We studied vigilance while sleeping in wintering Gadwalls (Anas strepera) in the Camargue (southern France) during the 1992–1993 and 1994–1995 winters. Vigilance while sleeping was reduced at the beginning and at the end of the winter. During mid-winter, birds in sleeping posture showed the highest level of vigilance, with longer peeks and shorter interpeek intervals. There was no effect of sex on the intensity of vigilant sleep, nor on changes in the pattern of vigilant sleep across the winter period. We discuss the results in relation to the behavioral ecology and energetic constraints of wintering Gadwalls in the Camargue. We suggest that the variation in sleep, measured by eye-closure, and vigilance, measured by eye-opening, in relation to winter period is a result of a trade-off between two opposite requirements: to save energy by sleeping and to monitor the environment by being vigilant.
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