Helper Effects on Variance Components of Fitness in the Cooperatively Breeding Red-Cockaded Woodpecker

Abstract

Studies of how avian helpers influence reproduction have concentrated on mean effects. We examined the effects helpers have on variance in reproductive success and breeder survival in the Red-cockaded Woodpecker (Picoides borealis). Variance in reproduction and survival are components of fitness, and increased variance results in decreased fitness. We found that helpers did not affect variance in breeder survival, but variance in reproductive success was affected by helpers at nests that produced young. When we exclude groups in which breeding failed or was not attempted, reproductive success is more variable among groups with one helper than among groups with no helpers or groups with two helpers, regardless of the age of the female breeder. A survey of the literature showed no clear pattern in other cooperative breeders, although the presence of helpers often was associated with higher variances. Like helper effects on mean reproductive success, helper effects on variance in reproductive success might be confounded by effects of habitat quality. Received 24 April 1995, accepted 6 September 1995.