Flapping flight is a highly effective form of locomotion which has permitted the radiation of birds into a wide range of niches. In this chapter I explore how the mechanics of flapping flight have molded the flight adaptations of birds. The paper has three main threads. First, I describe recent theoretical and experimental studies on flapping flight aerodynamics and demonstrate how the mechanical requirements of locomotion are reflected in wingbeat kinematics, in vortex wake structure, and in the action of the pectoral musculature. Next, I consider how flight performance varies with size; scaling has become a central tool in the analysis of flight in birds and has proved a useful means of predicting how different mechanical, physiological, and ecological parameters change in importance with size, morphology, and behavior. However, scaling is frequently misinterpreted: it is size-dependence of the constraints on adaptation which lead to allometric consistency in avian flight morphology, and many of these constraints can be related directly to flight mechanics. Finally, I use a multivariate analysis of wing morphology to demonstrate how these constraints interact to different degrees in different birds and underlie correlations among flight morphology, ecology, and behavior. These threads are then brought together in a discussion of the conjectural relationships between fitness and the evolution of specializations in flight morphology.
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