|
Biology and Genetics
Morphology
Bonelli's eagles are middle-sized raptors. The adults show a mostly dark underwing pattern, predominantly white underbody and generally dark brown upper parts with a white dorsal patch. A black sub-terminal band on the tail is also prominent. Regarding measurements, these range from 1.5 and 1.8 m for wingspan, from 65 to 72 cm for length and from 1500 to 2400 g for weight. Nonetheless, birds in Southern Portugal are suspected to be slightly larger and heavier than average.
Young individuals have a very distinct plumage from adults, mostly ochre to cinnamon brown with dark flight feathers. After their first year, they gradually moult into adult plumage, which becomes complete only in their 4th or 5th year. Females are larger than males, averaging nearly 20 cm more in wingspan.
This species may be confused with other eagles with mainly white underbody, like Booted or Short-toed eagles, but then the mostly dark underwing pattern is diagnostic of Bonelli's eagle. Also, this species frequently shows a faster and more agile flight action.
Breeding
Bonelli's eagles are one of the earliest breeding raptors in the Mediterranean region. In Southern Portugal, eggs can be laid in as early as the beginning of January. This precocious phenology pattern allows fledging to occur when food availability is at its peak, in April - May, when most potential prey species are actively breeding. It also allows successful second clutches, which replace previously failed incubations.
Clutch size is frequently of 2, less frequently 1 and seldom 3 eggs. This depends on the quality and abundance of prey, individual fertility and on weather conditions preceding the breeding season, which affects hunting success and, subsequently, the female physiological state. High precipitation during long periods before the breeding season reduces fertility, as well as continuous precipitation during the incubation phase may force females to interrupt it for too long, and therefore cause embryo death. Incubation lasts 39 days on average, and the development of nestlings takes approximately 65 days, until they finally fledge and the emancipation phase begins. Fledglings will then take as long as 5 months to definitively leave the proximity of the nest and of the parental territory and to initiate the dispersion phase (in September - October).
Territoriality
Breeding populations of this species are structured in monogamous and relatively stable pairs. These pairs maintain extensive territories (10 000 to 20 000 ha in the Southwest uplands; mean size ca. 14 000 ha) where conspecific eagles are generally kept away. This highly territorial behaviour also extends to other raptors, and the diversity and abundance of birds of prey within the Bonelli's eagle core distribution areas is reduced.
Demography and Genetics
Southern Portugal Bonelli's eagle population has been growing and expanding continuously during the last three to four decades, departing from a small number of pairs of the Southwest uplands and from the Southeast plains. That growth was enabled by a human exodus started in the 1960's, which increased the area available for the eagles. This scenario is backed up by the detection of a low genetic variability using polymorphic microsatellites, resulting from a "founder effect". This means that a limited number of individuals must have originated this population. After that, new eagle territories filled the existing gaps and occupied the periphery of the original nuclei. Eventually, the available habitat got saturated and eagles born within these source areas started being recruited as far as 150 km away.
This peculiar population is also characterized by marked genetic differentiation compared to its neighbouring populations. This reflects a small amount of gene flow between populations, i.e. breeding isolation, possibly the result of different nesting behaviour (use of trees vs. cliffs.) being imprinted to the individuals of each population. Therefore, mating probability between eagles of different origins might decrease significantly just by their choice of nesting features.
Since it is ecologically, behaviourally and genetically differentiated, the Southern Portugal Bonelli's eagle population must be treated as a particular management unit in terms of conservation.
|
