Age at first breeding and fitness in goshawk Accipiter gentilis

1. Age at first breeding has a large influence on fitness and hence is crucial to the evolution of life-history strategies. Goshawks Accipiter gentilis start breeding aged 1-4 years. Using 30 years of data and both lifetime reproductive success (LRS) and l(ind) as a fitness estimate for 74 female goshawks, I showed that the optimal age at first reproduction was 3 years in this population. 2. Females that started to breed earlier had lower LRS and l(ind), not because of reduced life span, but because of lower reproduction at early ages. 3. The constraint hypothesis, which states that foraging or other skills improve with age was the most likely explanation for the higher reproduction with increasing age. 4. Incorporating habitat heterogeneity provided the mechanism that explained not only the fitness cost to early maturity, but also why this cost was heterogeneous. Females starting to breed aged 1 suffered a very high fitness cost if they were in a bad-quality territory, but fitness costs were small when they were in a good-quality territory. This explains why I found evidence for a nonlinear selection pressure on age at first breeding. 5. Population density also affected whether a female started to breed early or not: over the study period, population density increased and the percentage of females starting to breed aged 1 decreased. 6. The optimal age at first breeding seems to be a trait affected by a complex interplay between cost and benefits of early reproduction mediated by habitat heterogeneity and population density.