdaøsk, and to explore possible factors dically make up ca 1% and 1Ð3,5% of the whole that may suggest why they differ in site selection. European winter population of these species (Mon- val & Pirot 1989, Michno et al. 1993, Meissner & Klawikowska 1993, Wetlands International 2002). Study area This winter area is therefore important on the scale The surveys were conducted in the area from the of the whole continent. Vistula mouth to the Rozewie Cape, an area including Data from the southern coast of Sweden (Nilsson the westernmost part of the Gulf of Gdaøsk, Puck 1972) show that wintering Tufted Ducks search for Bay, and the Hel Peninsula (Figure 1). The southern quiet and sheltered places, suitable for resting during coast between Vistula Mouth and Sopot is sandy and day-time, and gather in the flocks of above 1000 low. North off Sopot, there are cliffs that are crossed birds. They forage at night at sites that may be distant by river valleys (among others Reda and P«utnica from the daytime roost. On the contrary, the rivers). The Hel Pensinsula (37 km long) is charac- Goldeneye forages by day in smaller, more diffuse terized by a sandy coast on the open sea side and flocks and prefers shallow basins with stony bottom. shallow bays with narrow sandy beaches on the An earlier study on wintering habits of these inner side. There are a few hydrotechnical construc- species in the Gulf of Gdaøsk in the seasons of 1984/ tions in the area Ð harbours with piers in Gdaøsk

75 Figur. 1. Constancy and long-term mean number of Tufted Duck Aythya fuligula (black colour) and Goldeneye Bucephala clangula (grey colour) in mild and servere winters. Broken line Ð appproximate border of ice cover. Förekomstkonstans och medelantal för vigg (svarta symboler) och knipa (grå symboler) under milda och kalla vintrar. Streckad linje: ungefärlig gräns för isbeläggning.

76 Northern Harbour, Gdynia, Hel, and W«adys«awowo, the birds that were distinctly observed feeding or and costal breakwaters in the Wis«a Cmia«a Mouth, roosting on the water in a given sector were counted. in Gdynia, and between W«adys«wowo and Ku

Table 1. Number of Tufted Duck and Goldeneye observed in the consecutive seasons and the proportion of adult males. Antal av vigg och knipa som observerats olika vintrar samt andelen adulta hanar.

Species Year År Art 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999

Tufted Duck 3940 9296 15 211 4231 14 369 6259 8675 8656 18 029 14 347 7464 3572 % adult males 51,5 42,7 42,2 40,9 59,0 57,0 50,4 73,3 54,0 55,3 61,0 Ð Goldeneye 2557 3472 5767 5442 2659 3743 3281 15 589 9137 5781 2383 3366 % adult males 38,6 27,5 37,3 48,6 43,5 54,8 36,8 46,7 59,6 64,4 34,5 42,3

77 rest of the sectors larger concentrations were rare. Differences between spatial distribution of the species The Tufted Duck was particularly rare along the Hel In all studied seasons, the concentrations of Tufted Peninsula and in the southern part of the Gulf of Ducks and Goldeneyes were observed in different Gdaøsk. places (Table 2 and 3, Figure 1). In the mild winters, During the severe winters of 1996Ð1997, when ice the Tufted Ducks gathered in parts of the coast with covered the inner, northwestern part of the gulf, the hydrotechnical constructions like breakwaters, piers, Tufted Duck was concentrated in the southern part jetties, and in the river mouths. Goldeneyes preferred and also preferred sectors that were well protected the shallow water of the Gulf of Puck. During severe against wind and waves Ð north off Gdynia (along winters, when that part of water was covered with cliffs), Sopot pier, and Gdaøsk Northern Harbour ice, both species concentrated in the areas, which (breakwaters and piers). The number of individuals were free of ice-cover. But Tufted Duck and was similar in these three sectors (Kruskal-Wallis Goldeneye preferred different parts of the coast. test, H(2, N=6)=2; p>0,05). During one season birds Tufted Duck gathered in the available parts of the were observed close to other hydrotechnical coast that were well protected against waves and constructions Ð Hel harbour, near Jurata pier, and winds Ð near hydrotechnical constructions and in the ice-free areas in the mouths of Reda and Wis«a plots with cliff coast. The majority of Goldeneyes Cmia«a rivers. The concentrations were big (above was observed in the Vistula Mouth and in the plot 600 individuals) in four sectors. Concentrations of near Rewa (bordered with ice-cover). It was only a more than 100 birds were observed in only four few places where both species were noticed in greater sectors in one of the two winters. In the rest of the concentration (Table 3). sectors, as in the mild winters, less than 100 individuals were observed. Adult male ratio The proportion of adult males and female plumage Spatial distribution of Goldeneye birds is shown in Figure 2 for each of the nine larger, The constancy of occurrence and mean numbers of combined sectors from which data for a sufficient Goldeneye are shown in Figure 1 in the same way as number of birds were available. Proportions were for Tufted Duck. During the mild winters the Goldeneyes were strongly concentrated in the Puck Bay, northwest of a line between Gdynia and Jurata. During all ten mild winters, concentrations of more Table 2. Constancy during the ten mild and two severe than 100 Goldeneyes were found in two sectors Ð winters (number of winters with more than 100 birds in near Rewa and between W«adys«awowo and Cha«upy. a sector). For each combination the number of sectors The mean number of birds (308±237 sd and 439±221 is given. T=Tufted Duck, G=Goldeneye. sd) was similar in these places (Student t-test, t= Ð Förekomstkonstans under tio milda och två kalla vint- 1,276; df=18; p>0,05). Aggregation of above 100 rar (antal vintrar med mer än 100 fåglar i en sektor). Goldeneyes during 5Ð9 winters was observed in the För varje kombination ges antalet sektorer. T = vigg, G Vistula mouth, along the cliffs of the Gulfs of Puck = knipa. and Gdaøsk (north off Gdynia), and along the flat banks of Hel Peninsula. Number of years Ten mild winters Two severe winters During the severe winters, the Goldeneye flocks Antal år Tio milda vintrar Två kalla vintrar were observed at other places than in the mild winters, namely between Rewa and Sopot and at the TG Both T G Both Vistula mouth. More than 100 birds were observed in four sectors in both winters and in nine other 10 Ð 2 Ð sectors in one of the winters. The largest mean 8Ð9 2 1 Ð concentrations (more than 600 birds) were observed 5Ð7 5 8 Ð along the border of ice-cover near Rewa and in the 1Ð4 14 14 3 Vistula mouth. High concentrations (301Ð600 birds) Total 21 25 6 were observed also along cliffs between Or«owo and Sopot. In the rest of the sectors concentrations of 2 34Ð birds were smaller (less than 300 individuals). 1 591 Total 8 13 2

78 Table 3. Mean number of birds (in seasons when above attractive since they are well protected against western 100 individuals were observed). For each combination winds. The importance of harbours for resting Tufted the number of sectors is given. T=Tufted Duck, Duck flocks was highlighted in the studies from the G=Goldeneye. Gulf of Gdaøsk in the seasons 1984/1985Ð1986/ Medeltalet fåglar (under vintrar då mer än 100 fåglar 1987 (Michno et al. 1993), from Denmark (Joensen observerades). För varje kombination ges antal sekto- 1974) and Sweden (Nilsson 1972). Movements rer. T = vigg, G = knipa. Both = båda arterna. among harbour basins were observed in the Gdaøsk Northern Harbour. Birds probably wanted to avoid Mean numbers Ten mild winters Two severe winters rough waves (Gniotyøski 1988). Medeltal Tio milda vintrar Två kalla vintrar Data from this study confirmed the importance of the Vistula Mouth for wintering Tufted Ducks TGBoth T G Both (Michno et al. 1993). The concentration of birds was 101Ð300 5 16 1 4 7 1 observed in the part of Vistula River close to the 301Ð600 2 7 1 Ð 3 Ð mouth. This sector was well protected against wave >600 14 2 Ð 4 3 Ð and wind. Small concentrations of Tufted Ducks in the open parts of the coast were also noticed in Total 21 25 6 8 13 2 Sweden (Nilsson 1972). Even if sheltered roosting sites was the most important factor determining the distribution of the calculated if the number of birds was larger than 100. Tufted Ducks, it is possible that access to nearby During the mild winters, birds in female plumage foraging sites could be a contributory explanation. of both species predominated in the majority of the For example, Common Mussel Mytilus edulis, which combined sectors. In the Tufted Duck, adult males is a component of the Tufted Duck diet, occurs on the were more numerous in only three of eight sectors, surface of hydrotechnical constructions (Maøkowski namely in the Gulf of Puck north off Rewa, along 1975, Michno et al. 1993). Such constructions city beaches (between Gdynia and Sopot), and in the occurred at several of the concentrations sites Ð Ptasi Raj Lake. In some seasons the majority of birds Gdaøsk Northern Harbour, Gdynia harbour, and was observed in those three sectors and then the total breakwaters near Gdynia. In those places the four percentage of adult males was higher than the metres water depth line runs in a distance of one percentage of individuals in other plumages (Table kilometre from the coast. Tufted Ducks dive only to 1). In the Goldeneye, males predominated in three of 3Ð5 metres, so they can feed in the vicinity of seven sectors, namely in the open sea near roosting places. Wladyslawowo, south off Reva, and along city On the other hand they can fly to the shallow beaches between Gdynia and Sopot. waters Gulf of Puck, 15Ð30 km away. A few evening During the two severe winters, a predominance of observations in one sector in the vicinity of Rewa adult males was observed in the majority of the showed that Tufted Ducks flew far away into the sectors. In both species males predominated in three Gulf of Puck. This shallow water basin can be of four sectors. In the Tufted Duck, female plumage attractive for feeding birds. The lack of night data birds predominated only in the Reda mouth, and in about duck distribution makes it impossible to the Goldeneye only at the Vistula Mouth. confirm this presumption. Since Goldeneyes feed during day-time, when the counts were made, the connection between feeding Discussion sites and bird concentrations is more clear than for the Tufted Duck. The shallow shelf zone of the Gulf Spatial distribution of Puck was a good place for foraging Goldeneyes. The very large difference in spatial distribution In some areas of shelf zone high values of macro- between the two species could be an effect of their zoobenthos biomass were recorded in the 1990s behaviour. The Tufted Duck searches quiet resting (Osowiecki 2000). Additionally, the western part of areas during the day and feed at night, whereas the the Gulf is protected against western winds by cliff Goldeneye mainly forages during day-time. I will shore. Western and northwestern winds dominate in therefore first try to explain the spatial distribution of the Gulf of Puck (Augustyn 1985). Protection against each species separately and then compare them. winds can have positive influence for the water For the Tufted Duck, harbours and cliff shores are transparency.

79 Figure 2. Sex structure (long-term mean in %) of Tufted Duck Aythya fuligula and Goldeneye Bucephala clangula in mild and severe winters. Black colour Ð adult males, grey colour Ð females and immature bids. Sample size for each circle >100 indiv. Könsfördelning (medeltal i %) för vigg och knipa under milda och kalla vintrar. Svarta sektorer är gamla hanar, grå sektorer honor och unga fåglar. Stickprovsstorleken för varje cirkel är över 100 individer.

80 Concentration of Goldeneyes in all studied winters competition and can make Goldeneye’s feeding areas in the Vistula mouth can be explained by lack of ice- unattractive for foraging Tufted Duck. cover (it could be attractive for birds coming from But due to lack of complete data about age structure frozen, inland water bodies) and good protection and local distribution of zoobenthos in shelf zone of against wind and waves. the Gulfs of Gdaøsk and Puck, more precise Sewage in the Vistula River and from a sewage estimation of the influence of this factor on wintering plant outlet near Rewa could be an attractive source bird distribution is impossible. of food for foraging Goldeneyes. Sewage may cause The weather conditions affected the distribution increase of zoobenthos biomass in the shelf coastal of Tufted Duck and Goldeneye wintering in the Gulf zone by intensive eutrophication (Ossowiecki 2000). of Gdaøsk. Especially ice-cover limited availability Poles situated on the bottom near the collector mouth of many suitable places for foraging and resting and can be rich in Common Mussels (Maøkowski 1975). made birds gather in other places. Movement of Low concentration of Goldeneyes in the eastern Goldeneyes wintering in the Gulf of Gdaøsk from part of the Gulf of Gdaøsk can be caused by low the frozen areas to the Vistula Mouth and to the plots zoobenthos biomass. The bottom sediments there situated between Rewa and Gdaøsk was also observed are composed of banks of fine sand, which are in the seasons 1984/1985Ð1986/1987 (Meissner & shifted by currents and therefore are not overgrown Klawikowska 1993). by plants. Such a bottom type is not favourable for Human disturbance, tradition and social factors intensive zoobenthos colonisation (Osowiecki 1995, can also influence the distribution of wintering flocks 1998). (Joensen 1974, Nilsson 1972). However, in the con- Goldeneyes wintering in the southern Swedish trast to the Western Europe, human disturbance was coast (Scania) concentrated in zones with stony and not an important factor influencing distribution of muddy bottoms and avoided sandy parts (Nilsson wintering birds in my study area (Meissner & Klawi- 1972). There is stony bottom in the Gulf of Puck near kowska 1993). Rzucewo and in the Gulf of Gdaøsk near Gdynia. But only in the latter sector were bigger concentrations of Goldeneyes observed. The importance of river Adult male ratio mouths for Goldeneye was noticed also in Sweden During all studied winters, adult male ratio of Tufted (Pehrsson 1976). Major concentrations of Goldeneye Duck and Goldeneye was similar. Birds in female in Great Britain were noticed at sewage outfalls on plumage predominated in the majority the sectors in the coast (Campbell 1977, Duncan & Marquiss mild winters. During severe winters adult males 1993). Goldeneye can forage on wastes from sewage were the most numerous group. (Pounder 1976, Campbell & Milne 1977, Barett & High percentage of adult males in severe winters Barett 1985). could be a result of arrival of individuals from frozen There were some plots in the study area, where cooler wintering areas in northern Europe, where bigger concentrations of both Tufted Duck and high rate of adult males was observed (Nilsson 1970, Goldeneye were observed. Those places were well Owen & Dix 1986). Additionally, this effect could protected against waves and wind, and were good be caused by abandonment of the study area by feeding grounds as well. Therefore they were females and young birds. These birds are less resistant attractive by day for both roosting Tufted Ducks and to lack of food and bad weather conditions and could for feeding Goldeneyes. The two species did not leave unsuitable areas faster (Harrison & Hudson compete about food in such places because of different 1964, Bennet & Bolen 1978). Predominance of the daily activity rhythm. Additionally, both species are adult males in the observed flocks of Tufted Ducks specialised in foraging on somewhat different prey. and Goldeneye was also noticed in the Gulf of Both Tufted Duck and Goldeneye forage mainly on Gdaøsk in the severe seasons 1984/1985Ð1986/1987 mussels (Cramp & Simmons 1998). But the Golden- (Michno et al. 1993, Meissner & Klawikowska 1993). eye’s relatively small and smooth gizzard seems to In the present study, in the mild seasons, a higher be specialised in processing small and soft-shelled rate of Goldeneye adult males was observed in plots prey (Suter 1982). In southern Sweden, molluscs that were not protected against wind and waves were the main food of Tufted Duck in the non- (open sea sectors nearW«adys«awowo, along beaches breeding season. The Goldeneye diet consisted of beween Gdynia and Sopot). Females preferred molluscs, crustaceans, polychaets and fish (Nilsson shallower water, less exposed for adverse weather. 1972). Such specialisation can limit the interspecies This preference is a consequence of bigger energy

81 demands, less resistance to low temperatures and pracy terenowej i opracowania danych. Not. Orn. 34:23Ð food shortage, potential differences in efficiency of 30. foraging under suboptimal condition (e.g. deep and Meissner, W. & Klawikowska, M. 1993. Zimowanie gagola (Bucephala clangula) na Zatoce Gdaøskiej w sezonach fast flowing water), and intersexual competition for 1984/1985 Ð 1986/1987. Not. Orn. 34, 1Ð2:103Ð110. food (Sayler & Afton 1981). Michno, B., Meissner, W., Musial, K. & Koza-kiewicz, M. Data from the Swedish coast show that Goldeneye 1993. Zimowanie glowienki (Aythya ferina), czernicy females preferred foraging in shallower water than (Aythya fuligula) i ogorzalki (Aythya marila) na Zatoce males (Nilsson 1969, 1970, 1972). In this context Gdaøskiej w sezonach 1984/85 Ð 1986/1987. Not. Orn. 34, predominance of females and young birds in the 1Ð2:63Ð79. Monval, J.Y. & Pirot, J.Y. (eds.) 1989. Results of the Interna- Gulf of Gdaøsk in mild seasons seems to be caused tional Waterfowl Census 1967Ð1986. IWRB Spec. Publ. No by the big area of the shallow shelf zone (0.5Ð4.0 m). 8. Slimbridge. Nilsson, L. 1969. The migration of Goldeneye in northwest Europe. Wildfowl 20:112Ð118. Acknowledgements Nilsson, L. 1970. Local and Seasonal Variation in Sex-Ratios of Diving Ducks in South Sweden during the Non-Breeding I am especially grateful to all colleagues from WRG Season. Ornis Scand. 1: 115Ð128. KULING who took part in counts. I would like to Nilsson, L. 1972. Habitat selection, food choice, and feeding thank Dr W. Meissner for critical comments on the habits of diving ducks in coastal waters of south Sweden first version of the manuscript. This is paper no 103 during the non-breeding season. Ornis Scand. 3:55Ð78. of the Waterbird Research Group KULING. Osowiecki, A. 1995. Makrofauna denna Zatoki Gdaøskiej latem 1992 roku. Kruk-Dowg-Dowgiallo, L. & Ciszewski, P. (eds) Zatoka Gdaøska. Stan crodowiska 1992 r. Instytut References Ochrony ;rodowiska, Warszawa. Osowiecki, A. 1998. Macrozoobenthos distribution in the Augustyn, M. 1985. Charakterystyka klimatu poludniowego coastal zone of the Gulf of Gdaøsk Ð autumn 1994 and Baltyku. IMGW Gdynia. summer 1995. Oceanological Studies 4:123Ð136. Barett, J. & Barett, C.F. 1985. Wintering Goldeneye in the Osowiecki, A. 2000. Kierunki wieloletnich zmian w struktur- Moray Firth. Scottish Birds 13:241Ð249. ze makrozoobentosu Zatoki Puckiej. Crangon nr 3. Cen- Bennett, J.W. & Bolen, F. 1978. Stress response in wintering trum Biologii Morza PAN, Gdynia. Green-winged Teal. J. Wildl. Manage. 42:81Ð86. Owen, M. & Dix, M. 1986. Sex ratios in some common British Campbell, L.H. 1977. Local variations in the proportion of wintering ducks. Wildfowl 37:104Ð112. adult males in flocks of Goldeneye wintering in the Firth of Pehrsson, O. 1976. Regional, seasonal and annual fluctuations Forth. Wildfowl 28:77Ð80. of the Goldeneye Bucephala clangula (L.) on the Swedish Campbell, L.H. & Milne H. 1977. Goldeneye feeding close to west coast. Ornis Scand. 7:91Ð112. sewer outfalls in winter. Wildfowl 28: 81Ð85. Pounder, B. 1976. Wintering flocks of Goldeneye at sewage Cramp, S. & Simmons, K.E.L. 1998. The complete birds of the outfalls in the Tay estuary. Bird Study 23:121Ð131. Western Paleartic. CD version. Oxford CD. Oxford Univer- Sayler, R.D. & Afton, A.D. 1981. Ecological aspects of sity Press. Common Goldeneyes Bucephala clangula wintering on the Duncan, K. & Marquiss, M. 1993. The sex/age ratio, diving upper Mississipi River. Ornis Scan. 12:99Ð108. behaviour and habitat use of Goldeneye Bucephala clang- Strawiøski, S. 1983. Wintering of waterfowl on the Gulf of ula wintering in northeast Scotland. Wildfowl 44:111Ð120. Gdaøsk. Ornis Fenn. Suppl. 3:66Ð68. Gniotyøski, K. 1988. Pozal™gowe wyst™powanie ptaków wod- Strawiøski S. 1990. Methods of assesing number nych w portach Zatoki Gdaøskiej. Masters’s Thesis, Dept. from a ship based on counts from the Puck Bay. Baltic Birds of Vertebrate Ecology and Zoology, University of Gdaøsk. 5, Proc. of V Conference on the study and conservation of Górski, W. & Strawiøski, S. 1985. Wintering of waterfowl and migratory birds of the Baltic Basin. Riga 1987. gulls on the Polish Baltic coast. Comm. for the Study of Bird Suter, W. 1982. Vergleichende Nahrungsökologie von über- Migr. 17:80Ð98. winternden Tauchenten (Bucephala, Aythya) und Bläßhuhn Harrison, J. G. & Hudson, M. 1964. Some effects of severe (Fulica atra) am Untersee-Ende/Hochrhein (Bodensee). weather on wildfowl in Kent in 1962Ð63. Widfowl 14:26Ð Orn. Beob. 79:225Ð254. 32. Wetlands International 2002. Waterbird Population Estimates Joensen, A.H. 1974. Waterfowl Populations in Denmark 1965Ð Ð Third Edition. Wetland International Global Series No. 1973. Dan. Rev. of Biol. 9:1Ð206 12. Wageningen, The Netherlands. Manikowski, S. 1968. Obserwacje nad wyst™powaniem i rozmieszczeniem ptaków na Baltyku w okolicach Pólwys- pu Helskiego. Acta Orn. 11:45Ð60. Sammanfattning Maøkowski, W. 1975. Sukcesja obrastania przedmiotów pod- Faktorer som bestämmer olika rumslig fördelning wodnych przez rocliny i zwierz™ta w cyklach rocznych. Studia i Mater. Oceanol. 9, Biologia Morza 2. av övervintrande viggar Aythya fuligula och knipor Meissner, W. 1993. Liczenia ptaków wodnych zimujacych na Bucephala clangula i västra delen av Gdaøskbukten, Zatoce Gdaøskiej w sezonach 1984/85 Ð 1986/87 Ð metody Polen

82 Gdanskbukten är ett viktigt övervintringsområde för fåglar sågs alla tio milda vintrarna. De flesta regi- sjöfåglar. De grunda vattnen är attraktiva för både strerades vid Vistulas mynning och i norra hamnen vigg och knipa. De övervintrande bestånden uppgår i Gdansk. Andra viktiga områden var Plutnicas myn- till 1% resp. 1Ð3,5% av hela Europas vinterbestånd. ning, sjön Ptasi Raj samt hamn- och piranläggningar Viggar och knipor har olika dygnsaktivitet under i Gdynia och Sopot. Viggen var särskilt fåtalig runt vintern. Viggarna vilar dagtid i täta flockar på lugna Hel och längs södra kusten. Under de två kalla och skyddade platser men söker föda nattetid på vintrarna, då inre Puckviken var istäckt, fanns ett platser som kan ligga ett gott stycke från nattplatser- fåtal stora koncentrationer med över 600 fåglar i fyra na. Knipor söder föda dagtid i mindre och glesa av sektorerna. flockar på grunda vatten med steniga bottnar. Skill- Kniporna var starkt koncenterade till inre delen av naderna leder till att de två arterna dagtid koncentre- Puckviken under de milda vintrarna, nordväst om en ras till olika kustavsnitt. I denna uppsats försöker jag linje från Gdynia till Jurata. Mer än 100 knipor alla förklara skillnaderna i utbredning och de faktorer tio vintrarna noterades i två sektorer. Under de två som kan spela in. kalla vintrarna uppträdde kniporna på nya platser, Undersökningsområdet omfattar kusten från Vistu- främst mellan Rewa och Sopot och vid Vistulas las mynning till Rozewie udde, ett område som mynning. inkluderar västra delen av Gdanskbukten, Puckvi- Under alla vintrar observerades koncentratione- ken och halvön Hel (Figur 1). Kusten mellan Vistula rna av vigg och knipa nästan alltid på olika platser och Sopot är sandig och låg. Norr om Sopot är kusten (Tabell 2 och 3, Figur 1). Under milda vintrar låg klippig med några floddalar. Hel (37 km) präglas av viggarna främst vid de kustavsnitt där det fanns sandkust mot havet och grunda sandstränder in mot vågbrytare, pirar och kajer samt i flodmynningar. bukten. Det finns ett fåtal anläggningar i området, Kniporna föredrog i stället de grunda vattnen i hamnar med pirar samt några vågbrytare. Puckvikens inre. Även under de stränga vintrarna, då Det material jag analyserar kommer från de ordi- fåglarna uppehöll sig på andra platser än under de narie vinterfågelräkningar av sjöfåglar som utförts milda, låg arternas koncentrationer på olika ställen. av Waterbird Research Group KULING i mitten av Viggarna låg vid avsnitt som var skyddade från januari 1988–1999. Fåglarna räknades från land och vågor och vind, antingen vid pirar och hamnar eller ut till 1000 m från stranden. Viggar och knipor söker vid klippkust, medan kniporna koncentrerades till föda ner till ca fyra meter. I området går linjen för Vistulas mynning och till iskanten utanför Rewa. fyra meters vattendjup inom en kilometer från kus- Andelen adulta hanar och honfärgade fåglar visas ten och i Puckviken följer den ungefär isgränsen i i Figur 2 för de av de nio större områdena som hade Figur 1. Detta innebär att alla fåglar som övervintra- mer än 100 fåglar. Under milda vintrar dominerade de i området räknades. honor och ungfåglar hos båda arterna. Hos viggen På grund av kustens struktur och fågelkoncentra- var det fler adulta hanar än honfärgade fåglar i tre av tionerna delades området upp i 41 sektorer. Räk- åtta områden och hos knipan gällde detsamma för tre ningarna utfördes av 10Ð15 personer under 1Ð2 av sju områden. dagar varje vinter. Det maximala antalet fåglar som Undersökningen bekräftar tydligt den stora skill- räknats varje vinter visas i Tabell 1. De tolv vintrarna nad i platsval som finns mellan vigg och knipa. delades upp i två kategorier, tio milda och två stränga Viggen uppehåller sig främst på platser där de får vintrar (1996 och 1997). De stränga vintrarna var skydd mot vågor och vind under sin dagvila. Tyvärr bl.a. inre delen av Puckviken istäckt. För varje sektor är det inte känt var de uppehöll sig nattetid när de beräknades konstansen i förekomsten som andelen sökte föda, men enstaka observationer kvällstid in- (%) år med mer än 100 fåglar och medeltalet fåglar dikerade att de kunde flyga avsevärda sträckor. under vintrar med mer än 100 fåglar. Resultaten Emellertid är det också möjligt att de sökte föda nära visas i Figur 1, separat för de milda och stränga nattplatserna eftersom det fanns musslor och annan vintrarna. föda på de pirar, vågbrytare, hamnar och pålar som Honor och ungfåglar kan inte skiljas åt. Därför fanns i anslutning till nattplatserna. Eftersom knipo- kan endast relationen mellan adulta hanar och övriga rna söker föda under dagen då räkningarna genom- beräknas. För att få tillräckligt stora stickprov slogs fördes utgör deras uppehållsplatser också deras födo- närliggande sektorer samman till nio större områ- platser. Inre delen av Puckviken var det klart vikti- den, för vilka kvoten adulta hanar beräknades (Figur gaste området. Tidigare undersökningar har visat att 2). de grunda vattnen i detta område hyser en rik botten- För viggen fanns det ingen sektor där mer än 100 fauna. Området är också väl skyddat från de domine-

83 rande västvindarna, något som kan vara positivt även dock andelen adulta hanar högre i ett fåtal sektorer för sikten i vattnet, som inte grumlas när det finns med stora koncentrationer, vilket innebar att andelen skydd mot vågor och vind. En bidragande orsak till i det totala materialet kunde bli hög. Andelen adulta koncentrationen av knipor i Vistulas mynning kan ha hanar var ofta högst i områden som var exponerade varit utsläppen av avloppsvatten, som näringsberi- för vind och vågor medan fåglar i hondräkt domine- kar vattnet och ökat mängden bottendjur. Knipornas rade på grundare vatten i skyddade områden. En fåtalighet i övrigt längs södra Gdanskbukten beror förklaring till skillnaderna i andel adulta hanar kan på att bottnarna här består av rörlig sand som förhin- vara olika tolerans för kalla vintrar genom att de drar riklig förekomst av bottenorganismer. härdigare adulta hanarna drar sig söderut från nord- Andelen adulta hanar var i de flesta sektorer högre ligare områden i Skandinavien sådana vintrar och de än andelen fåglar i honlig dräkt. Vissa vintrar vad honfärgade fåglarna drar sig ytterligare söderut.

84 ORNIS SVECICA 13: 85Ð91, 2003

Foraging and diving patterns of the Great Crested Grebe Podiceps cristatus in a fishpond

MILAN VOGRIN

Abstract Diving and foraging patterns of the Great Crested Grebe Diving success varied significantly between groups, the Podiceps cristatus were studied on Vrbje fishpond in most successful were grebes with young. Adult grebes Central Slovenia. Three pairs and their 10 young were without young caught a higher proportion of large fish than studied. The duration of 348 dives of the six adults before the other groups did. Fish size was negatively correlated hatching, 295 dives of the three pairs of adults with young, with duration of dives in adult grebes without young and in and 185 dives of the immature birds were recorded during adults with young but not in immatures. the breeding season in 1998 (from June through October). Mean dive duration was 31 sec for adults before hatching, Milan Vogrin, Zg. Hajdina 83c, SI-Hajdina 2288, Slovenia 41 sec for adults with young, and 25 sec for immatures. E-mail: [email protected]

Received 28 August 2003, Accepted 2 October 2003, Editor: S. Svensson

Introduction The pond area is 13.5 ha and the average depth is Considerable interest has been shown in the diving about 1.2 m (0.5Ð2 m). Immersed (Typha latifolia, T. and foraging patterns of diving birds (see e.g. Cooper angustifolia) and floating (Potamogeton crispus, P. 1986, Trivelpiece et al. 1986, Burger & Wilson 1988, natans, P. spicatum, Myriophyllum spicatum, Elodea Croxall et al. 1988, Wanless et al. 1991). However, canadensis) vegetation covers between 40% and most of these studies have focused on marine birds, 60% of the surface of the pond (Vogrin 1999). The e.g. penguins and cormorants while data from pond is managed for fish rearing and it is usually freshwater birds, e.g. grebes are more scarce. discharged once a year (autumn/spring) for about 1 The Great Crested Grebe Podiceps cristatus is a to 5 months. Culturing of carp has always been predator that occurs in most freshwaters in Europe. carried out there and has been done with The behaviour and ecology of the Great Crested supplementary feeding. During the whole study Grebe have been studied extensively in several period the conditions of the pond (e.g. vegetation, countries (e.g. Cramp & Simmons 1977, Glutz von fish stock) were the same; however, we should take Blotzheim et al. 1987 and references therein), but the into account that fish also grow up during the study feeding habits and foraging patterns were poorly period as well as vegetation. For more information known at the onset of this study (but see Gwiazda about the study area see Vogrin (1996, 1997). Because 1997 and references therein). of the empty pond during early spring, waterfowl In this paper, I report on the foraging pattern of start to breed in June. adult and immature Great Crested Grebes during the breeding season. I show that swallowing the prey under the water by Great Crested Grebe depends on Methods the experience and breeding status. All observations were made from a car on the banks with most observations being made with naked eye, since during a dive the birds frequently moved out of Study area the field of view of the binoculars. However, Vrbje pond is situated in the Lower Savinja Valley binoculars and telescope were used to estimate the near the town Zalec (aproximatelly 46o14’ 15o09’). size of prey.

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