Flock Composition, Breeding Success, and Learning in the Brown Jay

Abstract

-Brown Jays are group breeders with helpers at the nest. In a montane population in Costa Rica, we found that flock composition by age class was highly variable. In this population the number of Old flock members predicted breeding success better than flock size. We suggest that experience may be important to the reproductive success of some cooperative breeders. This interpretation is supported by age-specific differences in nest attendance. Judged by the total number of feedings and the proportion of aborted feedings, the effectiveness of nest attendants increased with age. Further, Young birds improved significantly as nest attendants over one breeding season. To our knowledge, our findings offer the first quantitative support of Lack's hypotheses that young helpers are unlikely to breed successfully on their own and must learn to care for nestlings. The selective advantages of cooperative breeding in birds are often unclear. The significant positive correlation between the number of helpers in a flock and fledging success reported for a growing number of species (Brown 1978) implies that cooperative breeding is advantageous to breeding birds. In some cooperative breeders, however, the correlation between the number of helpers and fledging success is not significant (Zahavi 1974, Brown 1975); and in others the relationship approaches an asymptote at a given flock size (Woolfenden and Fitzpatrick, unpubl.). Brown (1978) has suggested that studies of the benefit from helpers should include examining other correlates of flock size, e.g., territory quality and/or flock composition (the demographic environment). What are the advantages to non-breeding helpers? Lack (1968) suggested that helping at the nest could evolve by natural selection if young birds, unlikely to breed successfully on their own, help raise nestlings to whom they are closely related. Helpers could be raising their inclusive fitness and also, by learning to care for nestlings, could be serving an apprenticeship that would improve their breeding success later. Field studies have shown that, in many cases, helpers are young and closely related to those they help (c.f., Alvarez 1975, Brown 1963, 1970, 1972, Woolfenden 1973, 1975, Woolfenden and Fitzpatrick 1977, 1978, Stallcup and Woolfenden, 1978). Data on the breeding success of young inexperienced birds are sparse (Woolfenden 1975: Table 2), and we know of no quantitative demonstration that young helpers do learn to care for nestlings. Testing hypotheses about learning and breeding success of young inexperienced birds in cooperatively breeding species is important for two reasons. First, if it can be shown that young birds are unlikely to breed successfully on their own and that they do learn to care for nestlings, then we may infer valid conclusions about the selective advantages of helping. Second, if it can be shown that the efficiency of helpers varies with age and sex, we can test Brown's hypothesis that the demographic environment, i.e., flock composition, is a better predictor than simple flock size of the effect of helping on breeding success. As part of a field investigation into the behavior of the Brown Jay (Psilorhinus morio), a cooperatively breeding species, we addressed the following questions: 1) To what extent do differences in breeding success between flocks reflect differences in age structure and/or flock size? We predicted that if experience is an important component of breeding success, then flocks with older members should produce more nestlings than flocks composed of young birds. 2) Do young flock members make more unsuccessful or aborted feeding trips than older birds? If so, does the frequency of aborted feeding trips change over the nestling period? If young birds are learning, they should make more unsuccessful or aborted