Body mass of six long-distance migrant passerine species along the autumn migration route

We analysed body mass and moult data of six passerine species along their autumn migration route from northern Europe to North Africa and derived hypothetical models of the organisation of their migration in terms of fuel store accumulation. We analysed data of 46,541 first-year birds from 34 trapping sites, sampled in a network of collaborating European and African ringing stations. After accounting for effects of time of day and size, there were marked differences between the six species examined in the change of body mass along the migration route and in the timing of moult. Garden Warblers (Sylvia borin) and Pied Flycatchers (Ficedula hypoleuca) underwent their postjuvenile moult prior to migration and increased their average body mass along the migration route. Sedge Warblers (Acrocephalus schoenobaenus) also increased body mass towards the south, but started the migration bout without further refuelling well before the Sahara and moulted mainly in the wintering grounds. Reed Warblers (Acrocephalus scircpaceus) and Whitethroats (Sylvia communis) migrated while still moulting and did not increase average body mass towards south. They accumulated the energy needed to fly over the Sahara just before it. Spotted Flycatchers (Muscicapa striata) behaved in the same way, but contrary to Reed Warblers and Whitethroats they did not accumulate much fat stores in North Africa, which might urge them to stop and fuel up regularly in the Sahara. In the course of the season average body mass of all species increased slightly, which enabled them to migrate faster. In general, average body mass of first-year birds in northern and central Europe during the migration period was comparable to that of adults during breeding.