Have Bird Wings Evolved into the Best Possible Shapes for Flight?
Albatrosses have long, slender wings that allow them to soar for extended periods by exploiting winds over the ocean. Hummingbird wings are suited to rapid changes in motion and can sustain hovering flight. Penguins lost the ability to fly long ago, yet their wings have been transformed into propulsive surfaces that allow them to manoeuvre rapidly underwater. These examples make it tempting to assume that natural selection has gradually driven the wings of different birds toward the optimal shapes for their respective flight styles.
The problem is that if we examine only living birds and then search among them for the "best-performing" wings, we are already assuming that the shapes found in nature are close to the best possible solutions. One study approached the question from the opposite direction. The researchers first constructed a theoretical morphospace (Note 1) containing wing shapes beyond those found in living birds. They then calculated the flight performance of these theoretical wing planforms and projected real bird wings back into the morphospace to determine how close they came to the theoretical optima.
The researchers collected wing data from 1,139 extant bird species representing 36 orders. They analysed the two-dimensional outline, or planform, of each wing when fully spread and viewed from above, and quantified its shape using Elliptical Fourier Analysis. They then expanded the range of observed shape variation by 20%, generating a 23 × 22 grid containing 506 theoretical wing planforms. Of these, 75 produced self-intersecting outlines and therefore represented physically impossible shapes, so they were excluded from the functional analyses.
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