Changes in fat mass affect the motivation to migrate in northern wheatears

To fuel their migratory endurance flights, most birds accumulate large quantities of fat prior to departure. It therefore seems logical that the decision depart on a flight depends stores, at least some extent; very small stores prohibit flight. However, studies linking migrants' estimated (or sometimes actual stores) departure likelihood or restlessness measured in captive (an accurate proxy for motivation from stopover) have provided inconsistent results. Fuel are not static though, but highly dynamic. Instead investigating given time, we focussed quantifying dynamic process fuelling and how it affects depart. We did so by temporarily caging wild northern wheatears Oenanthe oenanthe stopover site during autumn migration. Birds were kept under controlled conditions, receiving ad libitum food. Changes mass (measured quantitative magnetic resonance) closely monitored. found change was good predictor restlessness. Individuals accumulating little show much restlessness, while individuals showed strong increase Our study therewith provides clear support idea affect birds' migrate, shows migrating birds, increasing an important cue stopover. Fat accumulation affected same manner relatively lean birds. Additionally, food intake changes therein explain observed. hypothesize tissue itself signals energetic state, likely through production release hormones, affecting migrate.