Migratory restlessness increases and refuelling rate decreases over the spring migration season in northern wheatears

For migrating birds, late arrival at the breeding grounds reduces reproductive success. Hence, while en route, late migrants are expected to accelerate migration. Most time can be saved by shortening stopovers, periods of rest and fuel replenishment (refuelling) in between flight bouts. In several field studies late migrants have indeed been observed to shorten stopovers. The length of stopovers is assumed to depend, at least partly, on refuelling rate (fuel deposition rate, FDR). Therefore, shorter stopovers later in the spring migration season may be explained by a seasonal increase in FDR. We tested this idea by measuring both migratory restlessness (an accurate proxy for departure likelihood from stopover) and FDR in the same set of temporarily caged wheatears, Oenanthe oenanthe, a long-distance songbird migrant, throughout spring migration. We found that the amount of migratory restlessness increased with the progress of the season, indicating higher departure likelihood in late migrants. Clearly, however, late migrants in our sample did not accelerate migration by increasing their FDR; daily FDR was relatively constant during the season in lean birds, and even decreased over the season in fat birds. There was little seasonal variation in daily food intake in both lean and fat birds. Because we eliminated and/or corrected for environmental factors influencing departure likelihood, these results strongly indicate that an endogenous time programme is involved in the regulation of departure from stopover in spring.