Patterns in departure phenology and mass gain on African non‐breeding territories prior to theSahara crossing in a long‐distance migrant

Afro‐ P alaearctic migrants are declining to a greater degree than other European species, suggesting that processes occurring in A frica or on migration may be driving these trends. Constraints on food availability on the wintering grounds may contribute to these declines but little is known about when and where these resource constraints may occur. Sufficient resources are particularly important prior to spring migration, when migrants must cross the Sahara Desert. We examined mass gain and departure phenology in a long‐distance Palaearctic passerine migrant to determine the degree to which pre‐migratory fattening occurs in their long‐term non‐breeding territories in the G uinea S avannah region of A frica. We monitored 75 W hinchats S axicola rubetra for departure from their non‐breeding territories in one spring, and analysed mass data of 377 Whinchats collected over three non‐breeding seasons plus 141 migrating W hinchats caught in A pril over 8 years, all within the same few square kilometres of human‐modified G uinea S avannah in central N igeria. W hinchats left their winter territories throughout A pril, with males departing on average 8 days earlier than females. However, there was no evidence that time of departure from territory was linked to age, body size or mass at capture. Whinchats departed their territories with a predicted mass of 16.8 ± 0.3 g, considerably less than the c . 24 g required for the average Whinchat to cross the Sahara directly. Comparing departure dates with arrival dates in southern Europe showed a discrepancy of at least 2 weeks, suggesting that many W hinchats spend considerable time on pre‐migratory fuelling outside their winter territory prior to crossing the S ahara. Overwintering birds gained mass slowly during F ebruary and M arch (0.03 g/day), and non‐territorial or migrating birds at a much higher rate in A pril (at least 0.23 g/day), with up to 20% of migrating W hinchats in A pril potentially having sufficient fuel loads to cross the Sahara directly from central N igeria. Our results suggest that most W hinchats leave their winter territories to fatten up locally or, possibly, by staging further north, closer to the southern limit of the S ahara. Resource constraints are therefore likely to be particularly focused in W est A frica during mid‐ A pril and possibly at staging areas before the crossing of the S ahara D esert.