LENSINK R (2022) Variation in communal roosting of Great Cormorants Phalacrocorax carbo. LIMOSA 95 (4): 178-187.

Great Cormorants use communal roosts to spend the night. These roosts differ in size and when they are used. Relatively large roosts (maximum numbers >100 birds) are used year-round whereas smaller roosts are sometimes empty during the breeding period. Numbers peak in the non-breeding season, with peaks in early autumn (September), late autumn (October/November) and/or midwinter. These peaks presumably reflect different stages of the annual cycle such as breeding birds (and their young) leaving their breeding colonies (peak in September) and migration passage of northern breeding birds (peak in October/November). Interestingly, particular roosts are used most intensively during September, others are used most intensively during October/November, and others again are used most intensively during the winter. Differences in how roosts are used are more or less consistent between years. Changes in the habitat and weather can affect roosting behaviour. For example, birds temporarily leave the roost when their feeding habitat is covered by ice. Also one breeding colony was monitored. This colony was used for roosting only part of the year. Most importantly, it was not used after September. Breeding birds returned to the colony as early as December. Cormorants prefer a specific type of tree with space in the crown (Populus sp.). During winter, up to 20% of the birds arrive on the roost after sunset.

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