BREEDING ECOLOGY OF THE AQUATIC WARBLER ACROCEPHALUS-PALUDICOLA ON THE BIEBRZA MARSHES, NORTHEAST POLAND

Aquatic Warblers Acrocephalus paludicola were studied in a natural fen mire in the Biebrza River valley, the main breeding ground of the spe cies in its entire distribution range. The number of males present and singing at the sample plot changed considerably during the breeding s eason. Individual identified colour-ringed males sang with unchanged i ntensity throughout the breeding season. The period of daily singing a ctivity differed from other species of Acrocephalus: males sang at dus k rather than at dawn. Males took no part in rearing nestlings but rem ained on territory and showed vigilance behaviour. The density of male s in the breeding season ranged from 1.0 to 10.9 per 10 ha. The densit y of nesting females ranged from 1. 3 to 1 5.7 per 10 ha. In the most suitable habitat females were more numerous than males. The distributi on of females (nests) was clumped where potential food resources were higher. Nests were well hidden in places with deeper water between sed ge tufts and an abundance of old dry sedge. Females feeding nestlings collected most food within a 5-60-m radius (mean 31.7 m). The return r ate of males was higher than that of females. The results suggest a ma ting system that is intermediate between facultative polygyny and prom iscuity.