At one time students of bird migration believed normal migratory movements took place at heights above 15,000 feet. They reasoned, somewhat uncertainly, that flying became easier as altitude was gained. It has now been shown, through comprehensive radar studies, that 95 percent of the migratory movements occur at less than 10,000 feet, and the bulk of the movements occur under 3,000 feet. However, birds can and do fly well over 15,000 feet without apparent ill effects. The physiology of long-distance flight at high altitudes is of great interest but can only be touched on briefly in this discussion. Bird flight at 20,000 feet, where less than half the oxygen is present than at sea level, is impressive if only because the work is achieved by living muscle tissue. A Himalayan mountain climber at 16,000 feet was rather amazed when a flock of geese flew north 2 miles over his head honking as they went (Swan 1970). At 20,000 feet a man has a hard time talking and running or other rapid movements are out of the question; but those geese were probably flying at 27,000 feet and even calling while they traveled at this tremendous height. Accurate observations on the altitude of migratory flights is scanty, although altimeter observations from airplanes and radar are becoming more frequent in the literature. An example is the report of a mallard struck by a commercial airliner at 21,000 feet over the Nevada desert (Manville 1963). It is, of course, obvious that some birds must cross mountain ranges during migration and attain great altitudes. Numerous observations have come from the Himalayas (Geroudet 1954; Swan 1970). Observers at 14,000 feet recorded storks and cranes flying so high that they could be seen only through field glasses. In the same area large vultures were seen soaring at 25,000 feet and an eagle carcass was found at 26,000 feet. The expedition to Mt. Everest in 1952 found skeletons of a pintail and a black-tailed godwit at 16,400 feet on Khumbu Glacier (Geroudet 1954). Bar-headed geese have been observed flying over the highest peaks (29,000+ feet) even though a 10,000-foot pass was nearby. Probably 30 or more species regularly cross these high passes (Swan 1970).