Social structure of coal tits Periparus ater in temperate deciduous forest

Analysis of the impacts of social structure on the behaviour and life history of birds is a rapidly developing area of ornithology. Such studies commonly focus on the Paridae (tits and chickadees) as a model group, but detailed assessment of the basic social structure is lacking for many parids, particularly in the non-breeding season. Such baseline information is essential for understanding the organisation of bird populations, including associations between individuals within social and spatial networks. We assessed the non-breeding (autumn-winter) and spring (i.e. breeding season) social structure of Coal Tits Periparus ater in broadleaved forest in southern Britain. Coal Tits were found to be resident and sedentary in this habitat, with birds remaining close to their spring territories during the non-breeding season and associating in small groups averaging 3-4 individuals. Associations were variable, however, with no evidence of stable flock membership, group territoriality or strong pair bonds during autumn-winter. The non-breeding social structure did not change between years of high and low population density, being most similar to a 'basic flock' organisation. This differed from that expected of a food-hoarding parid and contradicted some earlier classifications, highlighting the potential plasticity of social behaviour between bird populations. Such variation can inform the understanding of the evolution of avian social structures and associated behavioural traits, such as food hoarding, and aid the interpretation of intraand inter-specific differences in behaviour.