Respiratory physiology of house sparrows in relation to high-altitude flight.

Some birds perform flapping flight at remarkably high altitudes. A commercial aircraft flying at 6400 m. (21,000 ft.) was struck by an object, evidently a mallard duck from the feathers recovered later from a dent in the aircraft (Manville, 1963). Radar echoes attributed to passeriform and charadriform birds have been regularly detected at altitudes of 6100 m. (20,000 ft.) or more during night migrations, although most birds migrate at altitudes below 1800 m. (6000 ft.) (Lack, i960; Nisbet, 1963). There are reports of birds flying at 6100 m. or more in mountains (Aymar, 1935; Meinertzhagen, 1955). Hunt (1954), describing the 1953 British expedition to Everest, mentions that birds were observed flying at altitudes above 7940 m. (26,000 ft.). Because of the properties of the atmosphere at 6100 m. and above, and human responses to these altitudes, it is difficult to understand how birds could fly at such altitudes. The atmospheric pressure at 6100 m. is 349 mm. Hg (Spector, 1956) with a partial pressure of oxygen (PO2) °f 73 Hg. These values are less than half those at sea level. Bird flight at sea level probably requires a rate of oxygen consumption 8 or more times that at rest (Lasiewski, 1963; Le Febvre, 1964; Tucker 1968). In humans, such an increase in oxygen consumption represents heavy work. Resting unacclimated men at 6100 m. are in a state of incipient hypoxic collapse after 10 min. (Armstrong, 1952), and men acclimated to this altitude can sustain oxygen consumption at 8 times the basal level for only 5 min. (Pugh, 1958). Coma and death occur in unacclimatized men exposed to 7000 m. (23,000 ft.) (Luft, 1965). How then, can birds perform the strenuous activity of flapping flight at these altitudes? In this paper I have examined some of the physiological problems of high-altitude avian flight by investigating oxygen consumption and oxygen transport by the circulatory system in resting house sparrows (Passer domesticus) exposed to a simulated altitude of 6100 m. and a temperature of 5 C. Although the sparrows under these conditions used less oxygen than during flight, oxygen consumption was more than twice the basal level. Also included are comparative data for white mice (Mus musculus) and observations on house sparrows and budgerigars (Melopsittacus undulatus) flying with and without supplemental oxygen at simulated high altitudes in a hypobaric chamber.