The form of a bird's wing is so basically important to the successful exploitation of an ecological niche that it inevitably yields many instructive examples of adaptive evolution. It also provides interesting examples of convergence, as is to be expected of a structure that contributes materially to such important functions as locomotion and the obtaining of food. It may therefore be profitable to examine the principal categories of wing type (using the word aerodynamically rather than anatomically) and to see how they correlate with special requirements. Space does not warrant a full discussion of the function of an airfoil, which is readily obtainable in adequate detail from elementary treatments of aircraft aerodynamics, such as those issued for pilot training; but attention must be drawn to some points that are vital to an appreciation of the various adaptions with which we are concerned.
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