Natural Climbers: Insights from Avian Physiology at High Altitude

Abstract

Parr, Nicole, Matt Wilkes, and Lucy Alice Hawkes. Natural climbers: insights from avian physiology at high altitude. High Alt Med Biol. 20:427-437, 2019.-High altitudes are physiologically challenging: the hypobaric hypoxia, cold, and increased ultraviolet radiation mean humans ascending to high altitude faster than they acclimatize risk life-threatening illnesses. Despite such challenges, birds can thrive at high altitudes and some even complete metabolically costly migrations across the world's highest mountain ranges. We outline the aspects of avian anatomy and physiology that confer advantages at each level of the oxygen transport cascade and compare them with those of human and nonhuman mammals. We also discuss additional adaptations that have been described for high-altitude specialist species of birds and how these are mirrored in high-altitude adapted mammals.

Extreme altitude changes between night and day during marathon flights of great snipes

“…The highest known altitude of any identified bird is the 11,000 m (37,000 ft) of a Rü ppell's vulture Gyps rueppellii that collided with an airplane. 29,35 Not only did one great snipe reach 8,700 m; it stayed above 8,000 m for several hours. With an estimated ambient temperature of À21 C, the additional chilling effect from an air flow proportional to flight speed, the very low oxygen and air pressures (350 hPa), and strong ultraviolet radiation, this is a truly inhospitable environment.…”

“…A complicating factor that needs to be given more attention is that by flying higher, migrants will often experience lower air humidity (Figure S1) and thus become exposed to an increased risk of dehydration. 21,29 Great reed warblers showed altitude changes between night and day during their crossing of the Sahara and the Mediterranean Sea (they traveled exclusively by night outside this region 6 ). The great snipes showed a prominent diel altitude cycle also over continental Europe and south of the Sahara.…”

“…The highest altitude to date is 8,200 m, a flock of whooper swans Cygnus cygnus flying between Iceland and Scotland. 29,34 In a study of bird migration by radar at the Negev desert in Israel, birds were regularly found migrating 5,000-7,500 m, with a single echo at ''almost 9,000 m a.s.l.'' 2 Based on wingbeat frequencies and direct observations, it was suggested that some of the high-altitude migrants were shorebirds.…”

Shorebirds wintering in Southeast Asia demonstrate trans-Himalayan flights

“…The physiological adaptations to flight and life at high altitude with thin air and low oxygen levels have been well studied in Bar-headed Geese and Andean Geese ( Chloephaga melanoptera ) 44 , 45 . Common features of birds that migrate at high altitudes include more effective gas exchange with greater hypoxic ventilatory response, larger lungs with higher capillarization, enhanced cardiac output, greater oxygen delivery to skeletal muscles due to increased haemoglobin oxygen affinity, and higher capillary to fibre ratios with increased oxidative capacity compared to low altitude migrants 45 , 46 . Migratory birds are notable for their physiological flexibility during migration, including anticipatory increases in mass, organ sizes and modulation of underlying biochemistry 47 – 49 .…”

The alveolar gas equation: do lessons from WW2 research into high flying pilots provide insights into the adaptation to high altitude flight in birds?

“…Since controlled hypocarbia is not recognized as an altitude adaptation in birds, many alternative explanations have been suggested (Parr N, et al, 2019). In terms of relative importance, it should be remembered that those various responses all share a common factor -the bird has to be alive and that requires a survivable PaO2, which in turn, requires hyperventilation.…”