Stable isotope analyses allow knowing eagle’s diet during the breeding season and over the years

The Conservation Biology Group, in collaboration with researchers from the University of Exeter (United Kingdom), has published a new study in the scientific journal IBIS where the diet of Bonelli’s Eagle cheeks is analysed by conventional diet methods such as pellet analysis and by stable isotope analyses of chick feathers.

Stable isotopes are non-radioactive atoms of a specific chemical element that have the same number of protons but differ in the number of neutrons. For instance, the most common carbon has an atomic weight of 12, with 6 protons and 6 neutrons (12C), but there are also stable carbon atoms with 6 protons and 7 neutrons (13C). Several studies in Ecology have shown that the proportion of certain stable isotope in a predator reflects its diet, as this proportion is transmitted from one level to another in the food chain in a predictable manner. In a previous study conducted by our research group this property was already used to have a first look at the diet of chicks by the analysis of stable isotopes of carbon, nitrogen and sulfur.

The main new features of the new study are determining which is the proportion of these isotopes in the main categories of prey such as rabbits, partridges, pigeons, seagulls, squirrels and passerines throughout the study area, as well as using more complex statistical models to quantify the range of consumption for each category of prey. The study includes data obtained from 28 territories in Catalonia between 2008 and 2011 thanks to a long-term monitoring carried out by our team with the collaboration of the Grup de Suport de Muntanya dels Agents Rurals collecting pellets from the nest and taking some feather samples.

During the field work to ring the chicks, some feathers were collected to about the eagle's diet.

The pellet analysis showed that, in terms of biomass, Rabbits were the main prey item (30.9%), followed by pigeons (26.9%), Yellow-legged Gulls (8.7%), Partridges (8.1%) and Squirrels (4.9%), together accounting for 79.5% of total biomass ingested. These results show an overall agreement with results obtained by stable isotope analyses, especially in terms of the most – and least – consumed prey, at both the population level and, to a lesser extent, the territory level. These results suggest that combining both methods, pellet analysis and by stable isotope analyses of chick feathers, is a useful tool for long-term monitoring of the diet of this raptor, and to assess differences between territories.

For further information:

• Resano-Mayor, J., Hernández-Matías, A., Real, J., Parés, F., Inger, R. & Bearhop, S. 2013. Comparing pellet and stable isotope analyses of nestling Bonelli's Eagle Aquila fasciata diet. Ibis. doi: 10.1111/ibi.12095
• Resano, J., Hernández-Matías, A., Real, J. & 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