Fuelling in front of the Sahara desert in autumn—an overview of Swedish field studies of migratory birds in the eastern Mediterranean

Birds must store fuel prior to the crossing of the Sahara desert, at least 1,500 km with few refuelling possibilities. A major question is how inexperienced birds know where to prepare for the oncoming barrier. Experiments with caged birds showed that information from the Earth’s magnetic field close to the desert might trigger extensive fuel deposition. Blackcaps Sylvia atricapilla trapped on Cyprus in September and October were much heavier than in Sweden during the early phase of autumn migration, typical for birds preparing for crossing the Sahara desert. There is a potential cost of being fat. Fat Sedge Warblers Acrocephalus schoenobaenus on Lesvos had much poorer take off abilities than lean birds, making them more vulnerable to predators. Swallows Hirundo rustica trapped at a roost site on Rhodes showed a synchronized body mass increase and this is in agreement with their wavelike pattern of passage on Crete. In recent years, studies of fuelling behaviour and stopover duration of first-year Garden Warblers Sylvia borin have been started on Crete with the aid of radio-transmitters.