Adaptations for Flight T he evolution of flight has endowed birds with many physical features in addition to wings and feathers. One of the requirements of heavier-than-air flying machines, birds included, is a structure that combines strength and light weight. One way this is accomplished in birds is by the fusion and elimination of some bones and the "pneumatization" (hollowing) of the remaining ones. Some of the vertebrae and some bones of the pelvic girdle of birds are fused into a single structure, as are some finger and leg bones -- all of which are separate in most vertebrates. And many tail, finger, and leg bones are missing altogether. Not only are some bones of birds, unlike ours, hollow, but many of the hollows are connected to the respiratory system. To keep the cylindrical walls of a bird's major wing bones from buckling, the bones have internal strut-like reinforcements. The pneumatization of bird bones led to the belief that birds had skeletons that weighed proportionately less than those of mammals. Careful studies by H. D. Prange and his colleagues have shown this not to be the case. More demands are placed on a bird's skeleton than on that of a terrestrial mammal. The bird must be able to support itself either entirely by its forelimbs or entirely by its hindlimbs. It also requires a deep, solid breastbone (sternum) to which the wing muscles can be anchored. Thus, while some bones are much lighter than their mammalian counterparts, others, especially the leg bones, are heavier. Evolution has created in the avian skeleton a model of parsimony, lightening where possible, adding weight and strength where required. The results can be quite spectacular: the skeleton of a frigatebird with a seven-foot wingspan weighs less than the feathers covering it! Not all birds have the same degree of skeletal pneumatization. To decrease their buoyancy and make diving easier, some diving birds, such as loons and auklets, have relatively solid bones. Those birds are generally less skillful fliers than ones with lighter skeletons. Birds have found other ways to lighten the load in addition to hollowing out their bones. For instance, they keep their reproductive organs (testes, ovaries and oviducts) tiny for most of the year, greatly enlarging them only during the breeding season. The respiratory system of birds is also adapted to the demands of flight. A bird's respiratory system is proportionately larger and much more efficient than ours -- as might be expected, since flight is a more demanding activity than walking or running. An average bird devotes about one-fifth of its body volume to its respiratory system, an average mammal only about one-twentieth. Mammalian respiratory systems consist of lungs that are blind sacs and of tubes that connect them to the nose and mouth. During each breath, only some of the air contained in the lungs is exchanged, since the lungs do not collapse completely with each exhalation, and some "dead air" then remains in them. In contrast, the lungs of birds are less flexible, and relatively small, but they are interconnected with a system of large, thin-walled air sacs in the front (anterior) and back (posterior) portions of the body. These, in turn, are connected with the air spaces in the bones. Evolution has created an ingenious system that passes the air in a one-way, two-stage flow through the bird's lungs. A breath of inhaled air passes first into the posterior air sacs and then, on exhalation, into the lungs. When a second breath is inhaled into the posterior sacs, the air from the first breath moves from shrinking lungs into the anterior air sacs. When the second exhalation occurs, the air from the first breath moves from the anterior air sacs and out of the bird, while the second breath moves into the lungs. The air thus moves in one direction through the lungs. All birds have this one-way flow system; most have a second two-way flow system which may make up as much as 20 percent of the lung volume. In both systems, the air is funneled down fine tubules which interdigitate with capillaries carrying oxygen-poor venous blood. At the beginning of the tubules the oxygen-rich air is in close contact with that oxygen-hungry blood; farther down the tubules the oxygen content of air and blood are in equilibrium. Birds' lungs are anatomically very complex (their structure and function are only barely outlined here), but they create a "crosscurrent circulation" of air and blood that provides a greater capacity for the exchange of oxygen and carbon dioxide across the thin intervening membranes than is found in mammalian lungs. Contrary to what was once believed, the rhythm of a bird's respiratory two-cycle pump is not related to the beats of its wings. Flight movements and respiratory movements are independent. The heart does the pumping required to get oxygenated blood to the tissues and to carry deoxygenated blood (loaded with carbon dioxide) away from them. Because of the efficiency of the bird's breathing apparatus, the ratio of breaths to heartbeats can be quite low. A mammal takes about one breath for every four and one-half heartbeats (independent of the size of the mammal), a bird about one every six to ten heartbeats (depending on the size of the bird). A bird's heart is large, powerful, and of the same basic design as that of a mammal. It is a four-chambered structure of two pumps operating side by side. One two-chambered pump receives oxygen-rich blood from the lungs and pumps it out to the waiting tissues. The other pump receives oxygen-poor blood from the tissues and pumps it into the lungs. This segregation of the two kinds of blood (which does not occur completely in reptiles, amphibians, and fishes) makes a bird's circulatory system, like its respiratory system, well equipped to handle the rigors of flight. The flight muscles of most birds are red in color ("dark meat") because of the presence of many fibers containing red oxygen-carrying compounds, myoglobin and cytochrome. They are also richly supplied with blood and are designed for sustained flight. Lighter-colored muscles ("white meat"), with many fewer such fibers, are found in pheasants, grouse, quail, and other gallinaceous birds. These are also well supplied with blood, are apparently capable of carrying a heavy work load for a short time, but fatigue more rapidly. If a quail is flushed a few times in a row, it will become so exhausted it will be incapable of further flight. Finally, of course, it does little good to be able to sustain flight or fly rapidly if you are always crashing into things. Although birds have found many ways to streamline, lighten, or totally eliminate unnecessary parts (like urinary bladders), they have not stinted on nervous systems. Birds have brains that are proportionately much larger than those of lizards and comparable, in fact, with those of rodents. The brain is connected to sharp eyes, and has ample processing centers for coordinating the information received from them. A bird's nerves can rapidly transmit commands of the brain to the muscles operating the wings. It is the combination of visual acuity, quick decision making, and high-speed nerve transmission along short nerves that permits a Golden-crowned Sparrow to weave rapidly among the branches of a thicket, escaping the clutches of a pursuing Sharp-shinned Hawk. SEE: Temperature Regulation and Behavior ; Metabolism ; Hawk-Eyed Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.