The expanding tree-nesting Bonelli´s eagle (Aquila fasciata) population of southern Portugal

Abstract

Bonelli's eagle is considered an umbrella and flagship species in Mediterranean ecosystems. In this thesis, we focus on the fact that the Bonelli’s eagle population in southern Portugal nests in trees – behaviour that sets it apart from the typical cliff-nesting populations that exist elsewhere in this eagle’s range – and on the ecological, demographic and conservation implications that this behaviour has for this population. The study of this phenomenon is key for providing guidelines that will assist in its conservation and in the management of species and/or populations with similar behaviour. It terms of biometry, no differences were observed between tree-nesting Bonelli’s eagles and those from the rest of the Iberian Peninsula and France. There were no gradual size variations with environmental temperature, although there were marked differences of statistical significance in some biometric measures (larger body length, wingspan, head length and width, tarsus width and tail length). These may represent an adaptation to forest environments or be an amplification of some of the original forms present in the small group of founder birds. Individuals in the Bonelli’s eagle tree-nesting populations prey more often on birds than birds that nest on cliffs. Taphonomy studies were conducted for the first time for Bonelli's eagle and showed that tree-nesting eagles of this species prey more often on birds. The way Bonelli's eagles consume and break up Leporidae species and birds’ bones differs from other predators, which allowed us to detect its presence at archaeological sites and discover whether former nesting sites belonged to this or another predator. This tree-nesting population occupies a novel ecosystem containing new combinations of species and landscapes that have emerged thanks to human action, namely, the planting of eucalypt trees. Their territories and nesting areas are generally on steep slopes and may also be related to the presence of adequate trees for nests in areas that are less affected by human disturbance and forest management. Newly colonizing pairs of Bonelli's eagles chose habitats that are structurally similar to those of the initial population nucleus, which may be due to imprinting of the original habitat conditions on young birds. Tree-nesting seems to be advantageous since this ecological novelty has permitted this species to colonize previously unoccupied areas and habitats, thereby obtaining important ecological and conservation benefits.