Migration routes of three closely related species lead to different non-breeding regions with contrasting environmental conditions

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Abstract
Understanding migration strategies in closely related species is essential to assess population-specific threats and to identify the ecological drivers of speciation. To compare migration strategies and environmental conditions across the annual cycle, we tracked Western Black-eared Wheatears, Eastern Black-eared Wheatears and Pied Wheatears. Individual migration routes were reconstructed from multi-sensor loggers and light-level geolocator data. Western Black-eared Wheatears breeding in Spain migrated south-west to Mauritania and Mali, overwintering at a single site in arid regions at the Sahara’s edge. Eastern Black-eared Wheatears breeding in Greece crossed the Mediterranean and Sahara to reach the Sahel, using single non-breeding sites in Nigeria, Chad, and Niger, much further west than previously assumed. Pied Wheatears breeding in Romania migrated via Turkey and the Arabian Peninsula, and overwintered in Somalia, with some individuals shifting sites during winter. Migration strategies differed: Eastern Black-eared Wheatears performed large-scale loop migration. Pied Wheatears used direct routes and did not follow their presumed former range expansion around the Black Sea. Environmental data suggested that non-breeding conditions contrasted stronger than breeding conditions: Western Black-eared Wheatears occupied arid non-breeding sites at the edge of the Sahara that remained dry throughout their stay, Eastern Black-eared Wheatears arrived in the Sahel at the end of the rainy season, and Pied Wheatears had two rainy seasons in the Horn of Africa. These results highlight distinct migration strategies and non-breeding conditions in three closely related wheatears, underlining the importance of species-specific or even population-specific tracking to refine non-breeding range delineation and inform conservation.

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