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Waterfowl (Anatidae) are a group of birds in which parental care is performed by both parents (a pair) or, less often, by females only. Alternative social systems in this group have been recorded only occasionally. This paper describes three unusual cases of prolonged association between a pair and an additional male (the offspring or sibling of the primary pair) in breeding Whooper Swans (Cygnus cygnus). In all instances the three birds displayed no antagonistic behaviour towards family members, participated in territorial defence, and bred successfully. These cases are unusual in light of what is currently known about the social system of the highly territorial Whooper Swan in particular and of Anatidae in general, and are typical of cooperative breeding. This shows that swans may occasionally form an alternative social system.

Prolonged association between a pair and a related male in breeding Whooper Swans (Cygnus cygnus)