Lack of Pair-bond: a Polygynous Mating System of the Japanese Bush Warbler Cettia diphone

Abstract

1) Mating system of individually marked Japanese Bush Warblers Cettia diphone was studied at Myokokogen (36°52′N, 138°11′E, alt. 700m), central Japan, during the 1990-91 breeding seasons. 2) Territories were maintained for short periods. Although 35 territorial males were confirmed within a census area in a breeding season, only 10-14 (x=11.7, n=9) of them were observed on each survey. 3) Intrusions into territories by non-residential males were observed frequently. These intruders never sang there. In M98's territory, 15 intrusions by floating males and five intrusions by territorial males were observed.4) In three territories, multiple females were observed breeding. In M98's territory, seven breeding attempts were made by 6 or 7 females within a season. 5) Females built nests and took the full share of parental care, including incubation, feeding nestlings, rearing fledglings, and mobbing predators. 6) Breeding failures, which might have been caused by predation, were observed frequently. There was a trend for females to leave a territory after one breeding attempt. 7) At least 21 females appeared in M98's territory, which suggests the presence of many potential mates for the male.8) Home ranges of breeding females were much smaller than male territories. Female ranges seemed to be restricted to within male territories. The ranges of females nesting in the same territories overlapped. Agonistic behavior between them was not observed. 9) Singing activity of males was vigorous throughout the breeding season irrespective of the nesting stages of breeding females. No contacts between males and nesting females, including escorting behavior of females by males, were observed.10) These results strongly suggested that the mating system of the Japanese Bush Warbler was advanced polygyny characterized by lack of pair-bonds.