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

Wings with an elongated shape or larger surface area are associated with increased flight efficiency in a wide range of animals from insects to birds.¹^,²^,³^,⁴ Inter- and intra-specific variation in these attributes of wing shape is determined by a range of factors-including foraging ecology, migration, and climatic seasonality⁵^,⁶^,⁷^,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 gradient⁵^,¹¹ because air density declines with altitude,¹² altering the aerodynamics of flight and driving the evolution of more efficient wings in high-elevation species to compensate for reduced lift.¹³^,¹⁴^,¹⁵ Although previous analyses have shown a tendency for longer or larger wings at higher elevations, at least locally,¹⁶^,¹⁷^,¹⁸^,¹⁹^,²⁰ it is difficult to rule out a range of alternative explanations since we currently lack a global synthesis of elevational gradients in wing shape for any taxonomic group. In this study, we use phylogenetic models to explore elevational effects on metrics of wing morphology linked to aerodynamic function in 9,982 bird species while simultaneously controlling for multiple climatic factors and ecological attributes of species. We found that relative wing elongation (hand-wing index) and wing area increase with elevation, even when accounting for latitude, temperature seasonality, body mass, habitat, aerial lifestyle, and altitudinal migration. These results confirm a pervasive elevational gradient in avian wing morphology and suggest that aerodynamic constraints linked to air density, perhaps coupled with oxygen deficiency, contribute to global patterns of trait evolution in flying animals.