Interview with Luis Palma about the biology and conservation of the Bonelli’s Eagle in Portugal

Date of the interview: 1 November 2009

The Bonelli’s Eagle is one of the most endangered raptors in Europe. In this interview, Luis Palma talks about the current situation of this species in Portugal.

Luis Palma

Luis Palma

Markus Jais: What is the current status of the Bonelli’s Eagle in Portugal in 2009 (number of breeding pairs, numbers of young fledged)?
Luis Palma: The global population totals 104 known pairs, plus a few more so far unconfirmed. The species is steady to slightly declining in the Northeast, steady in the Central East and fast growing and expanding in the South. Recent data on productivity is available only for the Southwest and the Southeast where in 2009 it was of 0.78 and 1.3 young fledged per territorial pair respectively.

Markus Jais: In what areas in Portugal do Bonelli’s Eagles nest? What habitat do they occupy?
Luis Palma: The breeding population occupies the Northeast (Upper Douro river basin – 24 pairs) and the Central East (Upper Tejo basin area – 15 pairs) mostly along the Spanish border; the Southwest low mountainous area (35 pairs) and the Southeast lowlands (13 pairs); the Western hilly country around Lisbon (5 pairs); and the lowlands of the Northern half of Alentejo from the Tejo river southwards to the town of Beja (12 pairs). Apart from the latter, where distribution is scattered and discontinuous, all other breeding clusters are continuous. In the Northeast and Central East they mostly occupy river canyons with steep rock cliffs and sparse vegetation; in the Southwest the habitat is Cork-oak woodland and scrubland with grazing and agriculture clearings; in the Southeast lowlands the habitat is steppe-like cereal and grazing land with low scrub patches; North of Lisbon the species occupies remnant patches of wooded and extensive farmland habitat amidst highly urbanized areas. Finally, in the lowlands of Northern Alentejo, the predominant habitat is open Holm-oak woodland crossed by small river valleys, with 3 pairs nesting in pinewoods bordering large estuaries.

Markus Jais: In Portugal, some Bonelli’s Eagles nest in trees. How many pairs of the whole population do nest in trees?
Luis Palma: Sixty seven tree-nesting pairs within the whole population (64%) while in the South 61 out of the 65 pairs (94%) are tree-nesters.
Trees used are tall emergent Cork-oaks (Quercus suber), tall Maritime and Monterey pines (Pinus pinaster and P.radiata), and large eucalyptus (Eucalyptus globulus and E. camaldulensis).

Bonelli's Eagle in tree nest in Portugal

Bonelli’s Eagle in tree nest in Portugal, © Hugh Jansman

Markus Jais: Is the breeding success in trees higher or lower than on cliffs? Is there a difference between tree species?
Luis Palma: Breeding success varies from area to area and seems independent of the nest type or the tree species used. It rather depends on prey availability in each area.

Markus Jais: How large are the territories of Bonelli’s Eagles in Portugal?
Luis Palma: The average home-ranges of 8 adults (5 females and 3 males) fitted with GPS/PTTs in the Southwest uplands are the following:

Kernel 95% – females: 8191.6 ha / males: 15272 ha
MCP (Minimum Convex Polygon) – females: 6610.2 ha / males: 12487.7 ha

Markus Jais: Bonelli’s Eagles can disperse over large distances as juvenile birds. Is the dispersal for Bonelli’s Eagles in Portugal known? Where do the birds settle to breed after dispersal?
Luis Palma: The dispersal patterns are known from 25 radiotracked and 2 GPS/PTT fitted juveniles. The main settling area is in the Southeast steppe-like lowlands. This area is also the home to 13 breeding pairs and lays adjacent to the Southwest uplands where 35 pairs breed.

Makus Jais: The population in southern Portugal shows a genetic differentiation compared to its neighbouring populations. Why is that? Is there an exchange with populations from other areas in Portugal and in Spain?
Luis Palma: Apparently, gene flow between this and neighbouring populations is very low and exchange seems rare. One possible explanation is that different nesting habits act as a behavioural isolation mechanism.

Markus Jais: What is the main food for Bonelli’s Eagles in Portugal? Is there enough food, for example, rabbits?
Luis Palma: Main prey species are the domestic pigeon, Wild rabbit and Red-legged partridge, and in some areas (open lowlands) also juvenile Hares. Apart from these, there is a large number of minor prey species, mainly birds, among which the Jay is the most common prey.

Markus Jais: What are the main threats to Bonelli’s Eagles in Portugal?
Luis Palma: There is apparently no active persecution of Bonelli’s eagles with the possible exception of some deliberate shooting of adults in the Northeast. More often, shooting is incidental during the hunting season although at a low rate. In the Southwest adult mortality affects 8% of the breeding population annually, more often males than females, which comes in support of the incidental character of shooting. Moreover, due to tree-nesting behaviour of the Southern population, forestry activities are the main source of threats, both in the form of the logging of some large trees but much more often as disturbance to reproduction.

Markus Jais: In Spain, electrocution is among the most serious threats to the Bonelli’s Eagle. How is the situation in Portugal? Are power lines made safe to protect the birds?
Luis Palma: In general, high tension power lines have recently been designed in a way to avoid serious impacts on breeding sites and those as well as many older ones have been fitted with BFDs (Bird Flight Diverter). Many lower tension lines in perilous locations have also been pylon-corrected and equipped with BFDs in recent years. Nevertheless, a few individuals have been found electrocuted.

Markus Jais: Is there competition between Bonelli’s Eagles and other species like Golden Eagles or Eagle Owls?
Luis Palma: Presumed competition with Golden eagles is known only from the Northeast population. There is no data on interaction with Eagle owls.

Markus Jais: There is a conservation project for the tree nesting Bonelli’s Eagle in Portugal. What is the goal of the project and what actions are taken?
Luis Palma: The project’s rationale is to act primarily on the factors that may negatively impact on the long-term preservation of old high-quality trees upon which the species depends for nesting and on the necessary tranquillity and safety of the breeding habitat, which can warrant good fecundity and low adult mortality. Forestry operations and hunting activities are closely followed and management agreements established with paper companies, private owners and hunting associations beyond emergency actions when required.

The project also seeks the mitigation of potential negative impact of power lines and wind farms on breeding areas and eagle safety through routine advising of promoters and managers within the scope of planning and design correction. The project further comprises, among other tasks, the field monitoring and satellite tracking of the eagle population, the census of staple prey species and a widescale multi-age public awareness.

Markus Jais: How do you see the future of the Bonelli’s Eagle in Portugal?
Luis Palma: The Northeastern population is endangered because of demographic isolation and low recruitment, low productivity and lack of food resources, and competition with Golden eagles and other cliff-nesting raptors. Pairs in the Lisbon area are very vulnerable because they live in a much humanised landscape, alongside with large urban areas, motorways, power lines and wind farms. Still, the situation seems stable, possibly due to immigration from source areas of the South where the species keeps steadily and rapidly expanding. Thus, provided that threats from forestry operations keep under control and conservation actions being carried out with the active participation of stakeholders, the future of the species seems promising in the region.

Markus Jais: Luis, thank you very much for the interview

Further Information

Conservation of tree-nesting populations of Bonelli’s Eagle in Portugal