Theoretical Morphology of Avian Wing Planform Reveals Variable Optimisation to Flight Style

Abstract

Bird wings exhibit a broad degree of functional and shape variation, though the exact nature of the form-function relationship is uncertain. Recent analysis suggests that functional variability is explained by linear non-shape-based traits and that shape variation is largely explained by phylogeny. We assay the relationship between wing planform shape and functional performance using a theoretical morphospace approach that eschews assumptions of the functional optimality of empirical morphologies. Empirical properties are considered post hoc relative to their positions in the functional performance landscapes. We produce a theoretical morphospace of wing planform shapes and compare the functional performance and optimality of theoretical shapes with 1139 extant taxa. These tests cover flight metrics and combinations linked with 7 flight niches. Metrics pertaining to agile flight strongly constrain shape, with hovering, diving and hawking birds developing optimal planforms. Marine soarers are suboptimal for metrics linked with efficient and manoeuvrable flight. Many taxa, principally passerines, remain suboptimal for all studied metrics and combinations. This suggests flight performance constrains planforms unevenly across birds. Phylomorphospace analysis suggests planform shape is only weakly influenced by phylogeny and functional optimality correlates closely with flight styles. This suggests wing shape remains a determining factor in how birds fly.