Morphological Adaptations to Migration in Birds

Migratory flight performance has direct or carry-over effects on fitness. Therefore, selection is expected to act on minimizing the costs of migratory flight, which increases with the distance covered. Aerodynamic theory predicts how morphological adaptations improve flight performance. These predictions have rarely been tested in comparative analyses that account for scaling and phylogenetic effects. We amassed a unique dataset of 149 European bird species and 10 morphological traits. Mass-adjusted aspect ratio increased, while mass-adjusted heart weight and wing loading decreased with increasing migration distance. These results were robust to whether the analyses were based on the entire species pool or limited to passerines or migrants. Our findings indicate that selection due to migration acts on wing traits that reduce the energetic cost of transportation to increase the flight range. Consequently, the demands for high ‘exercise organ’ performance might be low, and hence such energetically expensive tissues are not associated (pectoral muscle) or are inversely associated (heart) with migration distance.