Age at first breeding and fitness in goshawk Accipiter gentilis
Oliver Krüger, Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, UK. Tel.: +44 01223 336610; Fax: +44 01223336676; E-mail: firstname.lastname@example.org
- 1Age 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 lind as a fitness estimate for 74 female goshawks, I showed that the optimal age at first reproduction was 3 years in this population.
- 2Females that started to breed earlier had lower LRS and lind, not because of reduced life span, but because of lower reproduction at early ages.
- 3The constraint hypothesis, which states that foraging or other skills improve with age was the most likely explanation for the higher reproduction with increasing age.
- 4Incorporating 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.
- 5Population 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.
- 6The 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.