WEBL Nest Measurements

We examined the fitness consequences of helping behavior in the western bluebird (Sialia mexicana) at Hastings Reservation in Carmel Valley, California, USA, and tested hypotheses for how helpers benefit from engaging in alloparental behavior. Both juvenile and adult western bluebirds occasionally help at the nest. During a 12 year period, all adult helpers and most juvenile helpers were male. Helpers usually fed at nests of both their parents and rarely helped when only one parent was present. The frequency of pairs with adult helpers was only 7 percent, but nearly one-third of adult males helped among those with both parents on the study area. At least 28 percent were breeders whose nests failed. The propensity to help appears to depend upon parental survival, male philopatry, and the breeding success of potential helpers. Feeding rates were not increased at nests with juvenile helpers, apparently because breeding males reduced their feeding rates. In contrast, adult helpers increased the overall rates of food delivery to the nest in spite of a reduction in the number of feeding trips made by both male and female parents. Helpers did not derive any obvious direct fitness benefits from helping, but they had greater indirect fitness than nonhelpers due to increases in nestling growth rates and fledging success at their parents' nests. Helpers fledged fewer offspring in their first nests than did nonhelpers, suggesting that they were birds with reduced reproductive potential. Although we have not yet measured the effect of extrapair fertilizations on the fitness benefits of helping, we calculated the difference in fitness between helpers and nonhelpers as a function of the potential helper's paternity when breeding independently and his father's paternity in the nest at which he might help. In conjunction with constraints on breeding and indirect fitness benefits, we predict that relatedness of males to the young in their own as well as their parents' nests will influence helping behavior in western bluebirds.