Interspecific competition has long been recognized as driver of species diversity. To coexist, species must evolve mechanisms to reduce competition for limited resources. As carrion feeders competing for a limited and ephemeral resource, avian scavengers are ideal model organisms to study mechanisms of niche partitioning and co-existence. Much previous work pertaining to vulture coexistence has focused primarily on observations of species interactions and occurrence at carcasses. While valuable, the inclusion of behaviors beyond those observed at carcasses, such as movement patterns, may yield more comprehensive understanding of the mechanisms that reduce competition in avian scavengers. We used a combination of experimental carcass trials and GPS telemetry data to elucidate how sympatric black vultures (Coragyps atratus) and turkey vultures (Cathartes aura) are specialized to reduce direct competition. We illustrate how documented differences in physiological, morphological, and social foraging characteristics associated with each species manifest into interspecific variability in movement patterns, foraging habitats, and carcass use.
Results/Conclusions
Carcass trails demonstrated that relative to black vultures, turkey vultures had higher probabilities of foraging on smaller carcasses, and carcasses obscured by forest cover. Additionally turkey vultures were the first species to arrive at carcasses in 94% of trials. These results support the hypothesis that turkey vultures are able to use their well-developed olfactory capabilities to more effectively locate small and obscured food resources compared to black vultures, which are primarily visual foragers. GPS telemetry showed both species increased their movement rates (km traveled per hour) when soaring conditions were favorable, however turkey vultures exhibited greater movement rates in all conditions and at all times of day. These results indicate turkey vultures are able to forage over larger areas in a more energetically efficient manner than black vultures regardless of soaring conditions, likely as a result of flight advantages conferred by their lighter wing-loadings. Thus, it appears turkey vultures are able to minimize direct competition with black vultures, which are aggressive social foragers that commonly displace turkey vultures from carcasses, by using morphologically-based flight advantages to forage over larger areas, and physiologically-based olfactory advantages to efficiently locate food items not easily detected by foraging black vultures. Our study illustrates how the inclusion of movement ecology into niche partitioning studies provides a more complete understanding of interspecific specializations to resource exploitation by avian scavengers. This may have implications to conservation efforts such as supplemental feeding, aimed at increasing vulture populations in many parts of the world.