The social system of an individually-marked population of the fan-tailed warbler Cisticola juncidis was studied in Japan, over six breeding seasons from 1978 to 1983. More than 127 males established territories, some 50–70% were polygynists each year. Territorial males were replaced frequently within seasons. Females were less faithful than males to their first breeding sites. Perennial or seasonal pair bonds were rare, maintained over two successive breeding attempts, by only 13.6% of females. Half the females left the area after one breeding attempt. Frequent divorce, rapid and multiple remating of females, multiple breedings, and female movement over a wide area all combine to skew breeding sex ratio from unity and favor polygyny.