Fuelling strategies differ among juvenile Sedge and Reed Warblers along the eastern European flyway during autumn migration

Abstract

In this study, we investigated fattening strategies of juvenile Sedge and Reed Warblers during their autumn migration. We analysed fat scores of birds captured at five ringing sites situated between the southern Baltic Sea coast and Asia Minor. In Eastern Europe these two species had similarly low fat reserves. Their fat load increased in the Balkans. Remarkable differences between the species were noted in Asia Minor, where fat reserves of Sedge Warblers were more than two-fold higher compared to Reed Warblers. As high as 90% of Sedge and only 30% of Reed Warblers captured in Asia Minor had the potential capability to cross the Mediterranean Sea in one non-stop flight. Moreover, two-thirds of those Sedge Warblers were able to continue their long flight without refuelling and reach the southern edge of the Sahara desert, while in Reed Warblers only 6% of individuals were potentially able to use the same strategy. The results of the study show clear differences in potential flight ranges of the studied species, revealing different fattening strategies of the Sedge and Reed Warbler in the Balkans and Asia Minor.