Intra-sexual Relationships among Polygynously Mated Female Great Reed Warblers Acrocephalus arundinaceus

Abstract

1) Relationships among females within harems of the polygynous Great Reed Warbler Acrocephalus arundinaceus were studied in Kahokugata, central Japan, during the 1980-87 breeding seasons.2) During the earlier half of the breeding season, secondary females were more likely to settle when primary females were in the laying or incubation stage rather than in the pre-laying stage. Secondary females laid their first eggs on an average 14.1 days after the first egg date of the primary female.3) Nests of harem mates were hyperdispersed within a territory, at an average distance of 21.0 m.4) Aggressions between an already-settled female and a new female were frequently observed during a period just after the settlement of the new female.5) Males sang at high rates when they were unmated and in the egg-laying and incubation stages of his mate. In the pre-laying stage, males sang less frequently and consorted more often with newly-acquired females.6) The separation of harem mates in time is thought to be induced mainly by (i) reduced singing rates of males during the pre-laying stage of their mates and (ii) the repulsion between harem mates. The latter may also induce the overdispersion of nests.