SEASONAL AND INTERANNUAL OCCUPATION OF A FOREST ARCHIPELAGO BY INSECTIVOROUS PASSERINES

The distribution of insectivorous passerines was studied in 31 fragmen ts (0.1 to 350 ha) of a helm oak (Quercus ilex) Mediterranean forest a rchipelago in central Spain for two consecutive springs and winters (1 988 to 1990). The study aimed to assess the role of forest size, abund ance and other biological traits (nesting site and foraging substrata) of the species in their ability to occupy the fragments. The number o f fragments occupied by the species (f(i)) was correlated to their abu ndance (d(i)) in spring and winter. Spring-to-winter changes of di cor related positively to changes off, due to the incidence of summer migr ants but failed to show any significant pattern when restricted to spe cies occurring in both seasons. The between-winter variations of cl, c orrelated positively with changes in f, while the between-spring chang es of both parameters did not correlate, probably because of stronger effects of site fidelity on bird distribution in this period. Between- year persistence in the occupation of fragments was directly related t o bird species abundance and, for a given abundance, persistence decre ased in the smallest fragments. Hole-nesting species during the 1988 s pring and the exploiters of scarce trees or foraging substrata during both winters showed an apparent restriction to colonize the smaller fr agments due in parr to changes in habitat structure related to fragmen tation. These results support the hypothesis that abundance is a good predictor of the ability of species to occupy and persist in fragments . However, abundance alone does not fully explain the processes that d etermine these occupancy patterns, since other traits related to the s patial scale of analysis and the suitability of fragments for each bir d species are also involved.