Cooperative Enhancement Of Reproductive Success in Tropical Wren Societies

Stripe—backed Wrens, Campylorhynchus nuchalis, live in cooperative groups of up to 14 adults in which all members share in defense of the common territory, in construction and defense of roosting and breeding nests, and in the care of young produced by a single breeding pair. A 5—yr study of a completely marked population of 25—30 groups in the Venezuelan savanna shows that breeders (principals) assisted by two or more nonreproductive auxiliaries have much greater reproductive success than those with less help. Per capita production of independent juveniles in higher, as well. Only groups with at least two auxiliaries successfully rear two clutches in a season. The proportion of nesting attempts successful in producing fledglings is much higher for groups of four adults or more than for smaller groups, probably because of lesser predation on the nests of large groups. The success of large groups is not likely due to differences in territory quality of breeder experience. Rates of food delivery to nestlings and number of fledglings per successful nest are as high for pairs and trios as for larger groups. However, feeding of nestlings and fledglings by auxiliaries frees the principals from these tasks and probably makes multiple successful clutches possible. Auxiliaries directly defend the breeding nest from predators and competitors, although the principle male is most active in defense. Feeding efforts of auxiliaries are probably linked to increased nesting success primarily through enabling the principal male to defend the nest better. Aid—giving by auxiliaries is explained (1) by its effectiveness in improving the reproductive success of close kin through reducing risk of predation for nestlings, (2) by the probability of reciprocation of such effective help, and (3) by the low productivity of breeding outside a large established group. Data from this population of wrens constitute some of the strongest evidence to data that group defense against predators can provide the impetus for cooperative breeding.