Fat loads and estimated flight‐ranges in fourSylviaspecies analysed during autumn migration at Gotland, South‐East Sweden

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Abstract
The migratory capacity of four Sylvia species was investigated during their initial phase of autumn migration in Sweden. Fat loads of individual birds were estimated by first determining approximately the fat‐free body masses for birds of different size (as judged from body mass ‐ fat index relationships). Individual fat loads varied considerably both inter‐ and intra‐specifically. The mean fat load (%fat mass of total body mass) of different species was: Lesser Whitethroat 9.3%, Whitethroat 7.2%, Garden Warbler 16.4% and Blackcap 13.2%. Comparatively low fat loads in the first two species may be because they probably commence their autumn migration in this area before the post‐juvenile moult is completed. Our results indicate that several autumn migrating Garden Warblers and Blackcaps stopping over at Gotland have the energetic capability to almost cross continental Europe without refuelling This contrasts with the Whitethroat and the Lesser Whitethroat of which many individuals would only be able to cross the Baltic Sea.