FEMALE CHOICE AND THE CAUSES AND ADAPTIVENESS OF POLYGYNY IN GREAT REED WARBLERS

In a dense reedbed at Lake Biwa in Japan 30-80% of male Acrocephalus arundinaceus were polygynous each year; others remained monogamous or even unmated throughout the breeding season. Males that had established territories by mid-May (early males) were more likely to be polygynous than those that established territories after mid-May (late males). Second females were attracted to the territories of early males, even though unmated late males held territories nearby. Many late males attracted mates when all the first mates of early males either had not yet laid or had chicks. At these stages, males do not sing actively and hence did not attract females. Already mated early males attracted as many or more females as unmated late males as long as they were singing. Early males held large territories early in the breeding season and late males established their territories by carving out a part of the former's territory. Territories of early males always included an area of dense vegetation which was never yielded to later arrivals. Polygynously mated females bred at a higher nesting density and their males fed the chicks at variable rates, but 2nd females of early males fledged as may young as simultaneously nesting females mated with late males. Thus, polygyny was also adaptive to the female. Predation on eggs and chicks of females of late males was heavy, probably owing to the poorer nest cover in their territories. -from Author