When and where mortality occurs throughout the annual cycle changes with age in a migratory bird: individual vs population implications

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.

Figure 1

Characteristics of casualties of satellite/GPS-tagged kites. Death sites covered the whole annual range of the species, including the Spanish breeding quarters, the African wintering quarters and the regions covered by the migratory journeys (panel 1a). The percentage of casualties that occurred in each stage of the annual cycle varied through life (1b), so that mean age at death varied with the stage of the annual cycle in which an individual died (1c), while individuals of different ages died on average at different latitudes (1d). Error bars represent 1 se. The map was created through QGis Desktop 3.6.0 (http://qgis.osgeo.org).

Figure 2

Survival probabilities of satellite/GPS-tagged kites for different stages of the annual cycle. The relative risk of death was highest during the southward migration, followed by the northern migration, the breeding season and the African wintering stage. To avoid visual clutter, we show separately the first 20 days of exposure (panel a) and the overall period of exposure (panel b). Coloured bands around each line represent 95% confidence intervals. A version of the same figure but without coloured bands is available in Supplementary Fig. S.1.

Figure 3

Daily mortality rates of satellite/GPS-tagged Black kites for different stages of the annual cycle. Estimates were assessed through Kaplan-Meier procedures and multiplied by 10 for clarity of presentation, in order to avoid excessive numbers of zero-decimals. Error bars represent 1 se. The sample of tracked individuals for each bar is (from left to right): 32, 18, 16, 2, 54, 49, 52, 45, 30, 22, 19, 17, 25, 24, 24, 24.

Figure 4

Sensitivity of population growth rates to variation in mortality rates across stages of the annual cycle and across stages of the life cycle of satellite/GPS-tagged Black kites. The error bars represent uuncertainties in sensitivity values (1 se), calculated by bootstrap from 1000 population matrices with parameter values randomly taken from their respective estimated distributions.