Effects of Helpers on Nestling Care and Breeder Survival in Pygmy Nuthatches

Abstract

I studied nestling care in the cooperative-breeding Pygmy Nuthatch (Sitta pygmaea) in 1983 and 1984 to determine how helper participation in providing for nestlings affected breeding success and parental survival. Breeding in 1984 was about 3 weeks earlier than 1983. Mothers and helpers contributed equally, but significantly less than fathers in provisioning the nestlings. Between-nest differences in the onset of breeding determined most of the variation in nestling feeding rates; nests initiated early in the breeding season received more frequent feeding visits. Enhanced reproductive success by breeding units with helpers (Sydeman et al. 1988) was not related to increased feeding rates to the nestlings. Total feeding rates were no greater when helpers supplemented the feeding visits of both parents. At nests with helpers, however, parents and helpers were at the nest cavity more often. The feeding contribution made by helpers proved more beneficial to parents than nestlings, allowing parents to reduce feeding frequency, and presumably energy expenditure. However, this benefit did not lead to increased breeder survival.