DISSERTATION RESEARCH: Interspecific Competition Increases Cooperative Group Sizes in Brown-headed Nuthatches (Sitta pusilla).

Cornell University, Ithaca NY

Investigators

Abstract

Brown-headed nuthatches live and breed in family groups. These family groups appear to work together to defend nesting holes from other species that also need holes in trees to nest. Since nuthatches are small birds (10 g), they often lose their nests to larger species if they do not cooperate. This project seeks to understand if cooperative defense of nesting holes by brown-headed nuthatches increases the number of offspring they contribute to the next generation. Cooperative breeders often cooperate with relatives because outside opportunities for breeding are rare, and it is unclear whether this cooperation is an adaptive strategy or whether the birds are simply making the best of a bad situation. This project addresses those alternatives by testing whether cooperating nuthatches actually have more offspring than non-cooperators. This project therefore could uncover a potential new benefit of cooperative breeding, and it also provides information on the natural history of brown-headed nuthatches, a species facing the dual threats of global warming and urbanization. This project has already increased local population sizes of brown-headed nuthatches, and the Co-Principal Investigator will continue to participate in conservation efforts for this species by speaking informally at a farmer's market booth and encouraging locals to place nuthatch-sized boxes out on their property. This project tests the previously untested idea that interspecific competition functions as an important selective force on group size in the Brown-headed nuthatch (BHNU, Sitta pusilla). The key hypothesis is that competition among equally sized competitors influences the benefits of cooperative breeding and selects for larger group sizes. By uncovering how competition with other species impacts inclusive fitness for group members under differing competitive pressures, and at various group sizes, this project tests whether heterospecific competition can increase the fitness of BHNUs in groups, and thus select for increased cooperative group sizes. A traditional hypothesis for the existence of cooperative breeding is that ecological constraints prevent independent breeding and drive increases in group size, predicting that the inclusive fitness of individuals will decrease as group sizes increase. However, the motivating hypothesis of this project, that fitness increases as group size increases through helper efficacy in heterospecific defense, predicts exactly the opposite pattern. Genetic parentage determination, and the calculation of inclusive fitness, will allow this research to distinguish between ecological constraints and heterospecific defense benefits as drivers of increased group size in the cooperatively breeding BHNU.