PALACIOS, M.G.*; MARTIN, T.E.; Iowa State University, Ames; University of Montana, Missoula: Parasites, predators, and the evolution of incubation period in birds.

One longstanding question regarding life history evolution is why organisms vary so dramatically in the time required for development. Avian incubation provides an ideal model to study variation in developmental periods because incubation periods vary nearly an order of magnitude among species, and several interesting factors have been proposed to potentially explain this variation. However, the relative importance of these factors is not fully understood. Particularly interesting is the possibility that the length of the incubation period could be determined by interaction of antagonistic selective forces: 1) juvenile mortality in the form of nest predation is thought to favor fast development, whereas 2) parasites potentially favor long incubation periods to allow development of a better immune system for fighting disease. We tested these ideas through a comparative field study of incubation periods, nest predation, parasite infections, and immunocompetence among 14 coexisting passerine bird species. Species facing higher rates of nest predation tended to have shorter incubation periods, supporting the role of nest predation in accelerating development. Prevalence of blood parasites was positively associated with cellular immune response across species, suggesting that species facing greater selection pressure from parasites invest more in immunocompetence. However, blood parasite prevalence was not related to length of the incubation period and, contrary to expectations, immune response showed a negative relationship with incubation period. Therefore, we found no support for the hypothesis that selection from parasites favors longer incubation periods and counter-balances selection from nest predation for faster development.