FOOD FINDING AND THE INFLUENCE OF INFORMATION, LOCAL ENHANCEMENT, ANDCOMMUNAL ROOSTING ON FORAGING SUCCESS OF NORTH-AMERICAN VULTURES

I investigated foraging behavior and competition for food between Turk ey Vultures (Cathartes aura) and Black Vultures (Coragyps atratus) in southern Texas. Turkey Vultures usually located carcasses first, but o ften 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 li kely to visit large carcasses (>5 kg) than small ones. In addition, at very large carcasses (>100 kg), Black Vultures outnumbered Turkey Vul tures. Turkey Vultures apparently depend primarily on small items, whi ch they can consume quickly before Black Vultures usurp them. Both vul ture species roost communally and it has been suggested that their roo sts may function as information centers. However, because recent resea rch indicates that the information-center hypothesis is unlikely to be a sufficient explanation for communal roosting in vultures, I explore d other possible foraging benefits of communal roosting. Specifically, I incorporated field data collected on the foraging behavior of vultu res into two simulation models designed to evaluate the possibility th at communal roosting facilitates the use of local enhancement by roost members, either by promoting the formation of foraging groups (the as sembly-point hypothesis) or by concentrating birds in space (the spati al-concentration hypothesis). I conclude that birds depending on spati al-concentration effects occasionally may forage more successfully tha n noncommunally roosting individuals, but those that form foraging gro ups do best. Thus, communal roosting may be advantageous because it fa cilitates the formation of foraging groups.