A test of the importance of direct and indirect fitness benefits for helping decisions in western bluebirds

Abstract

In cooperative breeders, the extent to which helpers at the nest adjust their contributions in accordance with direct and indirect (kin-selected) fitness payoffs remains an open question. In a long-term study of the western bluebird, Sialia mexicana, helpers were exclusively male and helped at nests of both parents, a parent and stepparent, or a brother and unrelated female. This natural variation in the context of helping facilitated comparison of observational data on groups in which one type of fitness benefit (current direct, future direct, or indirect) varied, whereas the other two were constant. Helpers reduced their share of provisioning as they got older, so comparisons were restricted to groups with yearling helpers. When potential direct fitness benefits were identical, but relatedness was reduced by half owing to the presence of a stepparent, yearling helpers failed to reduce their share of feeding trips to the nest. The potential for future direct fitness benefits via possible mate and territory inheritance was low, and did not influence the helper’s share of provisioning in a comparison of groups with similar relatedness and opportunities for current direct fitness benefits. Even though cobreeding to gain current direct fitness benefits was infrequent (17% of nests with brother-helpers), it was associated with an increase in the helper’s share of provisioning, suggesting that a helper’s feeding allocation responds positively to increased opportunity for parentage in the nest. The current study demonstrates a useful framework for separating direct and indirect benefits with respect to helping decisions, and indicates that western bluebird helpers adjust their feeding rates in response to the potential for direct fitness benefits in the current nest, not indirect benefits or future direct fitness payoffs. Although past studies of this population showed that indirect benefits play a role in whether or not helpers help, the current study indicates that they do not play a role in how frequently helpers feed at the nest.