An indication of long-term group formation in Tufted Duck Aythya fuligula

Abstract

Many waterbird species assemble in large aggregations during winter. In most species, these aggregations are generally thought to be composed of individuals not familiar to each other. In this study, we analysed mark-recapture data of Tufted Ducks Aythya fuligula and Common Pochard A. ferina from four wintering places in Switzerland to assess whether individual birds were recaptured non-independently of each other one or more years later. We found that individual Tufted Ducks caught together during a previous winter were re-caught together more often than expected by chance. Our results suggest that winter flocks of Tufted Ducks may be socially structured to an extent unknown so far. We discuss a number of hypotheses that could explain our observations, but clearly, much more work needs to be done to confirm our observation and to understand its causes.