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 large woodlands along a band crossing the Iberian Peninsula from north to south. The variables were mean annual rainfall, mean annual temperature, density of trees <20 cm in diameter, density of trees > 30 cm in diameter, floristic composition (conifer or broadleaved trees) and geographical situation (distance from northern extreme of study band). The variable most highly correlated with bird density was mean annual rainfall (40% of the species had significant partial correlations with this parameter). The northern species (Chiffchaff, Goldcrest, Robin, Song Thrush, Bullfinch, etc.) generally correlated positively with mean annual rainfall, while the Mediterranean species (Sardinian Warbler, Bonelli's Warbler, Black-eared Wheatear, Rock Sparrow, Spotless Starling, 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 geographical situation (22%) largely determined the distribution of the northern and Mediterranean species. Finally, density of trees <20 cm in diameter (20%) and <30 cm in diameter (14%) generally showed a negative association with the majority of the species, particularly the Mediterranean species.