Avian reproduction occurs either in seasonal bursts prompted by seasonally predictable cues, or opportunistically when environmental conditions allow. For opportunistic breeders, the general environmental cues underlying the initiation of a reproductive attempt are well recognised. However, the factors underlying variation between individuals in the timing and the scale of their reproductive investment are much less understood. We hypothesised that individuals which are in better physiological condition would initiate breeding earlier than those in poorer condition and that consequently, across a breeding season these individuals would have higher reproductive success. We tested this in the zebra finch (Taeniopygia guttata) by repeatedly measuring physiological and behavioural traits in a group of males, before releasing these birds into an open aviary at the start of a breeding season. We then monitored the initiation of reproductive investments across the following breeding season and reproductive output. Whilst some individuals failed to breed at all, others produced up to four broods of offspring over the course of the 5month breeding season. There was little relationship between any of the physiological traits measured and overall reproductive output over the season. This suggests that the factors determining the initiation of breeding at the individual level are subtle. Although we did not identify any key physiological or behavioural traits which predict seasonal reproductive success, clearly both environmental cues and individual physiological state are important in controlling initiation of breeding of the group and individual fitness.