Improvement of reproductive performance with age and breeding experience depends on recruitment age in a long-lived seabird

Reproductive success increases with age in many species, however, the underlying mechanisms remain unclear and several hypotheses have been proposed to explain age-related improvements in reproductive output. In this contribution we investigated the effects of age, recruitment age, breeding experience and sex on reproductive performance during the early breeding career in the common tern Sterna hirundo using long-term individual-based data. We used measurements of performance, which spanned the entire breeding process: clutch size, hatching success, fledging success and fledglings per pair. Longitudinal analyses within individuals showed a clear increase with age in all performance measures. Furthermore, a significant change in reproductive performance was found between first time- and experienced breeders. Recruitment age had a strong influence on hatching and fledging success: two-year-old recruits had significantly lower reproductive success than birds which recruited at older ages, but the increase in breeding performance with experience was stronger in young recruits. Comparing age and experience effects, age effects were more pronounced during the first breeding attempts, whereas experience effects were also visible in subsequent breeding attempts. The degree of intra-individual improvements in reproductive performance is due to a complex interplay of age at first breeding and experience. The results strongly support the constraint hypothesis.