EFFECTS OF FOREST FRAGMENTATION ON A GUILD OF WINTERING PASSERINES - THE ROLE OF HABITAT SELECTION

This study analyses the winter colonization of an archipelago of 31 forests (0.1-350 ha) in central Spain by the guild of pariforms (Parus, Aegithalos, Regulus, Sitta and Certhia). Two hypotheses are considered (a) that birds with similar habitat preferences tend to disappear simultaneously with the reduction in forest size, leading to a 'nested' pattern of species distribution; or (b) that the species in the smallest forests are a random sample of those found in the larger ones. The results support hypothesis (a). The species that depend on relatively scarce resources, such as tree trunks and junipers Juniperus thurifera (Sitta europaea, Certhia brachydactyla, Parus cristatus and P.ater) only occupied the largest forests. On the other hand, species that exploit abundant, ubiquitous resources, such as helm oak Quercus ilex foliage (Regulus ignicapillus and Parus caeruleus), were distributed uniformly throughout all the fragments. These results emphasize the need for a better understanding of habitat selection by species when designing conservation strategies for fragmented populations.