Why do raptors take domestic prey? The case of Bonelli's eagles and pigeons

Abstract

Summary 1.Predator conservation management requires detailed understanding of the ecological circumstances associated with predation, especially that on economically valuable prey. We examined the mechanisms behind Bonelli's eagle Hieraaetus fasciatus predation on prey of domestic origin, using dietary data from 22 pairs breeding in south-west Portugal (1992Portugal ( -2001 together with information on landscape composition and prey availability. 2. Numerically, 42·7% (37·7% in biomass) of eagle prey comprised domestic species, about 70% of which were rural pigeons Columba livia and the remainder were racing pigeons Columba livia and domestic fowl Gallus gallus . Rabbits Oryctolagus cuniculus , red-legged partridges Alectoris rufa and jays Garrulus glandarius were the most frequent wild prey (43·1%; 50·8% in biomass). This dietary pattern was remarkably stable over a decade, but within each year the intake of pigeons almost halved over the course of the breeding season. 3. Landscape composition significantly affected the dietary proportion of wild and domestic prey items. This was particularly evident in territories dominated by eucalyptus Eucalyptus globulus plantations, where there was reduced consumption of rural pigeons and partridges, an increased intake of minor avian prey items and greater diversity in the diet overall. 4. Bonelli's eagles showed type II functional responses while preying on the most important wild (rabbit) and domestic (rural pigeon) prey, although the former was much stronger. Eagle predation on rabbits declined with increasing abundance of pigeons, and vice versa, but there was no switching in the traditional sense, as selection between these two species was inversely frequency dependent. 5. Synthesis and applications . Predation by Bonelli's eagle on domestic pigeons results from a combination of high vulnerability of the pigeons to eagles and a shortage of key wild prey such as rabbits and partridges, especially during the early breeding season. Given the relatively low economic value of rural pigeons and their importance in the diet of Bonelli's eagles, they could probably be used as a conservation tool to enhance food resources in breeding territories and to deflect predation from more valuable prey such as partridges and racing pigeons.

Does afforestation increase bird nest predation risk in surrounding farmland?

“…Eucalyptus plantations also appeared to have a positive effect on carnivores, probably because they are little disturbed by human activities and thus provide secure shelter in landscapes otherwise subject to periodic agricultural operations (Gehring and Swihart, 2003;Baker and Harris, 2006;Pita et al, 2009). Increasing cover by eucalyptus plantations, however, may ultimately have a negative effect on carnivores, as suggested by the decline in fox abundance in landscapes dominated by this exotic tree, where lagomorphs and other prey tend to be scarce (e.g., Palma et al, 2006;this study).…”

From local monitoring to a broad‐scale viability assessment: a case study for the Bonelli's Eagle in western Europe

“…This is the only population throughout the species' range in Europe to exhibit such a positive trend, which has occurred at the same time as an increase in its breeding range. Interestingly, this trend can be attributed both to high survival values and to the tendency of eagles from this population to nest in trees, which allows them to occupy new territories even in areas without cliffs (Palma et al 2006, Beja andPalma 2008).…”

Predator–prey relationships in a Mediterranean vertebrate system: Bonelli’s eagles, rabbits and partridges

“…For example, rabbit is the principal prey driving the spatio-temporal patterns in the eagle's diet on a continental scale (Moleón et al 2009). In addition, a type II functional response has already been found in an area of south Portugal, poor in wild prey (Palma et al 2006), and the positive selection of rabbits has been described at a local level in south Spain (Gil-Sánchez 1998; see Moleón et al 2009 for a complete list of arguments in favour of the rabbit-facultative-specialisation RFS hypothesis).…”

“…density of faeces) in a number of studies (e.g. Angerbjörn et al 1999;Gil-Sánchez et al 1999;Palma et al 2006) has prevented a more reliable, realistic combination of the numerical and functional responses as an absolute total response cannot be inferred.…”

“…In four cases with an overlap, the area considered was lower (5,411 ha for territories T6 and T7; 5,171 ha for territories T9 and T10; see Fig. 1), based on the assumption that eagles maintain and defend exclusive territories (Palma et al 2006), or that neighbouring pairs equally used the overlap zones. Given the longer time spent close to nests during the breeding season (Bosch et al 2010; personal observations), we considered another radius, which was half that of the previous one (radius = 2,209 m; area = 1,533 ha; this radius includes most of the 50% home-range probability kernel in this species and season; Bosch et al 2010).…”