Comparing pellet and stable isotope analyses of nestling Bonelli's Eagle Aquila fasciata diet

Abstract

Diet analyses are central to the study of avian trophic ecology, and stable isotope analyses have made an increasing contribution in the last two decades. Few isotopic studies have assessed the diet of raptor species, which are more frequently analysed by conventional diet methods such as pellet analysis. In this study, we compare prey consumption estimates of nestling Bonelli's Eagles Aquila fasciata from conventional pellet analysis (in terms of items and biomass) and stable isotopic mixing models (SIAR) using δ13C, δ15N and δ34S of feathers. The pellet analysis showed that European Rabbits Oryctolagus cuniculus, pigeons (mainly Common Wood Pigeons Columba palumbus and Domestic Pigeons Columba livia dom.), Red‐legged Partridges Alectoris rufa, passerines, Yellow‐legged Gulls Larus michahellis and Eurasian Red Squirrels Sciurus vulgaris were the main prey, so they were selected for diet reconstructions in SIAR. At the population level, mean prey consumption estimates were similar for pellets (both items and biomass) and SIAR. At the territory level, the weighted kappa statistic showed good ordinal scale agreement in main prey consumption between items or biomass and SIAR. Although the intraclass correlation coefficient showed poor method agreement when considering all prey in the same analysis, the intraclass correlation coefficients for each prey category showed significant agreement between pellets and SIAR when estimating the consumption of Rabbits, pigeons and Gulls, with lower agreement for passerines and Squirrels. Lastly, there was poor method agreement for estimates of Partridges. Our results suggest an overall agreement between the pellet analysis and SIAR when estimating nestling Bonelli's Eagle diet at both the population and, to a lesser extent, the territory level, supporting the usefulness of isotopic mixing models when identifying the terrestrial and marine components of raptor diets.

Multi-Scale Effects of Nestling Diet on Breeding Performance in a Terrestrial Top Predator Inferred from Stable Isotope Analysis

“…Prey tissue collections were supplemented with dead individuals collected in the surroundings of eagles’ breeding territories in Catalonia (see [42]). We did not find any evidence of prey isotopic differences among the three populations when comparing the main prey categories, i.e.…”

Diet–demography relationships in a long‐lived predator: from territories to populations

“…In western Europe, Bonelli's eagle predates on a wide range of prey that includes small‐to‐medium sized mammals (Lagomorpha and Rodentia), birds (Columbiformes, Galliformes, Passeriformes, Ardeiformes, Charadriiformes and others) and reptiles (mainly lizards) (Real , Moleón et al ). In this geographical area, marked differences in dietary patterns between territories have been described within single populations (Real , Palma et al , Resano‐Mayor et al ), a trend that increased after the outbreak of the Rabbit haemorrhagic disease in the late 1980s (Moleón et al ). Yet, despite the dietary differences between territories, European rabbits Oryctolagus cuniculus and, to a lesser extent, red‐legged partridges Alectoris rufa , are thought to be Bonelli's eagle's preferred prey item, as they are positively selected and high consumption of these prey species leads to a decrease in this eagle's diet diversity (Real , Moleón et al , ).…”

Using soil from pellets of White StorksCiconia ciconiato assess the number of earthworms (Lumbricidae) consumed as primary and secondary prey

“…In most cases each vertebrate prey item identified in each pellet was treated as a single item (Resano‐Mayor et al . ). However, when the remains clearly indicated the presence of multiple individuals of the same taxon, we treated them as separate items.…”

“…, Gillings & Sutherland , Resano‐Mayor et al . ). In most diet studies, researchers use indirect methods to describe diet.…”

“…To understand the energy requirements of animals living in changing or periodic environments, we need detailed data on the biomass and numbers of consumed prey items (Je z drzejewski et al 1994, Gillings & Sutherland 2007, Resano-Mayor et al 2014. In most diet studies, researchers use indirect methods to describe diet.…”