Many birds live and fly at extremely high altitudes, where the oxygen supply may be only one-fourth to one-third that at sea level. At such altitudes, which extend beyond 9,000 meters (29,500 feet), mammals including humans become oxygen-starved, cold, and comatose, especially if they try to exercise. The feats of high-flying birds are especially remarkable because the oxygen requirement for flight is 20 to 40 times greater than at rest, and greater than the oxygen demand for any other form of exercise. To understand fully the physiological ecology of birds, therefore, it is necessary to study them while they fly at high altitudes. No information presently exists about the physiological ecology of high-altitude bird flight, although some information has been obtained under resting conditions. To answer the questions--How do birds fly at high altitude, and how did they evolve this ability?--this project will investigate the mechanism of oxygen transport from regions of low supply (the atmosphere) to regions of high demand (the flight muscles). The research will study birds actually flying ( in a wind tunnel) under simulated high altitude conditions. For comparison, birds will also be studied while flying at low altitudes and while resting at simulated high or low altitudes. Most of the research will focus on the role of the blood, blood vessels, heart, and lungs. The spleen, a storage site for the red blood cells that carry oxygen, may increase the red-cell supply during flight, so this will also be investigated. Most birds previously studied, including the White-necked Ravens (Corvus cryptoleucus) and Rock Doves (Columba livia) to be used in this research, are never found at high altitude. Yet they tolerate exposure to extremely high altitudes while at rest, and their tolerance improves with increasing exposure. These acclimational adjustments turn out to resemble the ones observed in mammals, including humans, that exercise regularly. Does exercise in birds evoke changes similar to the ones caused by altitude acclimation? If so, then this may indicate that high- altitude tolerance in birds may have evolved secondarily to flight. To answer this question, the research will also address the effects of exercise conditioning. In addition to its contributions to our understanding of physiological ecology and evolution in birds, the research will shed light on mechanisms by which a prominent group of warm blooded air breathers tolerate oxygen-starving conditions. It is hoped that such knowledge may be useful in ultimately dealing with pathological oxygen starvation in humans, which often occurs in association with heart, vascular, lung, and blood diseases.
