When and where to fuel before crossing the Sahara desert – extended stopover and migratory fuelling in first-year garden warblers Sylvia borin

T. Fransson (correspondence), Bird Ringing Centre, Swedish Museum of Natural History, Box 50 007, SE-104 05 Stockholm, Sweden. E-mail:thord.fransson@nrm.se. C. Barboutis, Natural History Museum of Crete and Department of Biology, University of Crete, P.O. Box 2208,71409 Heraklion, Crete, Greece. R. Melleroth, Eldarva¨gen 8, SE-117 66, Stockholm, Sweden. T. Akriotis, Department of EnvironmentalScience, University of the Aegean, GR-81100 Mytilini, Greece.Large numbers of passerine migrants cross the Sahara desert every year on their way to-and-from wintering areas intropical Africa. In the desert, hardly any fuelling opportunities exist and most migrants have to prepare in advance. Acentral question is how inexperienced birds know where to fuel. Inexperienced garden warblers Sylvia borin were studiedin Greece just before the desert crossing in autumn. Body mass data collected at two sites indicate that most birds do notfuel for the desert crossing further north. For the first time, detailed information about stopover duration close to theSahara desert was studied by using light weight radio-transmitters. Results from Crete show that most first-year gardenwarblers arrive with relatively small fuel loads in relation to lean body mass (B30%), stay for 13 20 d and depart with anaverage fuel load of about 100%. Radio-tagged birds performed small scale movements initially and took advantage of figfruits. Birds trapped at fig trees were heavier than birds trapped with tape lures, showing that tape lures can bias thesample of migrants trapped. The precise fuelling pattern found indicates that first-year migrants must also includeexternal spatial cues to make the preparation for crossing the desert in the right area.