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 territor y area, resource levels, resource dispersion, group size, and group re productive success for 14 groups over a three-year period in Costa Ric a. Larger groups possessed larger territories containing more bull-hor n acacia trees (Acacia cornigera and A. collinsii), which provided a c ritical food resource during the dry season. On a per capita basis, ho wever, 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 amon g groups was influenced by two factors: (1) larger groups produced mor e successful nests per year, and (2) territories with a higher density of acacia trees fledged more offspring per successful nest. Magpie-ja ys bred in pasture and foraged primarily in woodland; both habitat typ es were patchily distributed. The dispersion of woodland and pasture p atches in the landscape appeared to constrain where jays could form te rritories, whereas the amount of acacia and other food resources deter mined the size that groups could attain. We conclude that ecological f actors are critical to understanding the White-throated Magpie-Jay soc ial system, along with other social and demographic constraints typica lly found in avian cooperative breeders.