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

  • Raymond Klaassen
  • Mikael Hake
  • Roine Strandberg
  • Ben J Koks
  • Christiane Trierweiler
  • Klaus-Michael Exo
  • Franz Bairlein
  • Thomas Alerstam
Publishing year: 2014
Language: English
Pages: 176-184
Publication/Series: Journal of Animal Ecology
Volume: 83
Issue: 1
Document type: Journal article
Publisher: Federation of European Neuroscience Societies and Blackwell Publishing Ltd

Abstract english

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).


  • Biological Sciences
  • animal migration
  • annual survival
  • ecology of death
  • long-distance migration
  • satellite radio-telemetry


  • ISSN: 1365-2656
Thomas Alerstam
E-mail: thomas [dot] alerstam [at] biol [dot] lu [dot] se

Professor emeritus

Evolutionary ecology

+46 46 222 37 85


Sölvegatan 37, Lund