|
|
|
Conservation
Threats
Bonelli's eagles are subjected to a diverse set of factors which threaten the stability of their populations, the most important of which are referred to in the following text.
Shooting
Direct persecution has many causes and affects mainly juveniles and immatures. One of the potential conflicts is related with hunting activities, since Bonelli's eagles often prey on small game. Another potential conflict has to do with the consumption of domestic prey (mainly pigeons and fowl). From 1990 to 1999, 70% of the known mortality cases regarding adult birds in Portugal were caused by shooting.
However, at least in the Southwest, the fact that adult mortality occurs only out of the breeding season, and also that male mortality largely outnumbers female mortality, leads to the conclusion that casual shooting during hunting, instead of intentional persecution, must be taking place.
Disturbance by human activities
The disturbance caused by forestry activities is the main cause of nest desertion during the critical periods of nest occupation and incubation. These activities include scrub clearing, timber harvesting, tree planting and the opening of firebreaks and dirt roads.
Human disturbance resulting from hunting activities is another potential cause of nest desertion and clutch failure. The most negative impacts are often caused by big game drives (mostly targeting at Wild boar Sus scrofa) which take place during the incubation period, potentially leading to clutch abandonment.
Habitat degradation
The main threats to Bonelli's eagle' habitats in Southern Portugal are: a general degradation of tree cover (whether cork-oaks, pines or eucalyptus) due to climate change, pests, forest fires, and intensive or inadequate forestry practices; the risk of felling nest-trees; the building of infra-structures like dams, roads and potential obstacles to flight, namely power-lines and wind farms. These factors can often be managed at a local or regional level. The lack of actions in this regard can lead to the reduction of the breeding potential of this population in the mid-long term. Factors like forest degradation, the felling of nest- trees and forest fires, acting along with the disturbance of nest-sites by human activities, subsequently lead to an increase of nest-site shifts, pushing the birds to use less stable, lower quality nest-sites. In turn, this favours the probability of the partial collapse of weak nest structures. These events have been responsible for as much as 20 and 13% of the total loss of eggs and nestlings respectively.
Food shortage
Bonelli's eagles base their diet in wild and domestic medium-sized birds and mammals. Where none of the staple items is abundant, the eagles prey range is more diversified, but nevertheless the territory's breeding productivity has been recorded to drop. Food shortage is associated with habitat degradation (e.g. where natural habitats gave place to intensive eucalyptus plantations), overhunting, epizooties and also with the loss of habitat diversity and food resources resulting from rural abandonment.
Juvenile mortality due to disease
Several pathologies might be at the origin of juvenile mortality. However, trichomonose induced by the flagellate protozoan Trichomonas gallinae is one of the main causes of nestling mortality, causing lesions in the upper digestive tract which can block food ingestion and ultimately starve young birds. Domestic pigeons are the main vector of the disease and at same time one of the topmost important diet items for many Bonelli's eagle pairs. The annual nestling mortality caused by this disease reaches 38% of the average annual nestling mortality, being the main cause for nestling loss in the Southwest population.
Power line and windfarm mortality
The vast network of medium and high voltage power-lines in South Portugal increases the risk of death by collision with cables, or by electrocution for a large sized bird such as Bonelli's eagles, especially within pre-adult settlement areas.
Wind farms are also dangerous flight obstacles, and pose an increasing great threat since there are several wind farms planned within Bonelli's eagle breeding areas.
Knowlegde gaps
Present knowledge gaps may have direct implications in the conservation of Bonelli's eagles, especially regarding adult mortality causes, population dynamics and trichomonose epidemiology.
Lack of public awareness
Bonelli's eagles are not well-known to most of the general public as a consequence of their secretive behaviour. Nevertheless, their conservation (as well as of most raptors) often depends on public awareness enhancement.
Local community members in the vicinity of the eagle territories, promoters, investors, politicians, hunting managers, land owners, hunters and other stakeholders should recognize the species, its ecological significance and the threats it is subject to. They must actively participate in its conservation, in order to attain the best possible results, and effectively preserve this species and its habitat.
|
|
|
Ecology
Breeding habitats
In Portugal, Bonelli's eagles breed in several habitat types. In the North they use exclusively rocky substrates, mostly in cliffs hanging over steep river canyons, but sometimes also on rocky crests topping mountainous areas. These situations are similar to what is commonly found along the Mediterranean basin. This type of habitat is also chosen throughout the international section of the Tagus river, however in this region tree nesting also occurs (2 pairs nesting on trees; 2 pairs both on rocks and trees). In the Estremadura (Central-West coastal belt), some pairs still occur in rocky habitats while most use dense woods for breeding. In the South, however, most nests are set on trees, and this type of substrate is the only one used in the uplands of the Southwest and in the Southeast plains. Nests are built on tall trees, which dominate relatively dense forest stands, otherwise in smaller forest patches or even on isolated trees. Trees used may be Cork oaks Quercus suber, Maritime pines Pinus pinaster, Monterey pines Pinus radiata or Eucalyptus Eucalyptus globulus and E. rostrata. Nests built on eucalyptus are at the greatest heights, mostly between 25 and 28 m above ground.
Diet
Bonelli's eagles feed on a wide variety of middle and small-sized prey, mostly birds. Nevertheless, their diet is considerably based in a few abundant middle-sized staple items. In the low mountains of the Southwest, 82.3% of the diet is made of only 4 species: first of all, domestic pigeons Columba livia var. domestica (39.2%), followed by European rabbits Oryctolagus cuniculus (18,4%), Red-legged partridges Alectoris rufa (17.2%) and Jays Garrulus glandarius (7,5%). The remaining 17.7% are shared by species as small as Goldfinches Carduelis carduelis and as large as Grey Herons Ardea cinerea. The importance of domestic pigeons raised in rural lofts in the eagle's diet is noteworthy, and compensates for the relative scarcity of traditional wild prey, namely rabbits and partridges. In territories where most usual prey is scarce e.g. where a high percentage of the area is occupied by poor habitats such as eucalyptus plantations, the prey range is consequently widened as a response.
|
|
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.
|
|
|
Distribution and population
Bonelli's eagles are distributed along most of Southern Portugal, while in the North and Central parts of the country their range is largely restricted to the international border regions of the Douro and Tagus rivers. Dense clusters exist in the Northeast, as well as in the low mountainous areas of the Southwest and in the cereal pseudo-steppes of the Southeast. These Southern nuclei hold ca. 70% of the Portuguese population of this species, estimated to be around 100 breeding pairs. Within the remaining distribution area - coastal limestone formations of the Portuguese Estremadura, the international course of the Tagus, the middle stretches of the Tagus, Sado and Guadiana river basins - the species occurs in small clusters or shows a more scattered pattern.
Settlement areas
Before reaching sexual maturity at its 3rd year of age, Bonelli's eagles go through a dispersive phase, when they use areas free of breeding territories and with abundant, most often in open or sparsely forested terrain. These are known as settlement areas.
The main pre-adult settlement areas in Portugal are located in the Southern plains, above all in the same region which includes the Southeast pseudo-steppe breeding territories. Studies developed in those settlement grounds reveal low mortality of non-breeding birds, probably due to low density of power lines, infrequent human persecution and high prey availability within Special Hunting Regime areas of that region. Most individuals born in Southern Portugal use this region before reaching sexual maturity.
|
|
|
|
|
|
|
 |
|
