The timing and synchronization of egg-laying in Great Cormorants breeding in coastal and inland colonies

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Great Cormorants Phalacrocorax carbo breed along coasts and inland in Japan where they feed on local coastal and freshwater fish. To examine factors affecting their timing of egg-laying and clutch size, their breeding ecology was studied at 4-6 colonies for 2-3 years in Aichi, Gifu and Mie prefectures in central-west Honshu, Japan. The mean date of egg-laying ranged over about two months (from March to May) among colonies and years. In the coastal colonies more than 20% of individuals laid eggs within one or two weeks and more than 40% of birds at most began to breed simultaneously, while such synchronization of egg-laying was rarely observed in inland colonies. The timing of egg-laying was not related to either rainfall or air temperature during winter. Clutch size differed neither between inland and coastal colonies, nor between years. We suggest that differences in food conditions around coastal and inland colonies influenced the degree of synchronization of egg-laying.
日本鳥学会の論文