Habitat utilisation, feeding tactics and age related feeding efficiency in the Caspian Gull Larus cachinnans |
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Authors: | Piotr Skórka and Joanna D Wójcik |
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Institution: | (1) Institute of Nature Conservation, Polish Academy of Sciences, Mickiewicza 33, 31-120 Kraków, Poland;(2) Institute of Systematics and Evolution of Animals, Polish Academy of Sciences, Sławkowska 17, 30-016 Kraków, Poland |
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Abstract: | The feeding behaviour of the Caspian Gull Larus cachinnans was analysed in southern Poland in 2001. During the pre-breeding period, most birds foraged on a refuse dump and some foraged
in a river valley. During incubation, similar numbers of birds foraged on fishponds, gravel pits and the refuse dump. During
the chick-rearing period, fishponds were the most important foraging grounds. The foraging success of three main foraging
tactics was analysed: digging on refuse, fishing and kleptoparasitism. We found that digging success was higher in juveniles
than in immature or adult birds. However, older birds moved and ate more items per unit of time than juveniles, which indicates
that older birds improved their energy gain simply by a higher speed of searching. The opposite was found for fishing success.
As juvenile birds made fewer attempts than immature or adult birds, fishing success was higher in adults. Adults and immature
birds interrupted more attacks than juveniles, which indicates that older birds were better able to assess the probability
of fish catching than juveniles. Kleptoparasitism was observed almost exclusively on the refuse dump during the pre-breeding
period. Young birds kleptoparasitised more frequently than adults, but they had a lower rate of success. However, the lower
success in young birds was due to victim choice, rather than differences in flight skills. Young birds kleptoparasitised Black-headed
Gulls Larus ridibundus and Jackdaws Corvus monedula more frequently than adults, but none of the attacks towards these species was successful. Generally, Caspian Gulls kleptoparasitised
conspecifics more often than expected from species frequency. Only attacks towards conspecifics yielded any success. |
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Keywords: | Optimal foraging Maturation Feeding success Kleptoparasitism |
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