Winter spacing and non-breeding social system of the Coal Tit Parus atar in a subalpine forest

In the genus Parus, non-breeding social territoriality has often been attributed to the presence of food hoarding which, by increasing the value of an area, is thought to favour the existence of exclusive territories. To test this, it is necessary to analyse the social structure of hoarding populations inhabiting different environments. I studied Coal Tit spacing patterns and winter social systems in a colour-ringed hoarding population inhabiting a mountain coniferous forest in the Pyrenees. Using data on home range, range overlap and social coherence, the social system was found to be based on adult birds that had previously bred together and which occupied relatively small home ranges. Large home ranges of resident juveniles overlapped extensively with each other and with those of adults. However, resident juveniles were resighted much closer to their ringing place than was expected at random, suggesting restricted floater ranges. Associations, composed mainly of juveniles, seemed to be rather large and loose, with little coherence. No territorial defence by any group of birds was detected during the study. Therefore, the social organization described for the present Coal Tit population is similar to the loose, non-territorial basic flock system of non-hoarding species. Resident social patterns seemed to be linked primarily to the availability of breeding resources, probably leading to the high site fidelity of adults and the floater life of resident juveniles. On the other hand, shortterm survival chances may attract transients to a rich food supply and thus affect flocking strategies of birds. High availability of resources allowing a high population density seems to be the main proximate factor influencing the spacing patterns described.