Abstract
The breeding habitats and the local population size of Japanese Marsh Warblers Locustella pryeri were surveyed annually from 2007 to 2014 in Watarase Marsh, Tochigi Prefecture. The warbler began breeding at the marsh in recent years, and the number of territorial males increased during the study period. Notably, the breeding population increased by eight to fifteen males after 2010, when large patches of reeds remained in the study site in March and April, instead of the reeds being removed by typical controlled burns. Two major factors are assumed to be related to the increasing warbler abundance in the study site: the general increase in the breeding populations at the major breeding sites in other parts of Japan, and the persistence of unburned reed beds in early spring, which is the crucial season for acquisition of territories by males. Large patches of older reed beds remained in the study site in 2010 due to wet weather, and in 2011 and 2012 due to the cancellation of the prescribed burning of the reeds. The vegetation in the study site was composed of tall monocotyledons such as the common reed Phragmites australis and the Japanese silver reed Miscanthus sacchariflorus. The reeds had a density of 16.2±9.1 stems / km2 at ground levle, and a height of 209.2 ± 32.7cm. The understory was composed of dense sedge cover. The warblers tended to occupy territories within a limited area of the study site, which suggests that the local population had not yet begun to disperse into habitat patches with lower quality.
