Individual migration strategies in Cormorants Phalacrocorax carbo passing through or wintering in western France.

The attendance of individually recognisable Cormorants has been studied at a roost site in western France together with the overall use of this roost. Only first and second year birds arrived at the roost before mid-October and almost only 1st-year birds did so after February. In the meantime birds of any age arrived at any time, and the arrival dates of birds of different geographical origin did not vary significantly. A majority of stays (58%) were less than 8 days, and 21% of the stays lasted for more than three months (mean duration 139 days) but there are indications that the true proportion of birds performing long stays, probably elsewhere, must be higher. The mean duration of stays was 27 days, hence the number of birds using the roost during the season was 3.9 to 6.2 times higher than the highest mid-month count. Short-stayers did not tend to return to the area in consecutive years, although part of them remained faithful to their overall migration route. Long-stayers were markedly site-faithful: 48% of the first year birds among them returned at the same wintering site in their second winter; the return rate thereafter was c. 76%, i.e. close to the survival rate of these birds, indicating that site-fidelity was the rule for them. The presented data disagree with an earlier statement that Cormorants are ,nomadic’ outside the breeding season. It is suggested that the strategy of long-staying and site-faithful individuals is of adaptive value.

Yesou P., ARDEA 83 (1): 267-274