Family Associations in Communally Roosting Black Vultures

-Observations of marked individuals in a partially marked population of Black Vultures (Coragyps atratus) show that immediate family members maintain close contact throughout the year. Associations between mates and between parents and offspring are primarily responsible for this. Mutual allopreening, feedings, and intercessions in fights occur almost exclusively within families. Adults known to breed within the study area (neighbors) associate more strongly than nonneighbors, although not as strongly as mates. Certain families associate preferentially with certain other families. Strength of interfamily associations is not related to distance between nest sites within the study area. I argue, therefore, that birds actively seek particular individuals as associates. Further knowledge of breeding dispersal of young of known parentage will provide clues to relatedness of highly associated neighbors. Strong associations among related individuals can help explain the use of avian communal roosts as information centers. Received 27 April 1984, accepted 10 June 1985. COMMUNAL roosts and breeding colonies could serve as "information centers" to which unsuccessful foragers return and follow successful individuals to a known food source (Ward and Zahavi 1973). Explaining the apparent aid-giving of successful foragers remains a difficulty of this hypothesis. Some workers emphasize the probability of high levels of relatedness within colonies (e.g. Erwin 1978, Waltz 1982) to help explain why successful foragers might tolerate any costs of being followed. By sharing food with closely related individuals, successful foragers could increase their inclusive fitness (Hamilton 1964). If mechanisms exist that allow preferential sharing with close relatives, inclusive fitness gains could outweigh costs of sharing. Black Vultures (Coragyps atratus) form large communal roosts each night throughout the year, although they nest in pairs in isolated, well-protected locations. Indirect evidence suggests that Black Vulture roosts may serve as food-finding information centers. For instance, morning roost departures are significantly clumped both in time and in direction, and these clumped departures cannot be explained by weather conditions (Rabenold 1983). A successful forager might return to the roost to recruit particular individuals with whom to I Present address: Department of Biological Sciences, Purdue University, West Lafayette, Indiana 47907 USA. share or cooperatively defend its find. If these associations of individuals are stable over time, it should be possible to detect relative differences in the strengths of associations between different individuals in communal roosting and feeding situations. There is evidence suggesting that parent-offspring ties persist for several months past fledging in Black Vultures (Jackson 1975, McHargue 1977, this study). This behavior would help explain the use of roosts as information centers. Preferential association and preferential behavioral interactions among individuals in a population could structure the distribution of benefits accruing from shared information at communal roosts. The maintenance of family associations outside the breeding season is the subject of this paper. To examine associations of individuals within Black Vulture communal groups, I ask specifically (1) whether mated pairs and individuals of known relatedness (parents and offspring or siblings) are seen together more often than predicted by chance; (2) whether these intrafamilial associations differ from associations of the same individuals with birds not known to be related (extrafamilial associations); (3) whether pairs nesting in an area associate more than expected by chance with other pairs nesting in the same area, and whether these neighborhood associations differ in degree from associations within families or between nonneighbors; and (4) whether strength of associations between neighbors is dependent on the distance between their nest sites. 32 The Auk 103: 32-41. January 1986 This content downloaded from 207.46.13.75 on Sat, 03 Sep 2016 04:08:06 UTC All use subject to http://about.jstor.org/terms January 1986] Black Vulture Family Associations 33