Sufficient food is critical for a long-distant migratory shorebird to advance migration phenology

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 food availability during spring body store deposition and prenuptial moulting phases, followed by release and automated tracking. With longer access to food in captivity, birds gained body stores faster and were heavier and further progressed in prenuptial feather moult, and after release departed earlier from the Wadden Sea. Our results show flexibility in migration decisions with an important role for food availability.