Experimental analyses of intra- and intersexual competition in red-winged blackbirds

Abstract

The roles of intra- and intersexual selection in determining harem size for many polygynous birds have proved difficult to separate. We designed experiments to feed only one sex to distinguish between intra- and intersexual selection in polygynous red-winged blackbirds. We then tested whether choices made by females benefit females. Our experiments were conducted on a number of different marshes, so we could evaluate whether females were choosing males within marshes or whether they are choosing their breeding situation primarily by making choices between different marshes. Females choosing between marshes strongly preferred marshes with males that had supplemental food and experimentally increased display rates. At the marsh level, some male displays were predictors of larger harem size and of greater nesting success. But while female choice was very strong among marshes, we detected only weak female preference for individual male traits within marshes. These results suggest that problems of scale probably affect studies of mate choice in avian breeding systems in ways that, to the best of our knowledge, have not previously been investigated by behavioral ecologists. Key words: Agdants phoenictus, female preference, mate choice, red-winged blackbirds, sexual selection. [Bthav Ecol 8:524-533 (1997)] I n resource-based breeding systems, females simultaneously acquire males and the resources males defend. Thus it is difficult to assess the degree to which females base their choice of a breeding situation on male quality versus the quality of the resources that the male controls. Even when experiments successfully tease apart the effects that male attributes versus male resources have on female choice, interesting questions remain. When females choose resources, what specific resources are being chosen, and how do these resources affect reproductive success? When females choose males, what is the importance of male investment versus male genetic quality? Red-winged blackbirds, AgeUdus photnictus (hereafter redwings), have long been a model for the study of such questions. Most studies agree that territory quality influences female mate choice in redwings, but the role of male quality still generates considerable debate (reviewed in Searcy and Yasukawa, 1995). Two studies indicate that supplemental feeding increases harem sizes in redwings (Ewald and Rohwer, 1982; Wimberger, 1988). In each experiment, both sexes were fed, so neither distinguished whether females were responding to increased male displays or whether females were responding to the extra food. In an earlier study, we used feeders that fed either males or females, but not both, to investigate the effects of sexual selection on the evolution of male body size and found that, under certain experimental conditions, male redwings with longer wings and tails and greater rates of several displays had larger harems (Rohwer et ai, 1996). Yet such correlations cannot tell us if females directly prefer certain males or if those males have an advantage in male-male competition and therefore win larger territories, indirectly leading to larger harems.