EXPERIMENTAL TESTS OF THE INFORMATION-CENTER HYPOTHESIS WITH BLACK VULTURES (CORAGYPS ATRATUS) AND TURKEY VULTURES (CATHARTES AURA)

In field tests of the information-center hypothesis (ICH) in south Tex as with black vultures (Coragyps atratus) and turkey vultures (Cathart es aura), large carcasses were provided and kept under continuous obse rvation. The use vultures made of these bait sites and their patterns of arrival were recorded to evaluate predictions derived from the ICH. Turkey vultures discovered most bait sites (30 of 31) first, but freq uently were displaced from the food by later-arriving black vultures. This competitive exclusion by black vultures limited subsequent feedin g opportunities for turkey vultures sufficiently that few (27%) return ed on subsequent days to bait sites they had previously visited. I fou nd no evidence that those turkey vultures that did return to bait site s acted as leaders for groups of naive birds and led them to bait site s - knowledgeable and naive turkey vultures did not arrive at bait sit es together, and groups arriving at bait sites were not larger on subs equent days than on the first days carcasses were available. In contra st, a significantly larger percentage (47%) of knowledgeable black vul tures returned to bait sites they had visited on previous days, and th e first groups of black vultures arriving at bait sites on subsequent days were significantly larger than the equivalent groups on first day s. Nine flocks of black vultures that arrived on subsequent days at ba it sites before sunrise (which suggests the birds had commuted directl y from a roost) contained knowledgeable birds, and two of these flocks contained both knowledgeable and naive individuals. Overall, 10 of 54 naive tagged black vultures (18.5%) arrived at bait sites under circu mstances that suggested they had followed conspecifics to the food fro m a roost. However, most black vultures apparently found carcasses thr ough independent search or by using local enhancement. Therefore, I co nclude that while following from roosts to food sites is a strategy us ed by black vultures, at this study site it is one they use relatively infrequently.