Differential survival throughout the full annual cycle of a migratory bird presents a life‐history trade‐off
Authors: Buechley Evan R. , Oppel Steffen , Efrat Ron , Phipps W. Louis , Carbonell Alanís Isidoro , Álvarez Ernesto , Andreotti Alessandro , Arkumarev Volen , Berger‐Tal Oded , Bermejo Bermejo Ana , Bounas Anastasios , Ceccolini Guido , Cenerini Anna , Dobrev Vladimir , Duriez Olivier , García Javier , García‐Ripollés Clara , Galán Manuel , Gil Alberto , Giraud Lea , Hatzofe Ohad , Iglesias‐Lebrija Juan José , Karyakin Igor , Kobierzycki Erik , Kret Elzbieta , Loercher Franziska , López‐López Pascual , Miller Ygal , Mueller Thomas , Nikolov Stoyan C. , de la Puente Javier , Sapir Nir , Saravia Victoria , Şekercioğlu Çağan H. , Sillett T. Scott , Tavares José , Urios Vicente , Marra Peter P. 
Published: March 2021

Long‐distance migrations are among the most physically demanding feats animals perform. Understanding the potential costs and benefits of such behaviour is a fundamental question in ecology and evolution. A hypothetical cost of migration should be outweighed by higher productivity and/or higher annual survival, but few studies on migratory species have been able to directly quantify patterns of survival throughout the full annual cycle and across the majority of a species’ range.

Here, we use telemetry data from 220 migratory Egyptian vulturesNeophron percnopterus, tracked for 3,186 bird months and across approximately 70% of the species’ global distribution, to test for differences in survival throughout the annual cycle.

We estimated monthly survival probability relative to migration and latitude using a multi‐event capture–recapture model in a Bayesian framework that accounted for age, origin, subpopulation and the uncertainty of classifying fates from tracking data.

We found lower survival during migration compared to stationary periods (β = −0.816; 95% credible interval: −1.290 to −0.318) and higher survival on non‐breeding grounds at southern latitudes (<25°N;β = 0.664; 0.076–1.319) compared to on breeding grounds. Survival was also higher for individuals originating from Western Europe (β = 0.664; 0.110–1.330) as compared to further east in Europe and Asia, and improved with age (β = 0.030; 0.020–0.042). Anthropogenic mortalities accounted for half of the mortalities with a known cause and occurred mainly in northern latitudes. Many juveniles drowned in the Mediterranean Sea on their first autumn migration while there were few confirmed mortalities in the Sahara Desert, indicating that migration barriers are likely species‐specific.

Our study advances the understanding of important fitness trade‐offs associated with long‐distance migration. We conclude that there is lower survival associated with migration, but that this may be offset by higher non‐breeding survival at lower latitudes. We found more human‐caused mortality farther north, and suggest that increasing anthropogenic mortality could disrupt the delicate migration trade‐off balance. Research to investigate further potential benefits of migration (e.g. differential productivity across latitudes) could clarify how migration evolved and how migrants may persist in a rapidly changing world.