Flight performance of migrating juvenile barn swallows in relation to fat load and wing aerodynamics

During Zugunruhen birds accumulate fat reserves that make them less agile and more vulnerable to predation, through effects on flight performance. We investigated the influence of variations of fat load and wing morphology on flight performance (velocity, acceleration and manoeuvrability) in migrating juvenile barn swallows (Hirundo rustica). Our results show, respectively, controlling for wing shape and fat load, that fat individuals and birds with more pointed wings preformed high-velocity flights. According to aerodynamics, fast flights are required to hold up huge body masses while pointed wings decrease drag and improve velocity. No manoeuvrability and fat load trade-off emerges in this study: birds probably optimize fat reserves in order to not negatively impact on flight abilities; this trade-off may emerge close to ecological barriers, where largest energy resources are crucially required.