Spring migration of the Garden Warbler (Sylvia borin)across the Mediterranean Sea

Information concerning the strategy adopted by passerine migrants in crossing ecologicalbarriers is still scant, especially as far as spring migration is concerned. With the aim ofinvestigating the pattern of desert and sea crossing in spring, capture frequency, biometryand physical conditions of Garden Warblers ringed at 18 stations spread over the Westernand Central Mediterranean were analysed as a function of the geographical position of theringing sites. The patterns of increasing capture frequency and wing length with longitude,matching those recorded within the species’ wide breeding range, suggest wide-frontmovements across the Sahara and Mediterranean. The progressively delayed trapping hourwith latitude indicates that Garden Warblers migrate actively during the day. Despite thepotentially favourable spring ecological conditions, the North African coast seems not tobe used substantially to restore the reserves depleted during the desert crossing. Accordingto recent estimates of flight range, fat levels recorded at our Mediterranean stations arecompatible with the absence of refuelling in North Africa, assuming a fat load at departureof about 27 g or lower fat load and some tail wind assistance.

Alessandra Grattarola, Fernando Spina and Andrea Pilastro, J. Ornithol. 140, 41%430 (1999)