COOPERATIVE BREEDING IN THE WHITE-THROATED MAGPIE-JAY - HOW DO AUXILIARIES INFLUENCE NESTING SUCCESS

Two fundamental questions in the study of cooperatively breeding birds are (1) do auxiliaries increase the reproductive success of a flock, and (2) if so, how? This paper addresses these questions in the white- throated magpie-jay, Calocitta formosa. This social corvid was studied for three consecutive breeding seasons in Santa Rosa National Park, C osta Rica. Detailed information was gathered on 16 breeding flocks att ending a total of 56 active nests. Auxiliaries were divided into three classes depending on their residency patterns and relative contributi ons to the breeding effort. The influence of auxiliaries was examined on eight measures of nesting success, and on five factors that influen ce reproductive outcome in this population: (1) onset of first laying, (2) predation, (3) hatching failure, (4) clutch desertion and (5) sta rvation. The results support three major conclusions. First, helpers d irectly enhanced nesting success at every stage in the nesting cycle. Second, all helpers did not help equally. Flock size alone bore no rel ationship to nesting success, but both the number of full-time helpers and helper score (a weighted measure of auxiliary contributions) were significantly related to reproductive outcome. Third, helpers increas ed breeding success by hastening the onset of first laying, reducing p redation on eggs and chicks, and decreasing hatching failure; helpers did not affect the incidence of clutch desertion or chick starvation. These findings provide strong quantitative support for the anti-predat or hypothesis and provide the first unequivocal evidence that helpers can enhance nest success by increasing hatchability. (C) 1996 The Asso ciation for the Study of Animal Behaviour