A comparison of the annual cycles in testicular size and moult in captive European starlings Sturnus vulgaris during their first and second years

There is considerable evidence that reproductive success improves with age in birds. It is often suggested that improved performance of older birds is because they are more experienced. Less consideration has been given to the possibility that improvements may be a consequence of age-related changes in reproductive physiology. One factor that consistently changes with age is laying date - first year birds lay later than older birds. In European starlings Sturnus vulgaris, older males begin reproductive activity earlier than first year males and dominate available nest sites. I monitored changes in testicular volume in captive starlings exposed to natural changes in day length and temperature. from their first autumn through the next two annual cycles. Testicular maturation was advanced by 3-4 weeks in birds during their second year compared to the first. and testicular regression occurred about 2 weeks later. The period of full sexual maturation was 50% longer during the second year. The timing of the post-nuptial moult was the same. A possible physiological mechanism to explain this is discussed. The results show that earlier reproductive activity in older birds can be explained, at least in part, by intrinsic physiological mechanisms. This does not preclude additional effects of experience of an individual in the improvement in reproductive performance between first and second year birds.