Comparative analysis reveals migratory swallows (Hirundinidae) have less pointed wings than residents

Abstract

The correlation between migration and wing pointedness, a pattern generally attributed to the need to reduce drag during powered straight-line flight, is well established in avian ecomorphology. However, most studies investigating this pattern have focused on relatively rounded-wing taxa, which employ different flight modes during foraging and migration. The basic assumption that migrants have comparatively pointed wings has not been questioned by analysing taxa with similar migratory and foraging flight. This study examines the correlation between migration and wing shape in swallows (Hirundinidae), a family with relatively pointed wings in which foraging flight resembles migratory flight. Using a phylogeny-based analysis, we compare the wing shape of species pairs with varying migratory habits in eight swallow genera. Surprisingly, migratory swallows have less pointed wings than sedentary species, and wing pointedness declines linearly with increasing migratory distance. This study represents the first published result documenting a reversal of the correlation between migration and wing pointedness found in other avian taxa. Interpreting this reversal requires a more nuanced understanding of wing ecomorphology; we hypothesize that inclement weather conditions on the breeding grounds and/or the roosting habits of migratory swallows create conflicting selective pressures that increase the cost of wing pointedness in migratory swallows.