COMPETITIVE INTERACTIONS WITHIN AND BETWEEN SPECIES IN A GUILD OF AVIAN SCAVENGERS MICHAEL P. WALLACE AND STANLEY A. TEMPLE Departmentof WildlifeEcology, University of Wisconsin,Madison, Wisconsin 53706 USA ABSTRACT.--Weobserved Andean Condors(Vultur gryphus),King Vultures (Sarcoramphus papa),Black Vultures (Coragypsatratus), Turkey Vultures (Cathartesaura), and CrestedCara- caras(Polyborus plancus) interacting at 217 animal carcassesat two sitesin northern Peru. At 53 carcassesfor which we knew order of arrival, Turkey Vulturesusually arrived first, Black Vulturessecond, and condorsthird. On the basisof our observationsof 8,066 aggressive encountersbetween birds, we constructeddominance hierarchies by calculatingthe propor- tion of encounterswon by an individualof one species,sex, or age during encounterswith an individual of another species,sex, or age. Within each speciesthere was a positive rela- tionship between a bird's dominanceand its age. In condors,males dominatedfemales of the sameage. Interspecificdominance was correlatedpositively with body mass.There are convergentsimilarities between the organizationsof guildsof Old and New World vultures. ReceivedI August1986, accepted24 November1986. WHENlimited resourcesare found in •9idely coramphuspapa), Black Vultures (Coragypsatra- dispersed,rich patchesthat are ephemeral and tus), Turkey Vultures (Cathartes aura), and unpredictable in spatial and temporal occur- Crested Caracaras(Polyborus plancus). We have rence, competition between consumers could shown previously that during our study sev- involve either differential exploitation or in- eral of thesespecies were being limited by the terference.Differential exploitationin suchsit- availability of carrion during a prolonged pre- uationsis often basedon the differentialability E1Nifio drought (Wallace and Temple 1987). of individuals to locate the resource, and inter- ference usually is achieved through the estab- lishment of dominancehierarchies among in- STUDY AREA AND METHODS dividuals that have found the resource (Maurer The field observationstook place between 1980 and 1984). Avian scavengersthat feed on carcasses 1984 in northern Peru in the Cerro Illescas region of large animals competefor just such a limit- andin thevicinity of •Iaupe. The Cerro Illescas isan ed, dispersed,rich, ephemeral, and unpredict- isolated mountain range of the Sechura Peninsula able food source, and differences in order of (6.0øS,81.0øW). Avian scavengersin this study area arrival at carcasses and dominance hierarchies fed almostexclusively on the carcassesof either ma- when birds are feeding together have been re- rine birds and mammals that washed ashore on the ported among members of scavengingguilds peninsula'sbeaches or fetal_ungulatesthat diedin (Kruuk 1967, Houston 1975). the surrounding desert.The Naupe area is about 150 km east of the Cerro Illescas in the western foothills The most detailed studiesof competitive in- of the Andes (5.35•S, 79.4øW).Avian scavengersin teractionsbetween avian scavengershave tak- this study area fed primarily on carcassesof domestic en place in Africa where large guilds of Old livestockthat died on the area'sdesert grasslands. World vultures (Accipitridae) feed on ungulate In both areaswe observedbirds interacting at 217 carcasses (Petrides 1959; Attwell 1963; Kruuk carcassesof large animalsat as many sites:129 burros 1967; Houston 1974, 1975). In contrast, there (Equusasinus), 57 dogs (Canisdomesticus), 12 sea lions have been few descriptions,most of them an- (Otaria byronia ), 10 goats( Capra h ir cus ), 4 horses( Equus ecdotalaccounts (e.g. Koford 1953,Stuart 1978), caballus),4 pigs (Susscrofa), and 1 green sea turtle of competitive interactions in guilds of New (Cheloniamydas). Each of these carcasseswas suffi- ciently large to be highly attractive to all members World vultures (Cathartidae). of the guild. We opened the largestcarcasses so that We observed patterns of differential exploi- they couldbe consumedby all guild members.If we tation and interference within and between 5 had not done this, smaller vultures would have been speciesthat are competingmembers of a guild forcedto wait until larger birds had first ripped the of avian scavengersin northern Peru: Andean thick-skinned carcassesopen. We watched these car- Condors (Vultur gryphus),King Vultures (Sar- cassesfor a total of 4,071 h from blinds positioned 290 The Auk 104: 290-295. April 1987 April 1987] CompetitionAmong Avian Scavengers 291 85 141 17 53 58 6 39 23 1.0 3O 2O7 131 0.5 -- 132 rr 141 85 ß -3 -2 -1 0 I 2 3 4 5 -2 -1 0 I 2 3 4 5 6 AGE OF INDIVIDUAL"A"MINUS AGE OF INDIVIDUAL'•' AGE OF INDIVIDUAL "A"MINUS AGE OF INDIVIDUAL 'B" Fig. 1. Proportionof encounterswon by a King Fig. 2. Proportionof encounterswon by a male Vulture of a certain age (individual A) when inter- AndeanCondor of a certainage (individual A) when actingwith a King Vulture of another age (individ- interactingwith a male condor of another age (in- ual B). Number above each point is samplesize. Age dividual B). Number aboveeach point is samplesize. differencesare in years. Age differencesare in years. nearby or, by using telescopes,from vantagepoints up to 0.75 km away. The number of scavengersob- During our observationswe alsohad opportunities servedat thesecarcasses ranged up to 240individuals to recordthe sequenceof arrival of Turkey Vultures, at a time. Black Vultures, and Andean Condors at 53 carcasses We identified all speciesthat visited the carcasses, we knew had not been visited previously by scav- and when possiblewe noted individual sexand age. engers.We positionedthese 53 carcassesin open areas AndeanCondors are sexuallydimorphic and undergo frequentedby all three speciesso that all birds had sequentialchanges in plumage, beak, and eye col- chancesof encounteringthem. oration until they are 6-7 yr old (K. C. Lint pers. comm., J. W. Carpenter pers. comm.). This allowed RESULTS us to distinguish7 age classesof condorsas well as their sex.King Vultures are sexuallymonomorphic, Use of carcassesby scavengers.--Weobserved but undergosequential changes in plumageand skin colorationwith age that allowed us to distinguish6 carcasseson 239 days. Turkey Vultures visited age classes(Heck 1968, C. Benevidiespers. comm.). the carcasseson 199 (83%) days, Black Vultures Turkey Vultures, Black Vultures, and Crested Cara- on 149 (62%) days, Andean Condors on 124 caras are also sexually monomorphic, but birds less (52%) days, King Vultures on 15 (16%) days, than 1 yr old were distinguishedfrom older birdson and caracarason 6 (3%) days. When Turkey the basisof plumage, skin, and beak color (Brown Vultures were present,they were accompanied and Areadon 1968). by Black Vultures on 73% of the days, by con- We recordedthe outcomesof 8,066direct aggres- dors on 58% of the days, by King Vultures on siveencounters between individuals that fed actively 8% of the days, and by caracarason 3% of the at carcasses.The winner was the bird that supplanted days.When BlackVultures were present,they the other individual, regardlessof whether the sup- planting involved physicalcontact or mere intimi- were accompaniedby Turkey Vultures on 95% dation. Becauseof the large number of Turkey and of the days, by condorson 75% of the days, by Black vultures that often interacted at the carcasses King Vultures on 10% of the days, and by cara- at the sametime, only about 50%of their interactions carason 4% of the days. Caracaraswere accom- were recorded.Condors and King Vultures, because panied by Turkey and Black vultures on 100% of their sizeand lower numbersat feedingsites, were of the days and by King Vultures and condors easier to track, and 95-100% of their interactions were on 66% of the days. On days when King Vul- recorded. From these data the frequenciesof inter- tures were present both Turkey and Black vul- actionsbetween different species,sexes, and age tures were always there; caracaraswere present classeswere calculated.Not all of the potential cat- on 47% of the days,and condorson 36% of the egoriesof interactionswere observed,but for those observedwe calculatedthe proportion of encounters days. When Andean Condors were present, won by an individual of one species,sex, or age dur- Turkey Vultures accompaniedthem on 95% of ing encounterswith an individual of a different the days, Black Vultures on 90% of the days, species,sex, or age.On the basisof theseprobabilities King Vultures on 9% of the days,and caracaras we constructed dominance hierarchies. on 6% of the days. 292 WALLACEAND TEMPLE [Auk,Vol. 104 45 40 34 16 136 64 29 11 29 18 5 1.0 92 ß ß ß ß 53 lOO 83 66 53 -5 -4 -3 -2 -1 0 I 2 3 4 5 6 MALE• AGE I,At•US FEMALE'S AGE 0 I 2 3 4 5 6 Fig. 4. Proportion of encounterswon by a male AGE OF INDIVIDUAL"A" MINUS AGE OF INDIVIDUAL'•" Andean Condor of a certain age when interacting with a female condorof anotherage. Number above Fig. 3. Proportionof encounterswon by a female eachpoint is samplesize. Age differencesare in years. Andean Condor of a certain age (individual A) when interactingwith a female condorof anotherage (in- dividualB). Numberabove each point is samplesize. spring. We never saw aggression between Age differencesare in years. adults and juveniles, even though they were often in close proximity. Order of arrivaL--Usually, Turkey Vultures King Vultures usually arrived in pairs or in arrived first at the carcasses,Black Vultures sec- what appeared to be family groups of 3, and ond, and Andean Condors last. Turkey Vul- landed in trees near the carcass.Although oc- tures