The ‘wintering strategy hypothesis’ predicts that the body mass of long-distance migratory dabbling ducks is low in early winter, after autumn migration, increases in mid-winter and decreases again in late winter because of the high energetic demands of the pairing period. However, this hypothesis is assumed rather than demonstrated, since very few studies have examined these predictions. Here, we tested the theory by examining changes in size-corrected body mass (Scaled Mass Index, SMI) in Northern Pintails Anas acuta wintering at rice fields of Extremadura, a key site in Southern Europe. In both females and males, SMI increased with rainfall and deceased with fasting time, peaking in midwinter and decreasing in late winter. Northern Pintails leaving Extremadura cover between 2600 and 5600 km to reach their breeding grounds and use a number of stopover sites during their northward migration. Together, our findings support the wintering strategy hypothesis and suggest that Northern Pintails wintering in Southern Europe follow an energy-minimizing migration strategy (i.e. taking advantage of the food stock found along the route and adaptively varying fat reserves in relation to the distance to cover). We discuss that this may enable them to spend more energy and time on activities such as moulting and pairing.