Breeding biology of the Eastern Great Reed Warbler, Acrocephalus arundinaceus orientalis

Abstract

1. This paper describes breeding biology of the Eastern Great Reed Warbler. Field work was carried out from 1962 to 1964 at Gamou coast near Sendai in the north-east Honshû, Japan. Study area consisted mainly of reed marsh with grassy and wooded area. The birds settled in reed marsh and use grassy and wooded area for feeding ground.2. The first date of spring arrival of male in different localities were discussed. At Gamou, spring arrival ranged from mid-April to early June, but most of the males arrived by mid-May. Males arrived at the study area in waves which coincided with warm waves in air temperature.3. Process of establishment of territory was described. Average size of territory decreased inversely with the increase of number of males and changed from about 2500m2. in early May to about 900m2 in early June when all males settled. As the species breeds at high density in the restricted reed marsh, it seems better to consider that the Great Reed Warbler breeds in grouped territories.4. The Great Reed Warbler is partial bigamy. At Gamou, 71.6 per cent of males were monogamous, 15.8 per cent bigamous and 12.6 per cent were unmated males. The earlier arrivals of males, the higher the pairing success. The average size of territory in unmated males was larger than that in mated males. It suggests that quality of territory may be important for pairing success.5. Egg-laying extended between mid-May and mid-July, with a peak in early June. Clutch-size ranged from two to six eggs and the commonest was five eggs. The number of young raised per nest was higher in clutch of 5 than the others. There were seasonal changes in average clutch and in number of yound raised per nest. These seasonal changes coincided with that in egg-laying.6. There were two kinds of reed marshes at the study area; Phragmites marsh and Rottboelia marsh. Pairing success of males and breeding success of pairs in the former were different from those in the latter. In the former, most of the males mated and some were bigamous. On the other hand, about a half of the males in the latter were unmated and the rest were monogamous. Similarly, the number of young raised per nest was considerably higher in the former than in the latter. Therefore, it is concluded that the Phragmites marsh is more favourable habitat than the Rottboelia marsh.