Stable isotopes of soil collected from feet of two species of migratory Acrocephalus give clues to stopover sites

Soil samples were collected from the feet of Marsh Warblers Acrocephalus palustris and Reed Warblers Acrocephalus scirpaceus caught soon after crossing the Sudan Red Sea coast of Africa. We measured carbon  (δ 13 C soil_feet ) and deuterium (δD soil_feet ) isotope ratios in these soils with the objective of identifying possible take-off sites of these birds. We collected soils from three sites in the Caucasus region, a potential refueling area for the flight to Sudan, and from the Sudan Red sea coast, and compared their deuterium (δD soil ) with δD soil_feet . There was a strong relationship between the arrival date of the birds and the isotope  signatures (δ 13 soil_feet and δD soil_feet ) of the soil they carried. Results suggest that warblers from different geographical regions or of different age groups might use different staging sites before reaching the Sudan. Data for precipitation deuterium (aDprec) together with aDsoil and δD soil_feet suggest that while early arriving birds had taken off from southeast Europe (picking up soil from this region), those arriving later had stopped off in Arabia. This indicates an intrinsic difference in strategy  between birds migrating at different times of the season. The isotopic compositions of biological tissues such as feathers have commonly been applied to track animal movement. But this is the first report of analysis of soil from birds’ feet: a novel approach to isotopic study based on material picked up and carried by an animal externally.

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