Variation in age-structured vital rates of a long-lived raptor: Implications for population growth

Age-structured variation in multiple vital-rates is a fundamental determinant of population growth, with important implications for conservation management. However, for many long-lived vertebrates such as birds of prey, such variation has been usually examined in shorter-lived species. Here, we investigate the pattern of age-related variation in fitness components and its repercussions on population growth for a migratory raptorial bird, the black kite (Milvus migrans), with a longer lifespan than most other previous model species. Both survival and offspring production varied along the lifespan in conjunction with the sequence of major life history stages: they were lowest during the initial years of life, increased steeply during the period of progressive incorporation of floaters in the breeding sector of the population (age 2-6), levelled off between 7 and 11 years of life, declined with senescence after age 12, and increased again for the few high-quality individuals capable of reaching age 18-25. This pattern was more gradual, asymmetrical and protracted than in shorter-lived species. Matrix modelling estimated a stationary growth rate, which was more sensitive to changes in survival in early life rather than to survival in adult life, contrary to expectations for long-lived species. Our results highlight: (1) a growing appreciation of the importance of juvenile survival for population dynamics, (2) the need for caution on the generalization that population-trends of long-lived species are primarily determined by adult survival, and (3) that the trajectory of the breeding populations of migratory species may be determined by environmental variation experienced in early life in staging areas located far away from breeding areas.