When and where does mortality occur in migratory birds? Direct evidence from long‐term satellite tracking of raptors

Abstract

Information about when and where animals die is important to understand population regulation. In migratory animals, mortality might occur not only during the stationary periods (e.g. breeding and wintering) but also during the migration seasons. However, the relative importance of population limiting factors during different periods of the year remains poorly understood, and previous studies mainly relied on indirect evidence. Here, we provide direct evidence about when and where migrants die by identifying cases of confirmed and probable deaths in three species of long-distance migratory raptors tracked by satellite telemetry. We show that mortality rate was about six times higher during migration seasons than during stationary periods. However, total mortality was surprisingly similar between periods, which can be explained by the fact that risky migration periods are shorter than safer stationary periods. Nevertheless, more than half of the annual mortality occurred during migration. We also found spatiotemporal patterns in mortality: spring mortality occurred mainly in Africa in association with the crossing of the Sahara desert, while most mortality during autumn took place in Europe. Our results strongly suggest that events during the migration seasons have an important impact on the population dynamics of long-distance migrants. We speculate that mortality during spring migration may account for short-term annual variation in survival and population sizes, while mortality during autumn migration may be more important for long-term population regulation (through density-dependent effects).

Convergence of broad-scale migration strategies in terrestrial birds

“…The migration of birds is a well-studied example, with ca 19% of extant bird species identified as migratory [2]. The hazards associated with migration can have negative effects on the survival and reproductive success of bird populations [3,4]. The resulting selection pressures have generated a unique set of morphological, physiological, and behavioural adaptations [5][6][7], whose quality varies based on the rigors of the migratory journey.…”

Conditions at autumn stopover sites affect survival of a migratory passerine

“…Nevertheless, a growing number of studies suggest that conditions encountered during migration may also be important for some species (Stokke et al 2005;Newton 2006;Bayly et al 2011), with a few studies providing evidence that mortality during migration may be higher than that experienced at the stationary stages (Ward et al 1997;Sillett and Holmes 2002;Klaassen et al 2014). These data coincide with occasional observations documenting substantial numbers of migratory bird deaths under adverse environmental conditions.…”

“…low food availability), it seems likely that a large proportion of the mortality which occurs in avian species is associated with this stage of the annual cycle. Several studies have provided evidence for this hypothesis (Newton 2006;Bayly et al 2011;Klaassen et al 2014, Hewson et al 2016. For example, Sillett and Holmes (2002) estimated that mortality rates of Blackthroated Blue Warblers Dendroica caerulescens were at least 15-fold higher during migration than during stationary periods.…”

Insights into the migration of the European Roller from ring recoveries

“…is crucial for understanding patterns of connectivity and predicting the response of migratory populations to environmental change (Cresswell 2014). Additionally, knowing where and under what circumstances migrants die contributes to our understanding of how population size is regulated throughout the annual cycle (Strandberg et al 2009;Klaassen et al 2014).…”