Territory and breeding density in the Eastern Great Reed Warbler, Acrocephalus arundinaceus orientalis

This paper describes territory and its function on the limiting of breeding density of the Great Reed Warbler in the favourable habitat. Field work was carried out at Gamou coast near Sendai in the north-east Honshû, Japan.2. The reed marsh was divided into two kinds in terms of species of reed; Phragmites communis marsh and Rottboelia latifolia marsh. In the Phragmites marsh, breeding success, including pairing success of males, and breeding density were higher than in the Rottboelia marsh. Hence the former was found to be favourable habitat.3. Reed marshes were not occupied by a number of males all at once, but were filled up gradually from late April to early June. There were three phases in the process of settlement of males; early migrants settled exclusively in the Phragmites marsh (first phase) and then new-comers settled in either marsh (second phase). Finally, the Rottboelia marsh was occupied by later arrivals only after the density in the Phragmites marsh had reached to the highest level (third phase).4. In the Phragmites marsh, the average size of territory decreased inversely with the increase of the density until the end of the second phase and did not decreased any more in the third phase.5. Intruders were found until the middle of June. The number of intruders reached a peak in the late half of the second phase and there were still several intruders in the Phragmites marsh during the third phase, although they could not settle there.6. The males defended its territory against intruders by means of chasings. The frequency of chasings was proportional to the number of the intruders. However, the chasing behaviour in the Phragmites marsh was more vigorous in the third phase than in the other phases.7. Therefore, it was suggested that the average size of territory in the Phragmites marsh decreased to a limit to which territory can be compressed, thus later arrivals were forced to settle in the Rottboelia marsh by the territorial behaviour of males that already settled in the Phragmites marsh.8. It is concluded that territorial behaviour limits the breeding density of the Great Reed Warbler in the favourable habitat.