Temporal and environmental drivers of pre-migratory fuel loads in songbirds in West Africa

Afro-Palaearctic migrants depart West Africa for breeding grounds in Europe during the transition from the dry to the wet season. Such timing can severely impact migration success because the onset of the rains varies between years, influencing food availability and migrants' body reserves, which are crucial for crossing the Sahara. We used a 23 year ringing dataset of seven migratory bird species from Nigeria to quantify changes in body mass and fat during the pre-migratory period as a function of recent and dry season rainfall and normalised difference vegetation index (NDVI). For a small subset of species, we also considered data from the wider non-breeding period. During the pre-migratory period, body reserves were positively correlated with pre-capture conditions in several species. Thus, body reserves increased after recent periods of high rainfall and NDVI, as well as with date. Effects of dry season conditions on pre-migratory body reserves were more variable, possibly linked to species-specific movement strategies in sub-Saharan Africa. During the non-breeding period, we found some support for reduced body reserves under good conditions, perhaps to enhance mobility. Limitations in fuelling due to poor recent conditions in West Africa could lead to birds departing later or in poorer condition, with potential carry-over costs to breeding.