Defenses of red-winged blackbirds against brood parasites and predators: The acquisition of threat recognition and the dynamics of group defense.

摘要

Brood parasites and their hosts represent a coevolutionary relationship, where each member has evolved characteristics to stay one step ahead of the other. One of those characters, host defense behaviors, and how they are acquired by individuals, has been the focus of my research.; Chapter 1 is a review of the brood parasites, their diversity, host selection, and adaptations.; Chapters 2 and 3 are reports of my research on the defenses of red-winged blackbirds (Agelaius phonecious) against taxidermic mounts of brown-headed cowbirds (Molothrus ater), a generalist parasite, common grackles (Quiscalus quiscala), predators of eggs and small nestlings, song sparrows (Melospiza melodia), a non-threatening species, and a blank perch. The goal of my research was to determine the roles of learned and innate factors in the acquisition of defense behaviors among redwings.; I studied the defense behaviors of three redwing populations, Wisconsin, New York, and Kentucky. Through cowbird censuses and knowledge of the redwing nesting history for many years prior to the experiments, I concluded that the Wisconsin and New York populations did not differ in the density of cowbirds present at the sites, but had dissimilar parasitism rates.; Among the populations I worked with, the two that were sympatric with cowbirds had redwings that showed significantly greater rates of defenses toward cowbirds than those of the population that was not sympatric with cowbirds. Differences in defenses between these sympatric and allopatric populations are explained by the differences in parasite and predator pressure the populations have faced. I concluded that redwing defenses toward cowbirds and grackles may still be heritable, but the behaviors are not species typical. These defenses are more population-specific and may involve learning.; In order to test if learning does play a part in the acquisition of redwing defense behaviors, I compared the defense behaviors between age classes within each population. Differences were seen in defense behaviors between age classes for both male and female redwings. These changes indicate that some learning is taking place during the lifetime of redwings.; The greater rates of defense behaviors by younger females provide evidence that predator and parasite recognition by females is innate within the sympatric populations of this study.; Chapter 4 reports on an experiment that determines the effects of individual differences on territory selection and the likelihood of participation in group-level defenses, by investigating some of the other variables that influence defensive reactions. I found that only threat type and proximity to the threat were consistent predictors of whether an individual defended within their own territory and participated in group defense. Participation in cooperative defenses was not dependent on the numbers of available neighbors. Individuals that chose territory sites with few neighbors typically had the same percentage of neighbors respond to threats within their territories as birds that had larger neighbor groups surrounding them. (Abstract shortened by UMI.)