Main page
Knowing about the Team

The scientific research is a fundamental tool to compile information about the biology, the ecology and the demography of Bonelli´s Eagle. In addition to several projects of specific research on this species, there is an annual monitoring of the state of the Catalan population.

The information obtained from the works of investigation has been of great importance to explain the conservation measures for this threatened species. Principal courses of investigation:

 Radioseguiment d'adults (Foto: Albert Tintó)


     Annual monitoring of the Catalan population

At the beginning of the 80s we started to do an annual monitoring of the population of Bonelli´s Eagle in Catalonia, together with other naturalists and administrations. Since 1991 the demographic parameters of a sample of 40-60 territories of eagles are followed annually.

The information gathered in the field is: the occupation of the territories, the valuations of reproduction and of adult mortality. From the obtained information it is elaborated a demographic data base of the Catalan population of Bonelli´s Eagle.

Go to "Investigation"

  Demography of the species

Task: Making of mathematical models to see the principal parameters involved in the demographic maintenance and conservation of diverse Iberian and French populations.

Principal results: The model allows to foresee how a population will evolve considering the principal demographic parameters: adult mortality, pre-adult mortality and productivity. The adult mortality has an effect 4 times higher than the pre-adult mortality and 10 times higher than productivity.

Bibliographical references :  A3, A11, A12, A29, A30, B9, C10, C13, C14, C15, D1, D2, E3, E5 and E10.

Go to "Investigation"

Diet and prey selection

Task: Description of the diet from collection of pellets, prey remains and fieldworks observations. Study of the selection of prey and of the plenty of nourishing resources in different geographical areas of the Catalan population.

Principal results: The rabbit seems to be the ideal prey of the eagles. When this one is scarce, the eagles have to extend his range of preys, which normally consists of species more difficult to capture and of minor weight and energetic value. In the last decades there has been detected a decrease of the presence of the rabbit in the diet of the eagles related to a decrease of this prey in the environment, because of changes of habitat, hunting pressure and diseases.

Bibliographical references : A10, A23, A31, B3, B6, B7 and E7.

Go to "Investigation"

Use of the territory and habitat selection

Task: Marking and radio tracking of adult individuals of different pairs of the regions of Barcelona, with the target to know the extension of their territories, the habitats that they select and the interaction with the human activities.

Principal results: The preliminary information indicates that the eagles concentrate their activity on areas near to the nest, provided that in these areas they realize the reproduction, though they also exploit areas quite far from the nests, in search of their ideal preys. Differences between the pairs exist with regard to the entire exploited area and the areas most used in the territory. The eagles select to hunt Mediterranean open habitats (garrigues and small crop areas of dry land with a human traditional activity), where they find a greater quantity and variety of preys.

Bibliographical references : A25, E11, E16 and E18.

Go to "Investigation"

      Breeding areas

Task: characterization of the breeding areas of the eagles according to demographic, climatologic, topographical, habitat and human activities parameters. Analysis of all these variables to determine the state of conservation of the breeding areas of their territories. Implementation of all this information in a Geographical Information System.

Principal results: the breeding areas of the unoccupied territories are usually more abrupt, they have a colder and humid climate, and the entropic parameters have more influence in the productivity than in the mortality. The entropic factors that affect the productivity more negatively are those related to human activities afoot realized next to the nest, and also the presence and proximity of roads and tracks.

Bibliographical references : D1 and D2.

Go to "Investigation"

Biology of the reproduction and behaviour

Task: Studies on the phenology and the factors that influence the reproductive success. Study of the behaviour of the adults during the reproductive period. Development of the methodology to determine the sex and the age of the chicks.

Principal results: The phenology of putting and the reproductive success of the eagles changes with the climatology, the diet quality and the age of the parents. The eagles that eat more rabbit and have more food lay the eggs earlier and produce more chicks. Non-adult eagles produce fewer chicks than adults. There exists a division of the adult tasks during the reproduction according to the sex. The females incubate and care for the chicks, and the males hunt, take food to the nest and defend the territory. From a young age, the female chicks are bigger than the males. The age of the chicks in the nest can be determined according to the length of the flight feathers.

Bibliographical references : A2, A6, A8, A9, D1 and D2.

Go to "Investigation"

Emancipation, juvenile dispersal and recruiting

Juvenile emancipation

Task: Marking of 5 juveniles with transmitters. Monitoring of their displacements during the 4 - 5 months following the abandonment of the nest.

Principal results: The juveniles leave the nest without being able neither to fly nor to manage by themselves. During the first month they learn the different types of flight, beginning with the simplest (flapping) up to the most complex (soaring). Until the second month the adults continue feeding the young eagles. From the third month it begins the independence period and the young eagles leave the parents territory.

Bibliographical references : A12, C6, C11, E2 and E6.

Juvenile dispersal

Task: Marking of 122 juvenile with rings and feather marks to locate the dispersal areas and to know the selection of the type of habitat.

Principal results: The dispersal areas where the juveniles concentrate have been located; these are territories where there are no adults (with adult absence), of soft relief and rich in preys. The young eagles that are born in Catalonia can fulfil two types of displacement: of short distance, principally in the areas of dry land of the central depression of Lleida, and of long distance, in different points placed out of Catalonia, such as the Sierra de Escalona (Alicante), Campos de Montiel (Albacete), region of Jerez and Laguna de la Janda (Cadiz), Valle del Tajo (Toledo) and other zones of the centre of the Iberian Peninsula and Extremadura. The habitats selected by the juveniles are dry farming areas that alternate with bushy areas that have a great plenty of rabbit and partridge. During this period it has been detected a high mortality caused by electrocution with electrical lines and direct persecution (shooting).

Bibliographical references : A13, A18, C18 and E4.


Task: Monitoring of the recruiting of the juveniles marked in the population of origin or in other areas.

Principal results: Non-adult individuals visit nesting areas and replace the casualties of adults in those territories with unpaired individuals. There is a certain tendency to return to the natal territories. During the dispersal period there is a high mortality, which bears a low valuation of recruiting.

Bibliographical references : A24, D1 and D2.

Go to "Investigation"


Comprovació de possible infecció per trichomoniasi (Foto: Santi Mañosa)

Mortality of the chicks for trichomoniasis

Task: Study of the trichomoniasis in chicks of Bonelli's Eagle and of the ecological factors involved in its spread.

Principal results: Trichomonas gallinae is a protozoan of which several birds are carriers. Despite the high occurrence of trichomonas in chicks of Bonelli´s Eagle, only a small percentage develops the disease and consequently dies during the nestling period. In the case of the eagles, the infection is mainly caused by the ingestion of doves, which are increasingly present in the diet of the eagles. This increase in the consumption of doves is related to their availability in comparison with other wild prey populations (decrease of the optimal preys) and to the incorporation of young eagles (non-experienced parents) to the territories. The disease seems to be more and more frequent in several populations of eagles such as the French, the Portuguese and some in the Spanish state.

Bibliographical references : A15.

Go to "Investigation"

Àliga electrocutada (Foto: Joan Real)

Causes of mortality in Spain

Task: Study of the causes of mortality throughout the decade of the 90s in the whole Spanish state.

Principal results: The principal causes of adult and pre-adult mortality are the electrocution in power lines and the direct persecution. The causes of mortality change according to the regions, the nesting or dispersal areas, and the age of the eagles. It stands out the high adult mortality for electrocution in Catalonia, which is related to the increase of dangerous power lines during the decade of the 90s and the substitution of (safe) wood supports for very dangerous metal supports. It is necessary to notice that in certain areas the persecution of the eagles for hunting reasons still continues.

Bibliographical references : A17 and A26.

Go to "Investigation"