Livelihood in anthropic landscapes: Stable isotopes as indicators of dependence of obligate avian scavengers on intensive animal farming

Abstract

The rise of intensive livestock farming poses a significant global threat to biodiversity, affecting scavenger species reliant on carcasses from these operations. Assessing avian scavenger dependence on such resources in human-modified environments is challenging using conventional methods. This study analyzes GPS-tracking and stable isotopes of 77 Eurasian griffon vultures (Gyps fulvus) in the Iberian Peninsula to explore variations in resource consumption. By means of Bayesian isotopic mixing models we examined the influence of home range size, degree of human transformation of landscapes, and individual characteristics on diet. Elevated δ13C levels indicate pig consumption, likely due to their reliance on corn-derived feed supplements. Populations in areas with higher Human Footprint Index, like the Ebro Valley, showed utilization of variated livestock resources (sheep and pig), broadening population trophic niches and individual heterogeneity. Conversely, in populations of less transformed (natural) environments, like the Cazorla Mountains, focal birds relied almost exclusively on extensive or semi-extensive sheep farming, with diet variations linked to home range size changes. Finally, the Cádiz mountains population heavily depended on pig remains, likely from vulture feeding stations and farms. We detected sex differences in diet probably linked to asymmetric behavioural responses between males and females to local changes in resource abundance and predictability. These findings demonstrate that Iberian griffon vulture populations heavily rely on intensive livestock from farming and supplementary feeding schemes, sometimes violating regional and European regulations. This dependence threatens individual health and population viability due to ingestion of veterinary drugs, pollutants, and direct mortality from infrastructure accidents, especially in humanized areas. Stable isotopes prove valuable in assessing the real avian scavengers’ reliance on carrion resources on a broad scale, surpassing estimates from widely-used methods.