Abstract
Breeding ecology of the Short-tailed Bush Warbler Cettia squarneiceps was investigated in western Hokkaido during five seasons (1990-1994). The first arrival and first egg-laying dates were similar among years. Clutch sizes varied between 4-6 eggs, but they decreased as the breeding season progressed. The mean incubation and nestling period was 12.9±0.5 days (SD, n=18) and 10.0±0.4 days (SD, n=17), respectively. Only females incubated eggs and brooded nestlings for a long period of time. Most of the feeding of the young was conducted by males. Different (beta or gamma) males from the resident (alpha) male appeared and sometimes fed the young in 48.1% of twenty five nests observed. Some males were captured at locations far from their own nests, while there were only few cases in which a female was captured at a place distant from her mate's song area. As it was often observed that a beta male was the neighboring territory holder and his own nest was in the incubation period, it is hypothesized that some males who are emancipated from the incubation, might visit other territories to seek extra-pair copulations (EPC). We have, however, not observed mating between a female and a beta male except in the case of a widow. Within the breeding season, the females usually selected the same partner for the second brood. Main reason why the divorce rate within the season is low for the warbler in our study area is probably the high success rate for the first brood.
