Abstract
We observed the breeding behaviour of the Yellow Bittern Ixobrychus sinensis at the Arakawa River, Saitama, Japan, during 1986-90. The area consists of paddy fields, reed beds, and cattail swamps.
1) Nests were built in swamps with the cattail Phragmites communis and bulrush Scripus lacustris, and in reed beds. Three to thirteen nests were found in the cattail swamp gregariously, while the nests in the reed bed were built solitarily.
2) The mean distance between the gregarious nests in the cattail swamp was 6.7 m; the nearest neighbouring nests was 2.5 m apart in 1990 when 12 nests were built simultaneously.
3) Egg laying started at early June, peaked at mid-June, and lasted to mid August. The clutch size was 5.95 (4-7 eggs). The incubation period was 18.7 days in average (17-20 days). Nestlings stayed in the nest from 5 to 8 days after hatching; thereafter, they temorarily leaved the nest when the observer aproached the nests. The nestlings never stayed at the nest after 20 days old.
4) The critical factor for nest site selection is water level under the nest. There were few nests in the reed bed, because of low water level.
5) The colonial nesting in the Yellow Bittern seemed to be induced by the shortage of good habitat free from predators.
