The Avian Flight System

Perhaps nothing in the biological world has captured the human imagination as much as a bird in flight. The beauty of a soaring gull, the agility of a sparrow landing in a tree, the ferocious grace of a falcon's attack all bespeak the amazing versatility of the avian flight apparatus. Scholars such as Leonardo da Vinci strove to understand and duplicate the intricacies of avian flight (Hart 1961), and some of the earliest work in zoology concerned avian flight. Perhaps what inspires us most about birds is their apparent ability to fly effortlessly. To a bird, all the intricacies of aerodynamics and wing control come naturally. Flying, however, is not as simple as it looks. In fact, we are only beginning to understand the physics and mechanics of avian flight (Rayner 1988, Norberg 1990, Goslow et al. 1990).

The key to flight is the generation of thrust through the production of a vortex wake during a downstroke, coupled with the shedding of the vortex during the downstroke-upstroke transition. The downstroke pushes the wing surface down through the air, which produces a forward thrust and generates lift. The upstroke moves the folded wing into the raised position for the next downstroke. While the upstroke is aerodynamically passive in low-speed flying, it takes on different aerodynamic functions during medium- and highspeed flight (Rayner 1988, Tobalske 2000).

For flight, a bird needs an airfoil, which is made of the feathers, and the ability to flex and extend the wing surface in order to complete an upstroke-downstroke cycle. Further, it needs to use these features in a way that makes flight possible. When coupled with an airfoil, the downstroke can generate a vortex wake that creates thrust and lift. The avian flight system is an intricate assembly of bones, ligaments, and muscles, to say nothing of the feathers. Feathers provide only the airfoil; the mechanics of the wing itself are provided by the bones, ligaments, and muscles.

To fly, an animal must be able to complete an upstroke-downstroke cycle, or flight stroke. The flight stroke includes the ability to make the transitions between upstroke-downstroke and downstroke-upstroke (or recovery stroke). To make these transitions, the animal must be able to fold and extend the wing surface. Different flying organisms have achieved this transition in different ways. Birds have a unique way of folding and extending their wings that is both coordinated and automatic. Because of the way that birds fold and extend their wing surfaces, eliminating the automatic flexion and extension system prevents birds from flying (Fisher 1957; Vazquez 1992, 1993, 1994).

This system is irreducibly complex in Behe's (1996) original sense: modern birds cannot fly unless all the parts of the automatic flexion and extension system are present and working together. (True, bats and pterosaurs fly without such a system, but the system in birds may still be irreducibly complex. The bacterial flagellum, after all, may be irreducibly complex even though humans move around without flagella.) To appreciate the point, however, it is necessary to consider the anatomy of the avian wing in detail.

Every time a bird flaps its wings, it executes a complex, interlinked series of skeletal and muscular movements. To start a flight stroke, a bird lifts its arm with its wing folded. At the top of the reach of its shoulder, it opens its wing by extending its forearm and hand (upstroke-downstroke transition). It then pulls its wing downward (downstroke), which provides the propulsive thrust of flight. At the bottom extent of the downstroke, the bird folds its wing (downstroke-upstroke transition) and lifts its arm upward (upstroke) till it reaches the upper extent of its shoulder motion. Then the cycle begins again.

The key steps in this sequence are the folding of the wing for the upstroke, which retracts the airfoil for the upstroke, and the unfolding of the wing for

Figure 5.1. The irreducible parts of the avian flight system: (1) radius, (2) ulna, (3) radiale, (4) semilunate trochlea of the carpometacarpus, (5) ulnare, (6) M. extensor metacarpi radialis, (7) M. extensor metacarpi ulnaris, (8) M. flexor carpi ulnaris, (9) feathers.

the downstroke. Moreover, the wing is folded and unfolded by an automatic, coordinated flexing and extension of the wrist with the elbow. In this system, when the elbow flexes the forearm against the humerus, the wrist flexes the hand against the side of the forearm, folding the wing. When the forearm is extended at the elbow, the wrist straightens the hand, extending or opening the wing. These motions occur simultaneously, governed by a combination of skeletal morphology, ligaments, and muscles that originate on the humerus and insert onto the metacarpals. Figure 5.1 illustrates the components of the avian flight system. Automatic, coordinated flexion and extension are necessary for the downstroke-upstroke transition in avian-powered flight, allowing the wing to be folded up to minimize resistance during the upstroke and then re-extended for the next downstroke (Coues 1871; Fisher 1957; Dial et al. 1988; Vazquez 1992, 1993, 1994). This automatic motion is the result of a kinematic chain involving a number of key bones, ligaments, and muscles; without any of them, the bird could not fly.

Essential to this kinematic chain are the bones. The key bones are the humerus, radius, ulna, radiale, ulnare, and the semi-lunate joint surface (tro-chlea carpalis) of the carpometacarpus. When flexed and extended, the elbow behaves as a hinge so that the distal end of the ulna rotates inward during flexion and outward during extension. In birds, the elbow, forearm, and wrist act as a functional unit. When the elbow is flexed, the wrist is automatically flexed with it. Conversely, when the elbow extends, the wrist automatically extends. In birds, this functional unity is critical to the wing-folding mechanism of flight and is due to a ligamentous and skeletal link between the elbow and wrist. The radius and ulna slide parallel to each other and drive a kinematic chain that pushes the wrist closed and pulls it open. This process has been clearly explained and well illustrated by Rick Vazquez (1992, 1993, 1994) but was originally described by Elliot Coues (1871) and later documented by Harvey Fisher (1957).

The kinematic chain starts with the enlarged radial condyle of the humerus. As the elbow flexes, the radius rides over the condyle and is pushed forward, sliding parallel to the ulna in a distal direction. This in turn pushes the radiale forward. Then in concert with a series of ligaments (l. radiocarpo-metacarpale dorsale, l. radiocarpo-metacarpale craniale, l. radiocarpo-metacarpale ventrale), the radiale slides along the trochlea carpalis of the carpometacarpus, automatically closing the wrist (Fisher 1957; Vazquez 1992, 1993, 1994). During wing extension, as the elbow opens, the radius is pulled back by the dorsal collateral ligament of the elbow, which in turn pulls the radiale back; the radiale along with the ligaments then pulls the trochlea carpalis back along the radiale, straightening the wrist (Fisher 1957; Vazquez 1992, 1993).

The second part of this chain involves the muscles that drive the flight stroke. The first muscle is the M. supracoracoideus. During the upstroke, the M. supracoracoideus lifts and rotates the humerus, pulling it into position at the top of the upstroke (Poore, Ashcroft, et al. 1997; Poore, Sanchez-Haiman, and Goslow, 1997). At the top of the upstroke, the triceps extends the elbow, which in turn functions with the M. extensor metacarpi radialis (EMR) to straighten the wrist. The EMR originates on the face and outer surfaces of the distal end of the humerus, and the muscle inserts onto the extensor process of the first metacarpal without attaching to the radius or radiale (Gadow 1888-93; Shufeldt 1898; Hudson and Lanzillotti 1955; George and Berger 1966; McKitrick 1991; Vazquez 1992, 1993, 1994). This muscle functions along with the radiale and wrist ligaments for the automatic extension of the hand with the elbow. This extends the wing for the downstroke.

The downstroke is principally driven by the M. pectoralis (the breast meat of the bird), the largest muscle of the arm system. Automatic flexion is governed by two muscles, the M. extensor metacarpi ulnaris and M. flexor carpi ulnaris. At the bottom of the downstroke, the biceps muscle flexes the elbow, which in turn flexes the M. extensor metacarpi ulnaris (EMU) and M. flexor carpi ulnaris (FCU) muscles. These muscles flex the hand back against the forearm, folding the wing. The EMU originates on the outside of the distal end of the humerus and inserts onto the dorsal surface of the second metacarpal of the carpometacarpus. The FCU originates on the inner side of the distal end of the humerus and attaches to the carpal bone called the ulnare (Gadow 1888-93; Shufeldt 1898; Hudson and Lanzillotti 1955; George and Berger 1966; McKitrick 1991; Vazquez 1992, 1993, 1994). The FCU acts in concert with the ulnare and the ulnocarpo-metacarpale ventralis ligament, which pulls the hand back against the ulna (Vazquez 1992, 1993).

The components minimally necessary to accomplish this automated upstroke-downstroke cycle have been experimentally tested (Fisher 1957; Goslow et al. 1989; Vazquez 1992, 1993, 1994; Poore, Ashcroft, et al. 1997; Poore, Sanchez-Haiman, and Goslow, 1997). The critical parts are the radiale, ulnare, radius-ulna, trochlea carpalis, M. extensor metacarpi radialis, M. extensor metacarpi ulnaris, and M. flexor carpi ulnaris (see figure 5.1). Remove any one of these skeletal or muscular components, and the system will not function.

The avian system also meets Dembski's and Behe's standard of irreducible complexity by having well-matched parts (Behe 2000, Dembski 2002b). The radius is attached by ligaments to the ulna and humerus, which guides the radius over the radial (dorsal) condyle; the radius fits against the radiale; the radiale fits into the trochlea of the carpometacarpus; and so on.

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