Experimental Food Supplementation at African Wintering Sites Allows for Earlier and Faster Fuelling and Reveals Large Flexibility in Spring Migration Departure in Pied Flycatchers

By travelling vast distances, migratory birds take advantage of earth's seasonality. Afro-Palearctic migrants can profit from lush spring conditions in temperate regions for chick rearing, but must also gain sufficient energy reserves to cross the Sahara. Rainfall during the dry season in Africa may influence the food available to birds to accumulate reserves. Conflicts of interests in resource exploitation at locations thousands of kilometres apart may occur if migrants encounter poor food conditions during these migratory preparations. Studying how wild birds adjust their fuelling and migration decisions to dynamic environments allows us to understand how flexible migrants can be, which is particularly important in an era of rapid change. We performed supplemental feeding prior to migration in individual Pied Flycatcher Ficedula hypoleuca wintering territories in Ivory Coast and remotely monitored their body mass change until they started their spring migration flight over the Sahara. We tested how access to extra food causally affects fuelling, departure mass and departure date. Seasonal fluctuations in natural arthropod availability prior to migration were monitored in two years, to explore how natural resource dynamics alters fuel accumulation. Birds that fully accessed extra food in March–April put on weight earlier and faster than birds without extra food supply, and departed 12 days earlier. Birds accumulated fuel loads that were higher than required for the Sahara-crossing, regardless of their access to extra food. Fuelling rates fluctuated in synchrony with natural conditions, as non-supplemented birds achieved the highest body mass gains at the time that natural arthropod availability peaked in the study area. Fuelling rates were lower in 2020, i.e. the year when the first rains after the dry season started late, than in 2019. Our study showed that Pied Flycatchers modulated fuelling rates – but not departure fuel loads – to food dynamics in West Africa, causing flexibility in the timing of departure. This strategy probably enhances a safe Sahara crossing, but may limit the possibilities of migrants to anticipate advancing spring conditions at breeding sites.