HOW WHITE-THROATED MAGPIE-JAY HELPERS CONTRIBUTE DURING BREEDING

摘要: We investigated the mechanisms by which helpers contribute to breeder re- production in a Costa Rican population of White-throated Magpie-Jays (Calocitta formosa). Helpers provided substantial proportion all feedings female breeders and their off- spring, proportionately more than most species cooperatively breeding New World jays. Breeding males typically fed females offspring less frequently expected, however. There was little evidence brood division sense individual provisioners (breeders or helpers) preferentially feeding particular fledglings within brood. The rate provisioning per recipient increased as function group size only during pre-incu- bation period (provisioning laying female). Provisioning rates nestling fledgling were not correlated with size, number fledged suc- cessful nest did increase size. reduce burden on breeders, however, occasionally primary care-providers fledglings, allowed renest. More successful nests produced groups many help- ers few, resulting young year. Mechanisms contributing this "helper-effect" included nesting attempts year higher likelihood renesting after attempt. conclude that contributions magpie- jay in- creased fitness, indirect direct benefits gained helping probably fa- vored its expression nonbreeding members. Received 30 October 1997, accepted 18 June 1998.