In 1996, following a relatively severe and prolonged winter, Great Cormorants started to breed at the Kąty Rybackie colony (NE Poland) one month later than in 1995 but breeding finished at the same time in both years. The estimated total food consumption of the Cormorants was lower during the shorter and more synchronised 1996 breeding season (737 tonnes) than in 1995 (805 tonnes) despite the larger population present in 1996 (5929 pairs) than in 1995 (4942). However, during the period of peak energy need in June the estimated total daily food consumption of Cormorants present in the colony was about 2 tonnes higher in 1996. In June 1996, after a couple of windy days, 24.3% of chicks died and the total fledging success was lower (2.19 fledglings/nest) than in 1995 (2.45). The observed mass chick mortality could be due to the combined effect of strong breeding synchronisation, decreased food availability, and increased costs of foraging due to strong winds. Large breeding colonies of Cormorants can only function successfully when the suitable breeding period is prolonged and breeding can start early. Long-term climate change due to global warming could have favoured the observed Cormorant population increase during the last decades and its expansion into NE Europe. Asynchrony could be adaptive towards alleviating the food requirements of both individual broods and the whole colony.
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