Nest and brood stage association between ducks and small colonial gulls in boreal wetlands

Abstract

Heterospecific grouping is often associated with reduced predation. One example of this phenomenon is birds breeding in association with more aggressive species. Here we report a study of the association between breeding ducks and small colonial gulls during the nesting and brood-rearing periods in boreal wetlands in Finland. Dabbling duck nests were rarely found within gull colonies, while Tufted Duck (Aythya fuligula) and Pochard (Aythya ferina) nests were exclusively found within gull colonies.During the brood stage, the broods of most duck species were found more often than expected within the colony areas of small gulls, such as the Black-headed Gull (Chroicocephalus ridibundus) and Little Gull (Hydrocoleus minutus). Dabbling duck broods in particular were associated with gulls. In diving ducks, Tufted Duck broods preferred gull defence areas, whereas Common Goldeneye (Bucephala clangula) broods did not. We suggest that colonies of small gulls may be much more important for ducks than previously thought. In recent years, Black-headed Gull populations have decreased in Finland, and the populations of Tufted Duck and Pochard have concurrently decreased.We recommend that the nesting site requirements of small colonial gulls should be taken into account in wetland restoration and when building new wetlands.