How White-throated Magpie-Jay Helpers contribute during breeding

We investigated the mechanisms by which helpers contribute to breeder reproduction in a Costa Rican population of White-throated Magpie-Jays (Calocitta formosa). Helpers provided a substantial proportion of all feedings to female breeders and their offspring, proportionately more than most species of cooperatively breeding New World jays. Breeding males typically fed breeding females and offspring less frequently than expected, however. There was little evidence of brood division in the sense of individual provisioners (breeders or helpers) preferentially feeding particular fledglings within a brood. The rate of provisioning per recipient increased as a function of group size only during the pre-incubation period (provisioning of the laying female). Provisioning rates per nestling and per fledgling were not correlated with group size, and the number of offspring fledged per successful nest did not increase with group size. Helpers did reduce the provisioning burden on breeders, however, and occasionally were the primary care-providers of fledglings, which allowed breeders to renest. More successful nests were produced in groups with many helpers than few, resulting in more fledged young per year. Mechanisms contributing to this "helper-effect" included more nesting attempts per year and a higher likelihood of renesting after a successful attempt. We conclude that the contributions of magpie-jay helpers increased breeder fitness, and the indirect and direct benefits gained by helping probably favored its expression by nonbreeding group members.