The diet of Bonelli’s Eagle in Europe and the influence of rabbit haemorrhagic disease

A new study written by the Biology Conservation Group and other researchers of Spain, France and Portugal, published the  Journal of Biogeography, focuses on the diet of the Bonelli’s Eagle in various regions of Europe over the period 1968-2006 and the changes observed in the wake of rabbit haemorrhagic disease (RHD), which was first recorded in 1988.

According to the study, the most common prey of the Bonelli’s Eagle before the outbreak of RHD was the rabbit, followed by pigeons and partridges. However, although in southern areas rabbits and partridges were the most frequently captured prey, in northern areas a wider range of prey species including pigeons and other birds was consumed, probably due to the lower abundance of rabbits and partridges. When RHD reached Europe in 1988, it extended across the Iberian Peninsula and the total rabbit population decreased by approximately 30%. This obliged eagles to diversify their prey items and hunt for other mammals, pigeons, corvids and other birds. If we consider that despite this impact, Bonelli’s eagles now consume a higher proportion of rabbits relative to the total population, it is clear that this is a key prey species in the survival of eagle populations.

The appearance of new species in the diet of the Bonelli's Eagle after the outbreak of RHD, such as gulls and starlings – subject to anthropogenic influence and possible carriers of pathogens and contaminants, as well as providing little in the way of energy – is a clear indicator of how emergent diseases can alter trophic relations and the general stability of ecosystems. The challenge now faced by researchers is to discover how these alterations may affect the life-cycle of Bonelli’s eagles and the survival of populations.

Rabbits, pigeons and Red-legged Partridges have generally been the main preys of the Bonelli's Eagle.

On the other hand, the Biology Conservation Group also carried on another article published in the Journal of Raptor Research, where they studied the diet of Bonelli's Eagle at 15 breeding territories in Catalonia, Northeastern Iberian Peninsula, through a conventional pellet analysis and stable isotope analyses (SIA) of nestlings' feathers, during the breeding season of 2008. Main objectives were to investigate the diet of Bonelli's Eagle nestlings and to determine whether SIA allowed accurate representation of their dietary patterns. The pellet analysis revealed a broad diet including pigeons (Columba spp.; 31.1%), European rabbits (27.9%), “other birds” (16.2%), Red-legged Partridges (13.1%), Eurasian red squirrels (Sciurus vulgaris; 5.2%), ocellated lizards (Timon lepidus; 2.6%), Yellow-legged Gulls (Larus michahellis; 2.2%) and “other mammals” (1.7%). Diet composition was heterogeneous and varied markedly among nestlings from different breeding territories. This study provided the first isotopic approach to the trophic ecology of Bonelli's Eagle nestlings, and concluded that δ13C, δ15N, and δ34S may be useful for assessing nestlings' dietary patterns in terms of main prey consumption and prey trophic level.

For further information:

• MOLEÓN, M., SÁNCHEZ-ZAPATA, J. A., REAL, J., GARCÍA-CHARTON, J. A., GIL-SÁNCHEZ, J. M., PALMA, L., BAUTISTA, J. AND BAYLE, P. 2009. Large-scale spatio-temporal shifts in the diet of a predator mediated by an emerging infectious disease of its main prey.  Journal of Biogeography, 36: 1502–1515. doi: 10.1111/j.1365-2699.2009.02078.x
• RESANO, J., HERNÁNDEZ-MATÍAS, A., REAL, F. & PARÉS, F. 2011. Using stable isotopes to determine dietary patterns in Bonelli’s Eagle (Aquila fasciata) nestlings. Journal of Raptor Research, 45 (4): 342-352. doi:  http://dx.doi.org/10.3356/JRR-11-13.1