Evolutionary Rates and Host Defenses Against Avian Brood Parasitism

Abstract

Experiments investigating host defenses against the brood parasitism of the brown-headed cowbird revealed that, within most species, nearly all individuals either accept or reject cowbird eggs. Therefore, species are easily designated as "accepters" or "rejecters". The results of these experiments differ somewhat from data on natural cowbird parasitism, where some species seem to be variable in their response. However, much of what appears in natural cowbird parasitism to be variable expressions of host defenses are probably not responses specific to cowbird parasitism but are manifestations of standard avian behavior patterns. I hypothesize that few intermediate species exist, because host defenses are selected so strongly that the time span over which any species shows an intermediate rejection rate is of short duration. Hence, the number of species in transition from accepter to rejecter is always low. Most accepters did show low rates (less than 20%) of apparent rejection to experimental cowbird parasitism, but it is not likely that all these species are currently beginning the transition to rejecter species. Like most observed rejections of natural cowbird parasitism, these rejections by the accepters were probably not expressions of evolved host defenses but of behaviors related to other facets of breeding. The hypothesis that species rapidly become fixed for rejection is tested by a model that derives the selection coefficient for the two alternative character states of acceptance and rejection. Parameters in the model are the probability of being parasitized, P, and values for the reproductive outputs of parasitized anal unparasitized accepter and rejecter individuals. The model is difficult to apply to present-day rejecters. However, the selective value that rejection will have when and if it evolves in current accepter species can be calculated by assuming that, once a bird removes a cowbird egg, its eggs have the same probability of success as if a cowbird egg had never been present. Selection coefficients for hypothetical rejection behavior in the four best-studied accepter species range from .14 to .34. Making certain assumptions about the genetic determinants of rejection behavior and using these selection coefficients to calculate rates of gene substitution results in a range of 20-100 years as the time necessary for these species to change from accepters to rejecters. Features of the present-day rejecter species indicate that the selection coefficients in these species could have been high enough to have resulted in similarly rapid rates of gene substitution. The high selection coefficients calculated and the rapid rates of gene substitution estimated support the hypothesis explaining the scarcity of species with intermediate rates of rejection of cowbird eggs.