Tests of hypotheses for the occurrence of polygyny in territorial birds using the red-winged blackbird (Agelaius phoeniceus).

Abstract

Several hypotheses have been proposed to explain the occurrence of polygyny in birds. The purpose of this study is to test those hypotheses using data from a marsh-nesting population of the red-winged blackbird (Agelaius phoeniceus). This study consists of two sections. The first section deals with theoretical aspects of the hypotheses and contains four chapters. I review all hypotheses and describe in detail their assumptions and predictions. I argue that the asynchronous settlement model cannot explain polygyny as stated and propose a new version of the model. I examine one assumption and four predictions of the sexy son hypothesis. Using examples, I demonstrate that the assumption and three predictions do not follow from the hypothesis. I argue that the the fourth prediction (mathematical model for the calculation of reproductive success of polygynous females) is flawed and propose a corrected version of the model. I review hypotheses that explain polygyny through "random" female settlement and propose a new ("status indifference") hypothesis. I conclude the first section by hierarchically classifying the plausible hypotheses by their key assumptions. The second section of this study presents the results of field experiments that tested the key assumptions. I tested female preferences for unmated or monogamously mated males by offering newly settling females a choice of two adjacent territories, one defended by an unmated male and the other by a monogamously mated male. Male mating status was randomized with respect to the variation in territory quality and male quality. Early in the breeding season, significantly more females settled with the unmated males than with the mated males. Although a similar trend was evident late in the breeding season, the difference was no longer significant. The consequences of polygamy on female reproductive success was then investigated. After randomizing harem size with respect to territory and male quality, I found that (1) monogamous females fledged as many young as bigamous females; (2) nestlings on monogamous and bigamous territories were provisioned equally often by both parents; (3) on bigamous territories, asynchronous broods were provisioned by males significantly more frequently than synchronous broods; (4) Ten-day-old female nestlings on monogamous territories were better developed than their bigamous peers. Finally, I establish whether females exhibit preferences for selected habitat characteristics. On a broad spatial scale (choice of a nesting area within a marsh), univariate analyses established that female selection of nesting areas was non-random with respect to six characteristics. However, a multivariate analysis revealed that females settle non-randomly only with respect to water depth. Females preferred nesting areas located in deep water to those in shallow water. On the narrow spatial scale (choice of a nest site within a nesting area), females preferred dense clumps of vegetation surrounded by sparse vegetation. (Abstract shortened by UMI.)