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 successfunel 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 typically found in avianc ooperative breeders.