Fifteen wintering roosts of Cormorants have been found in southwestern France (Atlantic coast) and northern Spain (provinces of Navarra, Guipúzcoa and Alava). The largest roosting population was observed in February (2373 birds). A second peak was observed in October. The population age structure changes significantly along the year: adults predominate in winter (82.4% in March) and yearlings in spring (47.0% in May and June). A different distribution by age has been found depending on roost location: in September more yearlings are found in coastal (30.3%) than in inland (16.4%) roosts, but not during the other wintering months. Such a finding could be attributed to the fact that the studied area is a migratory stage towards the south of the Iberian Peninsula. The proportion of adult birds is higher in coastal than in inland roosts in October (76.5% and 67.8% respectively), February (81.6% and 73.1%) and March (85.0% and 79.7%). Colour-ringed birds wintering in the studied area came from France (40.0%), British Isles (14.3%) and Denmark, the Netherlands and Poland (45.7% together). The carbo subspecies was found exclusively in the coastal roosts while the sinensis subspecies used both coastal and inland roosts.
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