Ecological Factors Affecting Group and Territory Size in White-throated Magpie-jays

--White-throated Magpie-Jays (Calocitta formosa) breed cooperatively and defend permanent, all-purpose group territories. We measured territory area, resource levels, resource dispersion, group size, and group reproductive success for 14 groups over a threeyear period in Costa Rica. Larger groups possessed larger territories containing more bullhorn acacia trees (Acacia cornigera and A. collinsii), which provided a critical food resource during the dry season. On a per capita basis, however, the number of acacia trees and territory area were the same for group members regardless of group size, and survivorship did not vary significantly with group size. Variation in reproductive success among groups was influenced by two factors: (1) larger groups produced more successful nests per year, and (2) territories with a higher density of acacia trees fledged more offspring per successful nest. Magpie-jays bred in pasture and foraged primarily in woodland; both habitat types were patchily distributed. The dispersion of woodland and pasture patches in the landscape appeared to constrain where jays could form territories, whereas the amount of acacia and other food resources determined the size that groups could attain. We conclude that ecological factors are critical to understanding the White-throated Magpie-Jay social system, along with other social and demographic constraints ypically found in avian cooperative breeders. Received 4 February 1997, accepted 20 August 1997. GROUP TERRITORIALITY is uncommon in birds and typically is associated with species that breed cooperatively. For most cooperativebreeding species that defend all-purpose territories (i.e. territories used for breeding and feeding; Schoener 1968, Brown 1969), it appears that a shortage of habitat of suitable quality has prevented nonbreeders from establishing territories of their own. These nonbreeders choose to be philopatric and help their parents breed and thereby gain various immediate and deferred benefits (Emlen 1991, Koenig et al. 1992). In most species of birds that defend allpurpose territories, juveniles disperse before the next breeding event, probably because they compete with their parents for breeding resources and food (Brown 1969, Brown and Orians 1970, Johnson and Gaines 1990, Koenig et al. 1992). These dispersers often are forced to live in marginal habitats as inconspicuous ubordinates or as nonbreeders in flocks while awaiting a territory vacancy (e.g. Smith 1978, Carmen 1988, Smith and Arcese 1989). Why can the territories of cooperative breeders upport more individuals than a pair and their • Present address: Department of Biology, University of California, 405 Hilgard Drive, Los Angeles, California 90095, USA. E-mail: [email protected] most recent offspring? And given that auxiliaries usually are competent and motivated to breed when territory vacancies become available (Pruett-Jones and Lewis 1990, Komdeur 1992), why are the same numbers of individuals not distributed in smaller groups on smaller territories, resulting in a higher density of territories (and breeders) for the same density of birds? These questions are difficult to answer because a full accounting of the factors affecting group and territory size must include any active benefits of grouping (including kin-related benefits), costs that result from forming groups (such as resource depletion), the dispersion and density of resources in the landscape, population-wide demographic pressures, variance in territory-holding ability among competing individuals, and the economics of territorial defense in competition with other groups (Bradbury and Vehrencamp 1976, Parker and Knowlton 1980, Brown 1982, Pulliam and Caraco 1984, Waser 1988, Koenig et al. 1992). We measured the variation among territories in ecological characteristics, group size, foraging rates, and group reproductive success for a population of the White-throated Magpie-Jay (Calocitta formosa), a cooperative-breeding species in which group sizes are relatively large and all-