Changes in fat mass and migratory restlessness 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 to depart on a migratory flight depends on fuel stores, at least to some extent; very small fuel stores prohibit migratory endurance flight. However, studies linking migrants’ estimated fuel stores (or sometimes actual fat stores) to departure likelihood have provided inconsistent results. Fuel stores are not static though, but highly dynamic. Instead of investigating fuel stores at a given time, we therefore focussed on quantifying the dynamic process of fuelling and how it affects migrants’ motivation to depart. We did so by temporarily caging wild northern wheatears (Oenanthe oenanthe) at a stopover site during autumn migration. Birds were kept under controlled conditions, receiving ad libitum food. Changes in fat mass (measured by quantitative magnetic resonance) and migratory restlessness (an accurate proxy for migrants’ motivatio...