Food Finding and the Influence of Information, Local Enhancement, and Communal Roosting on Foraging Success of North American Vultures

Abstract

I investigated foraging behavior and competition for food between Turkey Vultures (Cathartes aura) and Black Vultures (Coragyps atratus) in southern Texas. Turkey Vultures usually located carcasses first, but often were displaced by later-arriving Black Vultures, which used local enhancement to locate the carcasses. Turkey Vultures occurred equally frequently at carcasses of all sizes, but Black Vultures were more likely to visit large carcasses (>5 kg) than small ones. In addition, at very large carcasses (> 100 kg), Black Vultures outnumbered Turkey Vultures. Turkey Vultures apparently depend primarily on small items, which they can consume quickly before Black Vultures usurp them. Both vulture species roost communally and it has been suggested that their roosts may function as information centers. However, because recent research indicates that the information-center hypothesis is unlikely to be a sufficient explanation for communal roosting in vultures, I explored other possible foraging benefits of communal roosting. Specifically, I incorporated field data col- lected on the foraging behavior of vultures into two simulation models designed to evaluate the possibility that communal roosting facilitates the use of local enhancement by roost members, either by promoting the formation of foraging groups (the assembly-point hy- pothesis) or by concentrating birds in space (the spatial-concentration hypothesis). I conclude that birds depending on spatial-concentration effects occasionally may forage more success- fully than noncommunally roosting individuals, but those that form foraging groups do best. Thus, communal roosting may be advantageous because it facilitates the formation of foraging groups. Received 29 September 1995, accepted 10 December 1995.