Seasonal and Interannual Occupation of a Forest Archipelago by Insectivorous Passerines

The distribution of insectivorous passerines was studied in 31 fragments (0.1 to 350 ha) of a holm oak (Quercus ilex) Mediterranean forest archipelago in central Spain for two consecutive springs and winters (1988 to 1990). The study aimed to assess the role of forest size, abundance and other biological traits (nesting site and foraging substrata) of the species in their ability to occupy the fragments. The number of fragments occupied by the species(f i ) was correlated to their abundance (d i ) in spring and winter. Spring-to-winter changes of d i correlated positively to changes of f i due to the incidence of summer migrants but failed to show any significant pattern when restricted to species occurring in both seasons. The between-winter variations of d i correlated positively with changes in f i, while the between-spring changes 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 to bird species abundance and, for a given abundance, persistence decreased in the smallest fragments. Hole-nesting species during the 1988 spring and the exploiters of scarce trees or foraging substrata during both winters showed an apparent restriction to colonize the smaller fragments due in part to changes in habitat structure related to fragmentation. 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 determine these occupancy patterns, since other traits related to the spatial scale of analysis and the suitability of fragments for each bird species are also involved.