Ardea
Official journal of the Netherlands Ornithologists' Union

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Van Eerden M.R. & Munsterman M.J. (1995) Sex and age dependent distribution in wintering Cormorants Phalacrocorax carbo sinensis in western Europe. ARDEA 83 (1): 285-297
The winter distribution of the Cormorant shows a clear example of partial migration, some birds staying close to the colonies whereas others migrate some 2500 km south. Counts on the wintering area gave an estimate of 165 000 - 210000 birds compared to about 180000 as derived from the breeding census and young production during summer (1990). Countries taking up many Cormorants were France (18%), Spain (15%) and Tunisia (15%). The demographic pattern of the winter distribution of the Cormorant shows that adult males stay closest to the breeding grounds whereas juvenile females migrate furthest to the south in the Mediterranean. Scans on roosts throughout the wintering range in western Europe combined with the numbers present indicate an overall male surplus, being most prominent in adults (1 male:0.66 female for adults and 1:0.86 for juveniles). All three currently available hypotheses (body size, social dominance and arrival time) apply to the system of differential migration as observed in this species. It is argued that the unequal sex ratio may reflect different mortality rates in relation to the cost of migration, being highest in females and juveniles.


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