Fuel accumulation advances nocturnal departure: a migratory restlessness study on northern wheatears at stopover

Migrating animals typically make stopovers to rest and replenish the fuel used during previous travel bouts. In birds, fat or fast refuelling individuals are generally more likely to depart from stopover than lean or slow refuelling birds. The departure decision, however, involves more than the day-to-day departure likelihood. Also, the time of night (or day) at which migrants depart is relevant, as this largely determines the duration of the flight bout and thereby affects the speed of migration. Because stopovers serve to replenish fuel stores, longitudinal data are most informative regarding the relationship between fuel stores and nocturnal departure time, but no such data exist. To fill parts of this gap, we caught and temporarily caged migrating northern wheatears, Oenanthe oenanthe, at a spring stopover site. We related the birds' evening fuel stores to their start of nocturnal migratory restlessness. In northern wheatears, this is a reliable proxy for their actual departure time under free-flying conditions. We observed a strong negative within-individual effect of fuel stores on the start of migratory restlessness. With an increase in fuel stores birds advanced their start of restlessness. This clearly shows that evening fuel stores are important for migrants' timing of departure at night. Hence, next to the regulation by innate rhythms, the timing of nocturnal departures during spring migration also depends on cues from fuel stores.