1. On the basis of juvenile:adult ratios among netted birds and studies of moult, inferences are drawn about seasonal and annual variation in the breeding output of a bullfinch population studied near Oxford, in southern England.
2. In most years during 1962–66, the juvenile:adult ratio among netted birds increased from July or August to October, and then declined. The ratio in October, taken as a measure of annual productivity, varied between 2·8 and 5·5 in different years.
3. In the Oxford area, egg-laying among bullfinches occurred between mid-April and late August, so that the last young of the year left the nest in late September/early October. However, not all pairs continued breeding this late in the year. Individual bullfinches started to moult when they finished breeding. The proportion of adults which started moult after 20 August (implying successful breeding from eggs laid after mid-July) varied between 7 and 68% in different years (or between 19 and 63% using a different method of estimation). Juveniles from these late nests, identified as those starting moult after 31 August, formed 24–61% of the total juvenile population in different years, the rest having been produced from clutches begun before mid-July.
4. In the 2 years when the greatest proportions of adults started to moult after 20 August and the greatest proportion of juveniles started after 31 August, the juvenile:adult ratios in October were highest. By implication, the two-fold variations in overall productivity between years were associated primarily with variations in the amount of late breeding.
5. This study, based on moults and age-ratios among netted birds, provided information on seasonal productivity that was practically impossible to obtain in this species by conventional nest finding. An implication was that an interaction between the seasonal pattern of breeding in particular years and a seasonal trend in predation on nest contents, had major effects on overall annual productivity.