Anna Buczma, Michał Goc, Wojciech Kosmalski
Ornis Polonica 2011, 52: 231–246
https://doi.org/10.12657/ornis.2011.4.0
Abstract: Breeding phenology of the Great Cormorant was studied between 2003 and 2011 in a colony in Kąty Rybackie. On three study plots, post-hatching eggshells were collected once a week, allowing for estimation of both the onset and the progress of hatching. The beginning of hatching between the earliest (2008) and the latest (2006) breeding seasons differed by about one month. The onset of breeding (earliest broods) varied among studied plots within the colony, and, at the same time, the sequence in which Cormorants started to breed was repeatable. The earliest breeding was always found in the oldest part of the colony, while the latest – at the plot where Cormorants and Grey Herons Ardea cinerea co-occur. These differences were smaller in seasons, when breeding was later overall. Both the onset and median dates of breeding were significantly correlated with the dates of melting of the ice cover from the Vistula Lagoon and this relationship was less marked in warmer seasons, when ice cover melted earlier. This confirms the importance of Vistula Lagoon as the basic foraging area for Cormorants early in the season. At the same time, it may explain why parts of the colony located close to the lagoon (the oldest parts of the colony) are the most attractive for birds.
