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

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)] In resource-based breeding systems, females simultaneouslyacquire 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 qual-ity of the resources that the male controls. Even when exper