The risk of breeding failure often varies strongly over the breeding season. If birds optimize production of offspring accordingly, investment in reproduction should be lowest when food for young is scarce or nest predation risk is highest. Evidence for this hypothesis is mixed and stems mainly from studies on northern temperate birds, and it is unclear whether these patterns apply to southern temperate birds with “slow” life histories that resemble those of tropical birds. We studied seasonal variation in reproductive investment and nesting success in a Neotropical temperate suboscine, the Firewood-gatherer (Anumbius annumbi). Pairs produced an average of 2.8 clutches over the very long (August–April) breeding season. Clutch size (mean = 5.1 eggs, range: 3–8) was very variable between and within pairs and showed an initial increase followed by a decline, the typical pattern of variation for multibrooded temperate species. The main determinant of breeding success was predation. Although the probability of nest failure increased over the season from 70% to >90%, clutch size was not correlated with failure risk. Egg volume did not show consistent seasonal patterns. Our results suggest that reproductive investment is not optimally adjusted to the seasonal variation in nest predation risk, and more research is needed to assess the influence of food availability.