Factors affecting large-scale distribution of the Bonelli's eagle Aquila fasciata in Spain

We analyzed the environmental determinants of the regional distribution of Bonelli's eagle (Aquila fasciata) in Spain (ca. 500,000 km(2)), taking into account its frequency of occurrence on UTM blocks of 50 x 50 km. We found that the distribution pattern of Bonelli's eagle was a highly predictable phenomenon based on climate, vegetation and interspecific relationships. The proportion of sunny, anticyclonic days, per year (i.e., high levels of solar radiation) was the main environmental predictor explaining the distribution and abundance pattern of the Bonelli's eagle. Sparse plant formations (mainly shrublands) had also a positive effect, while altitude, agricultural land and deciduous forests had a negative influence. The relative abundance of one of its main preys, the Red Partridge (Alectoris rufa), favored the probability of occurrence of Bonelli's eagle, but only in the less sunny areas located in the north of Spain. The relative abundance of its main competitor, the Golden eagle (Aquila chrysaetos), was slightly but positively correlated with both the distribution and the abundance of the Bonelli's eagle in the Spanish portion of the Iberian Peninsula, probably due to similarities in their habitat preferences. Finally, we did not find further regional effects of the variables describing the degree of human pressure (density of roads, urban cover, and length of power lines).