How White-throated Magpie-Jay Helpers contribute during breeding

We investigated the mechanisms by which helpers contribute to breeder repro duction in a Costa Rican population of White-throated Magpie-Jays (Calocitt a formosa). Helpers provided a substantial proportion of all feedings to fe male breeders and their offspring, proportionately more than most species o f cooperatively breeding New World jays. Breeding males typically fed breed ing 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 g roup size only during the pre-incubation period (provisioning of the laying female). Provisioning rates per nestling and per fledgling were not correl ated with group size, and the number of offspring fledged per successful ne st did not increase with group size. Helpers did reduce the provisioning bu rden on breeders, however, and occasionally were the primary care-providers of fledglings, which allowed breeders to renest. More successful nests wer e produced in groups with many helpers than few, resulting in more fledged young per year. Mechanisms contributing to this "helper-effect" included mo re nesting attempts per year and a higher likelihood of renesting after a s uccessful 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.