Coherence, Composition and Territories of Winter Social Groups of the Willow Tit Parus montanus and the Crested Tit P. cristatus

Abstract

Among Willow Tits as well as Crested Tits conspecifics united into small, coherent, and territorial social groups (mean size, 2.9 ind for Crested Tit and 4.1 ind for Willow Tit) during the non-breeding season. Members of the same group were not closely genetically related. Throughout the winter each group contained a stable set of individuals. Adults from different breeding pairs rarely participated in the same group, but groups without adults were rare. The groups were formed and their individuals composition largely set by late summer as dispersing first year birds became resident. At settlement, the first year birds spaced among groups and distributed themselves so as to form even pairs. Most likely, the food storing behaviour necessitates the stationary life, and thus may explain the stable content of the groups. Food competition probably accounts for the spacing of conspecifics among flocks. Differential costs and benefits to group members from the social behaviour probably have to be considered to explain group size and dynamics.