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

Area of Science:

• Ornithology
• Aerodynamics
• Evolutionary Biology

Background:

• Wing shape and size influence flight efficiency across diverse animal taxa.
• Factors like foraging, migration, and climate drive variations in wing morphology, potentially creating latitudinal gradients.
• An elevational gradient hypothesis suggests wing shape evolves to compensate for reduced air density and lift at higher altitudes.

Purpose of the Study:

• To globally investigate elevational effects on avian wing morphology.
• To test if wing shape and area increase with elevation, independent of other environmental and ecological factors.
• To synthesize global patterns of wing shape evolution in birds.

Main Methods:

• Phylogenetic models were used to analyze wing morphology in 9,982 bird species.
• Metrics of wing morphology, including relative wing elongation (hand-wing index) and wing area, were assessed.
• Analyses controlled for latitude, temperature seasonality, body mass, habitat, aerial lifestyle, and altitudinal migration.

Main Results:

• A significant positive correlation was found between elevation and relative wing elongation (hand-wing index).
• Wing area also demonstrated a significant increase with elevation across the studied bird species.
• These elevational gradients persisted even after accounting for various climatic and ecological variables.

Conclusions:

• Avian wing morphology exhibits a pervasive elevational gradient, with wings becoming more elongated and larger at higher altitudes.
• Aerodynamic constraints related to decreasing air density at higher elevations appear to be a key driver of these morphological changes.
• These findings suggest that air density and potentially oxygen availability play a significant role in shaping the evolution of flight in birds and other flying animals globally.