Food subsidies shape age structure in a top avian scavenger

Abstract

Human activities and recent changes in sanitary regulations are currently shaping the availability of carrion resources across ecosystems. How changes in regulations influence demographic parameters in avian scavengers is still poorly known. We combine photographic observations gathered by citizens and observational data from research projects in northern Spain to examine if the age structure of Eurasian Griffon Vulture (Gyps fulvus) populations at different trophic resources (natural randomly-distributed carcasses, predictable resources [supplementary feeding sites and farms], and landfills) varied in relation to modifications of sanitary regulations from 2004 onwards. We found that the proportion of immature birds increased significantly after the introduction of new European sanitary regulations allowing farmers to dispose of livestock carcasses in the field, rather than incinerating them. Also, we found that the age structure varied significantly between food resources, such that we detected a higher fraction of immatures at landfills, as well as in sites where carrion was highly clumped. These findings reveal that loss of natural randomness in carrion availability may elicit age-dependent effects on the spatial distribution of the vultures at the mesoscale which may ultimately affect population structure. Our findings shed light on challenges on how to manage food subsidies to preserve avian scavenger populations in an increasingly anthropized world.