Seabirds are central place foragers during breeding, and the quantity and quality of the surrounding foraging grounds is therefore likely to be a very important factor for parents’ provisioning of offspring. We studied changes in breeding success, diet, feeding rates and chick body condition of Great Cormorants Phalacrocorax carbo sinensis in the old Danish Vorsø colony as it quadrupled in number of breeding pairs and later declined markedly. Some of these parameters were also studied in two neighbouring colonies. We found marked declines in breeding success in the old colony while breeding success remained high in one of the neighbouring colonies. This decline was the result of an increase in the proportion abandoning nests during incubation and in the proportion of chicks dying. Changes in the early phases of breeding may relate to a decline in the Bull Rout Myoxocephalus scorpius stock, which formerly was exploited by the Cormorants during pre-laying and incubation. Increased chick mortality seems to be caused by a decline in feeding frequency of the chicks, the result of longer foraging flights and carry over effects from low food availability early in the season. Estimated foraging density was negatively correlated with chick body condition, particularly when foraging range was assumed to be 20 km rather than 30 km. We conclude that geographically determined access to nearby high-quality foraging areas together with non-cormorant related variation in food availability were the major factors behind colony and inter-annual variation in chick body condition.
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The 8th International Conference on Cormorants, 2011