Competitive interactions within and between species in a guild of Avian scavengers

-We observed Andean Condors (Vultur gryphus), King Vultures (Sarcoramphus papa), Black Vultures (Coragyps atratus), Turkey Vultures (Cathartes aura), and Crested Caracaras (Polyborus plancus) interacting at 217 animal carcasses at two sites in northern Peru. At 53 carcasses for which we knew order of arrival, Turkey Vultures usually arrived first, Black Vultures second, and condors third. On the basis of our observations of 8,066 aggressive encounters between birds, we constructed dominance hierarchies by calculating the proportion of encounters won by an individual of one species, sex, or age during encounters with an individual of another species, sex, or age. Within each species there was a positive relationship between a bird's dominance and its age. In condors, males dominated females of the same age. Interspecific dominance was correlated positively with body mass. There are convergent similarities between the organizations of guilds of Old and New World vultures. Received 1 August 1986, accepted 24 November 1986. WHEN limited resources are found in Widely dispersed, rich patches that are ephemeral and unpredictable in spatial and temporal occurrence, competition between consumers could involve either differential exploitation or interference. Differential exploitation in such situations is often based on the differential ability of individuals to locate the resource, and interference usually is achieved through the establishment of dominance hierarchies among individuals that have found the resource (Maurer 1984). Avian scavengers that feed on carcasses of large animals compete for just such a limited, dispersed, rich, ephemeral, and unpredictable food source, and differences in order of arrival at carcasses and dominance hierarchies when birds are feeding together have been reported among members of scavenging guilds (Kruuk 1967, Houston 1975). The most detailed studies of competitive interactions between avian scavengers have taken place in Africa where large guilds of Old World vultures (Accipitridae) feed on ungulate carcasses (Petrides 1959; Attwell 1963; Kruuk 1967; Houston 1974, 1975). In contrast, there have been few descriptions, most of them anecdotal accounts (e.g. Koford 1953, Stuart 1978), of competitive interactions in guilds of New World vultures (Cathartidae). We observed patterns of differential exploitation and interference within and between 5 species that are competing members of a guild of avian scavengers in northern Peru: Andean Condors (Vultur gryphus), King Vultures (Sarcoramphus papa), Black Vultures (Coragyps atratus), Turkey Vultures (Cathartes aura), and Crested Caracaras (Polyborus plancus). We have shown previously that during our study several of these species were being limited by the availability of carrion during a prolonged preEl Nifio drought (Wallace and Temple 1987). STUDY AREA AND METHODS The field observations took place between 1980 and 1984 in northern Peru in the Cerro Illescas region and in the vicinity of Naupe. The Cerro Illescas is an isolated mountain range of the Sechura Peninsula (6.0?S, 81.0?W). Avian scavengers in this study area fed almost exclusively on the carcasses of either marine birds and mammals that washed ashore on the peninsula's beaches or feral ungulates that died in the surrounding desert. The Naupe area is about 150 km east of the Cerro Illescas in the western foothills of the Andes (5.35?S, 79.4?W). Avian scavengers in this study area fed primarily on carcasses of domestic livestock that died on the area's desert grasslands. In both areas we observed birds interacting at 217 carcasses of large animals at as many sites: 129 burros (Equus asinus), 57 dogs (Canis domesticus), 12 sea lions (Otaria byronia), 10 goats (Capra hircus), 4 horses (Equus caballus), 4 pigs (Sus scrofa), and 1 green sea turtle (Chelonia mydas). Each of these carcasses was sufficiently large to be highly attractive to all members of the guild. We opened the largest carcasses so that they could be consumed by all guild members. If we had not done this, smaller vultures would have been forced to wait until larger birds had first ripped the thick-skinned carcasses open. We watched these carcasses for a total of 4,071 h from blinds positioned 290 The Auk 104: 290-295. April 1987 This content downloaded from 157.55.39.45 on Thu, 01 Sep 2016 05:57:50 UTC All use subject to http://about.jstor.org/terms April 1987] Competition Among Avian Scavengers 291 8 141 17 53 1.0 132 0 0 0 >_ 0 m~~~~~~~~~