Abstract
1) Interspecific relationships between sympatric Great and Schrenck's reed warblers Acrocephalus arundinaceus and A. bistrigiceps were studied at Sugadaira (36°20′N, 138°20′E), central Honshu, during the breeding seasons in 1982-87. An intensive observation on the breeding schedule, home range and interspecific behavior was made in 1987.
2) Great Reed Warblers usually arrived at the breeding ground earlier than Schrenck's Reed Warblers. The earliest arrivals were male Great Reed Warblers, followed by female Great, male Schrenck's, and finally female Schrenck's reed warblers.
3) The mean staying period of mated males in the study area was about 60 days for both species. The home ranges of Schrenck's Reed Warblers were unstable as compared with those of Great Reed Warblers; Schrenck's often changed their home ranges.
4) Breeding success (fledglings/egg laid) in the study area was 40.4% for Great and 61.1% Schrenck's reed warblers in 1987.
5) In establishing home ranges the two species were segregated by time and space. Great Reed Warblers usually establish their home ranges earlier than Schrenck's. Schrenck's Reed Warblers had their home ranges either in the neutral area among the treat's home ranges or in the area distant from them.
6) The habitat preferred by the two species differed horizontally, although both species were overlapped in their preference for reed beds. Mean nest height was 1.02 m for Great and 0.63 m for Schrenck's reed warblers.
7) Three types of territorial behavior were observed: counter singing, chasing, and fighting. In Great Reed Warblers counter singing was noticeable. Great Reed Warblers often preferred tall trees for singing post, while Schrenck's selected reed stem for it.
8) Sizes of home ranges and singing areas of Great Reed Warblers were considerably larger than those of Schrenck's. Some males of Schrenck's Reed Warblers travelled from their home ranges to further places and sometimes established a second home range.
9) Territorial Great Reed Warblers were always dominant against intruding Schrenck's, whereas the latter could not successfully chased out the intruded Great Reed Warblers.
10) The two sympatric reed warbler species enabled to avoid competition not by interspecific territory, but by different time schedule for breeding and establishing home ranges. Schrenck's Reed Warblers had their home ranges in the neutral area between neighboring territorial males of Great Reed Warblers and made their breeding successful by non-territorial sneaking behavior
