Age-dependent phenological plasticity in a wild bird

Life-history traits are often plastic in response to environmental factors such as temperature or precipitation, and they also vary with age in many species. Trait variation during the lifetime could thus be partly driven by age-dependent plasticity in these traits. We study whether plasticity of a phenological trait-the egg-laying date-with respect to spring temperature, varies with age, and explore whether this variation relates to changes in breeding success throughout the life cycle. We use data from a four-decade long-term monitoring of a wild population of blue tits in Corsica, to estimate age-dependent plasticity of reproductive phenology and annual reproductive success. We show that both laying date plasticity and annual reproductive success vary with age: young and old females are less plastic, and fledge fewer offspring, than middle-age females. Furthermore, in contrast to young and prime-age females, in old females fledging success does not depend on laying date. Phenological plasticity is a major mechanism for coping with rapid environmental variation. Our results suggest that understanding its role in adaptation to climate change and population persistence requires integrating the age structure of the population.