Phenological Trends of Palearctic Passerine Bird Species Wintering in The Gambia and Nigeria

The timing of avian migration for Palearctic passerine birds across the Western Africa region remains relatively unexplored. This study investigates the changing wintering phenology of nine Afro-Palearctic migratory species in relation to their time spent in Western Africa. We examined the annual first arrival and last departure dates for these nine migratory species within their non-breeding ranges using two datasets: The Gambia (1964–2021) and Nigeria (1974–2021). We employed phylogenetic linear mixed models to explore trends in pre-breeding (European spring) departure, post-breeding (European autumn) arrival, and duration of stay on the non-breeding grounds. Our results show that pre-breeding timing is advancing for migrant birds departing from The Gambia compared to those from Nigeria. Migrant species in The Gambia are departing earlier than their counterparts in Nigeria. This finding suggests that population in The Gambia may be responding to different environmental cues than those in Nigeria. The duration of stay was longer for species wintering in Nigeria than for birds in the Gambia. This difference could be driven by local environmental factors but might equally be driven by changes in another period and area during the migratory year. Post-breeding arrival for populations in both The Gambia and Nigeria were similar, commencing in late August. This suggests that post-breeding migration patterns may be affected by similar factors en route to African wintering areas. We discussed the phenological differences in pre-breeding departure and duration of stay of bird populations using the two counteries. These differences highlight the complexity of migratory timing and suggest how local environmental conditions might shape migratory behaviour. Our findings suggest that phenological changes in migratory species can vary significantly across the West African wintering grounds, reflecting the spatio-temporal phenological shifts observed on their breeding grounds.