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

Migratory birds have evolved the ability to accumulate, at appropriate times during the annual life-cycle, large amounts of fat to fuel their migratory journeys. However, increased fuel load leads to an increased body mass, which is expected to make small migratory birds less agile and more vulnerable to predation. Predation risk depends on body mass through its influence on flight performance (flight velocity and acceleration) and especially flight manoeuvrability, traits that are vital in the escape response. In addition, individuals vary in wing aerodynamic properties, which are also expected to affect manoeuvrability and flight performance. We investigated the effects of interindividual variation in fat load and wing aerodynamic traits (wing pointedness, convexity and aspect ratio) on short-term flight performance in 43 juvenile barn swallows (Hirundo rustica) during autumn migration. Individuals were captured at two roosting sites located near Warsaw (Poland) in September 2014. The evaluation of short-term flight performance (maximum velocity and acceleration) and flight manoeuvrability was performed using flight tunnels. Information about fat load was collected using a TOBEC body scanner. Controlling for wing aerodynamic traits, we found that fat individuals reached higher velocity compared to lean ones. On the other hand, controlling for fat load, wing pointedness positively predicted maximum velocity, and negatively predicted the time needed to reach the maximum acceleration. Conversely, fat load or wing aerodynamic traits did not predict flight manoeuvrability. The higher flight velocity reached by birds with larger fat load is in line with the expectations, since comparative evidence suggests that heavier birds can reach higher fly speed than lighter ones. In fact, according to fundamental aerodynamics, the lift force generated on a wing is positively related to flight velocity, and therefore a faster flight is required to hold up more mass. Moreover, by optimizing wingtip vortices, more pointed wings decrease the induced drag and thus could improve flight performance both in terms of speed and acceleration. The lack of any association between fat load and flight maneuverability was against the expectations. In fact, we predicted that birds could trade-off manoeuvrability with fat load. However, experiments were carried out in the initial stages of migration, and none of the individual we tested showed the highest fuel loads that can be observed in this species. This finding suggests that on the initial stages of the migratory journey, when birds start to accumulate fat, they may optimize fuel load in order not to negatively impact on flight performance. However, a trade-off between fuel load and manoeuvrability may be expected to emerge before large ecological barriers that must be crossed with sustained flights, such as deserts and sea, where swallows show the highest fuel load.