Age-related improvement in reproductive performance in a long-lived raptor: a cross-sectional and longitudinal study

In numerous iteroparous species, mean fecundity increases with age. Such improvement has been explained by: a) progressive removal of inferior breeders from the breeding population (selection-hypothesis); b) delayed breeding of higher-quality phenotypes (delayed-breeder-hypothesis); c) longitudinal enhancement of skills associated with age per se (age-hypothesis); d) progressive improvement in the capability to conduct specific tasks facilitated by accumulated experience (breeding-experience-hypothesis); and e) increasing parental investment promoted by declining residual reproductive values (restraint-hypothesis). To date, there have been few comprehensive tests of these hypotheses. Here, we provide such a study using a long-term dataset on a long-lived raptor, the black kite Milvus migrans (maximum lifespan 23 yr). Kites delayed breeding for 1-7 yr and all measures of breeding performance increased linearly or quadratically up to 11 yr of age. There was no support for the delayed-breeder-hypothesis: superior phenotypes did not delay breeding longer. Superior breeders were retained longer in the breeding population, consistent with the selection-hypothesis. All measures of breeding performance increased longitudinally within individuals, supporting the age-hypothesis, while some of them increased with accumulated previous experience, supporting the breeding-experience-hypothesis. Some analyses suggested the existence of trade-offs between reproduction in the early years of life and subsequent survival, partially supporting the restraint-hypothesis. The pattern of age-related improvements in breeding rates observed at the population-level could be ascribed to the combined effect of the progressive removal of inferior phenotypes from the breeding population and the individual-level lack of specific skills which are progressively acquired with time and experience. It was also compatible with a longitudinal increase in reproductive investment. Results from previous studies suggest that different mechanisms may operate in different species and that age-related improvements in reproduction may be frequently promoted by the complex interplay between longitudinal improvements and changes in the relative frequency of productive phenotypes in the breeding population.