Abstract
The annual cycle of most animals is structured into discrete stages, such as breeding, migration and dispersal. While there is growing appreciation of the importance of different stages of an organism’s annual cycle for its fitness and population dynamics, almost nothing is known about if and how such seasonal effects can change through a species lifespan. Here, we take advantage of the opportunity offered by a long-term satellite/GPS-tracking study and a reliable method of remote death-detection to show that certain stages of both the annual and life cycle of a migratory long-lived raptor, the Black kite Milvus migrans, may represent sensitive bottlenecks for survival. In particular, migratory journeys caused bursts of concentrated-mortality throughout life, but the relative importance of stage-specific survival changed with age. On the other hand, the balance between short-stages of high mortality and long-stages of low mortality made population-growth similarly dependent on all portions of the annual cycle. Our results illustrate how the population dynamics of migratory organisms can be inextricably linked to ecological pressures balanced over multiple stages of the annual cycle and thus multiple areas of the globe, suggesting the frequent need for challenging conservation strategies targeting all portions of a species year-round range.
Highlights
The annual cycle of most animals is structured into discrete stages, such as breeding, migration and dispersal
While much progress has been made, there is a need for additional robust studies based on detailed estimates for all stages of the annual and life cycle, in order to complete our picture of seasonal effects
We exploit the opportunity offered by a long-term satellite GPS-tracking study and a new method of remote death-detection to examine for the first time how variation in vital rates through all stages of the annual cycle and life cycle may affect the population dynamics of a migratory, long-lived raptor, the Black kite Milvus migrans
Summary
The annual cycle of most animals is structured into discrete stages, such as breeding, migration and dispersal. Incorporating a full life cycle perspective into full annual cycle studies seems critical, because age-structure in vital rates is a fundamental determinant of individual fitness and population growth[10,11] This is especially so for long-lived species, which are subject to threats that may change through life, as individuals proceed from initial juvenile stages, to immature non-breeding phases, to adult breeding careers and to the final, senescent portions of their life. For these species, identifying sensitive bottlenecks in both their annual cycle and life cycle is fundamental for efficient management and conservation[12]. Because the study included non-breeding individuals, to avoid confusion we mainly refer to the post-breeding migration as “southward migration” and to the pre-breeding migration as “northward migration”
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