Age-specific variation in demographic traits of sympatric seabirds with divergent population trajectories

Abstract

**Abstract:** Age and experience affect key life-history traits, influencing evolutionary processes in a variety of ways. Timing of breeding in relation to optimal environmental conditions (e.g. peaks in prey availability) can also influence breeding success, and studies of closely-related, allochronic species may help disentangle the effects of phenological mismatch on ageing patterns. Here, we test for the effects of age and experience on the survival and reproduction of two long-lived seabirds with different timing of breeding and divergent population trajectories. At South Georgia, populations of the northern (NGP, Macronectes halli) and southern giant petrel (SGP, M. giganteus), are increasing and stable, respectively. In early adulthood, first-time breeders of both species had lower breeding success and annual survival. Breeding success improved markedly with age until around 20 years, driven by within-individual factors, but was lower in SGP which also experienced earlier reproductive senescence. Pre-fledging body mass in NGP showed a broadly quadratic relationship with parental age, suggesting that the age-specific pattern in breeding success was partially related to variation in foraging performance. Furthermore, pre-fledging body mass increased rapidly with parental age among first-time breeders in both species, but breeding success did not, further demonstrating the importance of foraging performance rather than breeding experience per se. Due to allochrony, NGP rear their chicks when the availability of seasonally-limited, high-calorie food (seal and penguin carrion) is higher. This would explain their delayed reproductive senescence and, potentially, improved performance in adulthood due to better neonatal nutrition. In addition, it contributes to the divergent population trajectories of the two species at South Georgia. These findings provide empirical evidence for the influence of phenological mismatch on reproductive senescence. **Authors:** Dimas Gianuca¹, Richard Phillips², Andrew Wood², Deborah Pardo², Stephen Votier³ ¹Projeto Albatroz, ²British Antarctic Survey, ³University of Exeter