Brood Parasite Population Increase Facilitates Interspecific Brood Parasitism of Mute Swan (Cygnus olor) by Greylag Goose (Anser anser)

Brood parasitism is a well-known behaviour in wildfowl, however highly territorial species, like geese and swans, are only occasionally subjected to parasitic behaviour. Population increase in areas with low number of suitable breeding territories could facilitate parasitic behaviour. The study presents information about interspecific brood parasitism exhibited by Greylag Goose (Anser anser) toward the Mute Swan (Cygnus olor) during population development in central Poland, in years 1999-2022. Brood parasitism was observed in less than 5% of Mute Swan nests annually. The majority of parasitic eggs hatched successfully but the fledging success was very low (goslings from two broods survived until fledging). The intensity of brood parasitism increased in years with considerable expansion of Greylag Geese. Interspecific brood parasitism as predicted by conditional strategy hypothesis could act as adaptive behaviour in dense populations where vacant territories are scarce and occupied by other wildfowl species.