FACTORS INVOLVED IN THE DISTRIBUTION OF FOREST BIRDS IN THE IBERIAN PENINSULA

The relationships between a series of environmental variables and the abundance of individual species of forest birds were examined in 58 la rge woodlands along a band crossing the Iberian Peninsula from north t o south. The variables were mean annual rainfall, mean annual temperat ure, density of trees <20 cm in diameter, density of trees > 30 cm in diameter, floristic composition (conifer or broadleaved trees) and geo graphical situation (distance from northern extreme of study band). Th e variable most highly correlated with bird density was mean annual ra infall (40% of the species had significant partial correlations with t his parameter). The northern species (Chiffchaff, Goldcrest, Robin, So ng Thrush, Bullfinch, etc.) generally correlated positively with mean annual rainfall, while the Mediterranean species (Sardinian Warbler, B onelli's Warbler, Black-eared Wheatear, Rock Sparrow, Spotless Starlin g, Azurewinged Magpie, etc.) exhibited an inverse pattern. Mean annual temperature (36%) was also closely associated with bird distribution, but did not show clear tendencies according to biogeographic groups. Floristic composition (26%) was a strong determinant of the abundance of species distributed throughout the whole Iberian Peninsula and geog raphical situation (22%) largely determined the distribution of the no rthern and Mediterranean species. Finally, density of trees <20 cm in diameter (20%) and <30 cm in diameter (14%) generally showed a negativ e association with the majority of the species, particularly the Medit erranean species.