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.Les apports alimentaires déterminent la structure d'âge chez un charognard aviaire au sommet de la chaîne alimentaire RÉSUMÉ. Les activités humaines et les récents changements dans la réglementation sanitaire façonnent actuellement la disponibilité des ressources en charognes dans les écosystèmes. La manière dont ces changements de réglementation influent sur les paramètres démographiques des charognards aviaires est encore mal connue. Nous avons combiné des observations photographiques recueillies par des citoyens et des données d'observation provenant de projets de recherche dans le nord de l'Espagne pour examiner si la structure d'âge des populations de Vautours fauves (Gyps fulvus) se nourrissant de ressources différentes (carcasses naturelles réparties de façon aléatoire, ressources prévisibles [sites d'alimentation supplémentaires et fermes] et décharges) a varié en fonction des modifications de la réglementation sanitaire à partir de 2004. Nous avons constaté que la proportion d'oiseaux immatures a augmenté de manière significative après l'introduction de nouveaux règlements sanitaires européens permettant aux agriculteurs d'éliminer les carcasses de bétail en les laissant dans les champs plutôt que de les incinérer. Nous avons également constaté que la structure d'âge variait de manière importante selon les ressources alimentaires : une fraction plus élevée d'immatures a été détectée dans les décharges, ainsi que dans les sites où les charognes étaient fortement concentrées. Ces résultats révèlent que la perte du caractère aléatoire naturel de la disponib...

Age-Specific Demographic Response of a Long-Lived Scavenger Species to Reduction of Organic Matter in a Landfill

“…In addition, in some ecological systems and species, feeding on landfills could have a detrimental effect on chick and juvenile survival since individuals of these ages are more susceptible than adults to the abovementioned risks (e.g., [36,37]. Nevertheless, for long-lived scavengers such as vultures, the overall trade-off seems to be positive, as current evidence supports the idea that landfills represent an important food source that may partially support some populations [27,30,[38][39][40].…”

“…This synchronized reduction in food resources could have significant consequences for demographic rates and ultimately threaten an important food source for numerous species that rely on these facilities and, in particular, for the griffon vulture population in the northeast Iberian Peninsula. These vultures are highly specialized and feed on carcasses originating from extensive livestock farming and landfills [26,38] and younger age classes being particularly dependent on landfills, especially during periods of wild food scarcity such as winter [27,28,30,31,100]. As a result, the reduction in food and the potential closure of landfills may lead to a dramatic shift in trophic strategy among scavenger species, forcing them to rely more on less predictable food sources (and, to some extent, predictable ones like supplementary feeding sites for scavengers), as previously predicted for Egyptian vultures (Neophron percnopterus) [40].…”

“…Landfills acting as PAFS represent a continuous food source that benefits numerous opportunistic species [17]. This surplus food enhances population survival by providing sustenance year-round and, importantly, for specific age cohorts during periods of natural food scarcity (e.g., juvenile vultures during winter since the use of PAFS is energetically less demanding than searching for wild carrion) [15,[27][28][29][30][31]]. Yet, feeding in landfills though is hazardous, and may only provide low-quality and polluted food, with the associate risk of exposure to solid waste ingestion (e.g., plastic, rubber, glass or metals) and pathogens [32][33][34][35].…”

“…Thus, with declining organic waste, we first predicted that the proportion of adult and non-adult (i.e., juveniles and immatures) transients captured in the landfill would increase, meaning that the site would progressively become less attractive and that newly marked individuals would be more prone to becoming transients after the installation of the WTC [49]. Moreover, PAFS tend to attract and benefit non-adult individuals [15,18,30,[50][51][52] since this age fraction is less constrained in their foraging behavior as they are non-breeders and not bound to a specific territory. Consequently, a reduction in food at the landfill may lead to increased density-dependent intraspecific competition at the site, resulting in a higher rate of permanent emigration (reduced site fidelity) since non-adults are less competitive and experienced than adults [52][53][54].…”

Validating the concept of top scavenger: the Andean Condor as a model species

“…Indeed, the fact that vultures have such a dominant influence on the use of carrion and the ecological interactions around it, disproportionate compared to that of other scavenger species [14][15][16][17], has led to them being referred to as top or apex, scavengers [e.g. 14,[18][19][20][21][22][23].…”