Parentage and reproductive success in the white-throated magpie-jay, Calocitta formosa, a cooperative breeder with female helpers

I conducted a 3-year study investigating parentage in a Costa Rican population of white-throated magpie-jays, a cooperatively breeding corvid inhabiting the tropical dry forests of Central America. This species is unusual among avian cooperative breeders in that helpers are mostly females. Using 12 microsatellite loci, I estimated parentage for 110 offspring in 32 broods and eight social groups. Primary breeders were not the sole producers of offspring within a brood. At the 95% confidence level, female helpers within the group produced 16% of young, and floater males sired approximately 30% of young. Helpers that bred mated either with the primary male (three cases) or with floaters (five cases). When the analysis was limited to the most reliable data, the 20 nests and 63 offspring where all candidate mothers had been sampled, at the 95% confidence level only one helper produced young, while floater males sired 22% of offspring. The results of this study cast doubt on the simple classification of white-throated magpie-jays as cooperative breeders and suggest a more complex combination of cooperative and parasitic behaviours.