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, Costa Rica. Detailed information was gathered on 16 breeding flocks attending a total of 56 active nests. Auxiliaries were divided into three classes depending on their residency patterns and relative contributions to the breeding effort. The influence of auxiliaries was examined on eight measures of nesting success, and on five factors that influence reproductive outcome in this population: (1) onset of first laying, (2) predation, (3) hatching failure, (4) clutch desertion and (5) starvation. The results support three major conclusions. First, helpers directly enhanced nesting success at every stage in the nesting cycle. Second, all helpers did not help equally. Flock size alone bore no relationship 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 increased breeding success by hastening the onset of first laying, reducing predation 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-predator hypothesis and provide the first unequivocal evidence that helpers can enhance nest success by increasing hatchability. (C) 1996 The Association for the Study of Animal Behaviour
