Roost Attendance and Aggression in Black Vultures

-In a 5-yr study of a partially marked population of Black Vultures (Coragyps atratus) in North Carolina, turnover among marked birds in a large winter roost averaged 34% each night. Roost attendance and areawide resighting rates differed among age classes; juveniles remained site faithful while young adults ranged far, and adults were intermediate in movement. Local breeders were more consistent in year-round attendance at roosts near their nest sites than were other marked adults using these roosts, and local breeders were likewise overrepresented on initial days at baited feeding sites. During aggressive interactions at roosts and at food, adults won over young birds and juveniles lost to older birds. Breeding adults won more fights close to their nest sites than farther away. Winners of fights at roosts were more likely to be present on subsequent evenings than were losers. The results suggest that food-finding adults, primarily local breeders, can to some extent control local roost membership by means of aggressive interactions in roosts. Successful foragers can reduce the costs of being followed and sharing food by limiting the number and identity of potential followers through aggression. Received 9 September 1986, accepted 11 April 1987. BLACK Vultures (Coragyps atratus) gather in large roosting groups throughout the year but nest in dispersed, isolated locations. Although roost membership is fluid, relatively stable units of nuclear families and allied families co-occur predictably (Rabenold 1986). High levels of relatedness make active recruitment of followers to food more likely (Waltz 1982) from roosts serving as food-finding information centers (Ward and Zahavi 1973). Evidence that foraging is enhanced for Black Vultures by following from roosts includes: (1) the temporal and directional clumping of roost departures (Rabenold 1983), (2) the return of successful foragers to food visited the day before, (3) the tendency of birds ignorant of the location of food to follow others from roosts, and (4) the larger roosting group sizes and greatly enhanced secondday recruitment to food during the season when food is least abundant (Rabenold 1987). Even with such evidence, it is difficult to embrace the hypothesis that following from the roost is an important mechanism for enhancing foraging. The advantages of roost participation are not obvious for finders of food that are followed from the roost by competitors for food. Sharing selectively with family members and other al1 Present address: Department of Biological Sciences, Purdue University, West Lafayette, Indiana 47907 USA. lies helps explain the recruitment of followers (Rabenold 1986), but a family group comprises only a small portion of potential followers in this system. Costs of sharing with unrelated followers could be minimized if there existed an effective means of prohibiting following or enforcing reciprocation, and if the cost of these controls were less than the cost of tolerating competitors (Waltz 1983). I studied aspects of the social organization of Black Vulture roosts that could structure the distribution of benefits from sharing information. I examined patterns of roost attendance, food-finding abilities, and competitive skills in Black Vultures by age class and breeding status. Further, I attempted to determine whether agonistic interactions in a roost influence roost membership. If regular food finders were not only dominant at food but could also limit followers depending on their relatedness, their likelihood of reciprocation, or the local abundance of food, then cooperation in potential information centers is more easily understood. MATERIALS AND METHODS Study area and population.-The study area was located in Chatham Co. in central North Carolina. Its 250 km2 are 60% agricultural and 40% woodland; there are no towns (Reeves et al. 1970). In addition to tobacco, local farms produce mainly poultry, swine, and cattle. Carcasses of these animals are the principal food of the vultures (Rabenold 1987). I studied the 647 The Auk 104: 647-653. October 1987 This content downloaded from 207.46.13.33 on Wed, 12 Oct 2016 04:54:28 UTC All use subject to http://about.jstor.org/terms 648 PATRICIA PARKER RABENOLD [Auk, Vol. 104