Research

Common terns are long-lived migratory seabirds. Using data from a long-term study population located in the Banter See at Wilhelmshaven on the German North Sea coast, initiated by Prof. Peter Becker, my co-authors and I investigated the age-specificity of various traits. We found that while survival probability monotonously declined with age, various phenological and reproductive traits gradually improved with age, before levelling off. These age-related improvements were mostly due to within-individual changes in trait expression, and while effects of selective appearance and disappearance provided evidence for quality differences between individuals, these effects were small. Correcting for changes in trait expression with age, we found within-individual variation in phenology and reproductive success to be associated with mortality risk. Specifically, we found that if individual birds arrived and bred earlier than they did on average and laid larger clutches and eggs than they did on average, their mortality risk increased. We therefore showed that when quality differences between birds are accounted for, survival costs of early and successful reproduction can be observed. Finally, we combined performance measures to calculate age-specific reproductive values, and found evidence for senescence. The fitness cost of this senescence was, at ~20%, slightly higher than reported for other bird species.

Great tits are small, short-lived passerines that readily breed in nest boxes. Within the context of my PhD project, I used a longitudinal analysis approach and a 49-year data set of breeding attempts by female great tits inhabiting Wytham Woods, Oxfordshire, UK to study age-specific performance. With my co-authors, I provided evidence for three aspects of senescence: (i) an increased likelihood of skipped breeding, or complete early nest/brood failure, (ii) reduced local recruit production due to increasing declines in hatching success, fledging success and post-fledging survival of offspring, and (iii) a decline in adult local survival probability. Combined, these senescence effects caused a deterioration of 87% in female reproductive value between the age of 1 and 9 years, although the total fitness cost in terms of reproductive value at the onset of reproductive life at age 1 was only 4%. Further work suggested patterns of age-specific reproduction to be robust to environmental and life-history variation between the Wytham population and a Dutch island population of great tits, but identified three factors underlying variation in rates of reproductive senescence between females within Wytham. Rates of reproductive senescence were accelerated in birds hatched from older mothers, in immigrant compared to locally-born birds, and in birds with high levels of early-life reproductive performance. Variation in basal metabolic rate in winter was not found to relate to maternal age, immigrant status and early-life reproductive performance, but was also not related to metabolic rate in spring, and therefore seemed unlikely to underlie the variation in rates of reproductive senescence between females.