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

Abstract

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.