Theoretical morphospace reveals mixed optimisation of the avian wing planform for flight style.

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. Hypothesised empirical properties are considered post hoc relative to their positions in performance surfaces. We produce a theoretical morphospace of wing planform shape and deduce the functional performance and optimality of 1139 extant taxa. Functional tests cover metrics and combinations with a hypothesised link to 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 low cost of transport and manoeuvrable flight. Many taxa, principally passerines, are suboptimal for all studied metrics and combinations demonstrating uneven constraint on flight performace 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.