Experimentally increased food availability allows for earlier departure in a long-distance migratory shorebird

The timing of animal migrations is an interplay between migration programmes and physiological and environmental conditions. The rate of body store deposition underlies many timing options, with a key unresolved question being whether the rate of body store deposition and resource constraints limit advancements in migration departure.

To study limits of deposition on departure timing, we captured red knots (Calidris canutus islandica) wintering in the Wadden Sea and manipulated hours of food access per day during spring body store deposition and prenuptial moulting phases. These birds were then released and tracked until spring migration departure from the Wadden Sea.

When birds were allowed to feed more hours per day in captivity, they did not start body store deposition and plumage moult earlier, but gained stores faster, were heavier and had further progressed in prenuptial feather moult at the moment of release. After release, birds that had ad libitum access to food in captivity departed earliest from the Wadden Sea, and departure timing was explained by the rate of mass deposition in captivity.

We experimentally demonstrate flexibility in the timing of annual-cycle processes during spring migration. Red knots are flexible to adjust the rate of mass gain and moult to food availability, in turn allowing them flexibility in departure timing. This indicates that improvements in foraging conditions at staging sites will facilitate earlier departures, which will help migratory birds to keep pace with global warming.