Elevational constraints on flight efficiency shape global gradients in avian wing morphology

Wings with elongated shape or larger surface area are associated with increased flight efficiency and dispersal ability in a wide range of animals from insects to birds 1-4. Inter- and intraspecific variation in these attributes of wing shape is determined by a range of factors - including foraging ecology, migration and climatic seasonality 5-8 - all of which may drive latitudinal gradients in wing morphology 9,10. A separate hypothesis predicts that wing shape should also follow an elevational gradient because air density and oxygen supply decline with altitude 11, altering the aerodynamics of flight, and driving the evolution of more efficient wings in high-elevation species to compensate for reduced lift 12,13. However, previous analyses have found only mixed support for the thin-air hypothesis 14-18, and we currently lack a global synthesis of elevational gradients in wing design for any taxonomic group. In this study, we use phylogenetic comparative models to explore elevational effects on wing morphology in 9986 bird species, while accounting for multiple climatic and ecological attributes, including latitude, temperature seasonality, body mass, aerial lifestyle and migration. We found that relative wing elongation (hand-wing index) and wing area increase with elevation, particularly in the upper montane zone (>4 km above sea level). These results confirm a pervasive elevational gradient in avian wing morphology, highlighting the role of aerodynamic constraints as key mechanisms shaping global patterns of trait evolution in flying animals.