Most migrating birds fly at altitudes within the range 150–600m (490–1,970ft). However, some species are known to climb considerably higher, particularly if their migration routes take them across mountain ranges. For example, in the 1950s, an expedition to Mount Everest found skeletons of Northern Pintail (Anas acuta) and Black-tailed Godwit (Limosa limosa) at an altitude of 5,000m (16,000ft) on the Khumbu Glacier.
So what is the world’s highest flying bird? Well, the record is indisputably held by an unfortunate Rüppell's Vulture (Gyps rueppellii). This hapless bird was sucked into the jet engine of a plane flying over Ivory Coast at an altitude of 11,300m (37,000ft) above sea level on 29 November 1973.
But soaring birds such as vultures can take advantage of the slightest upward air current to reach great heights with little effort. So which is the highest flying bird that depends mainly on muscle-power? The award must go to — insert drum roll here — the Bar-headed Goose (Anser indicus).
Anecdotal reports suggest that Bar-heads can fly even higher than this. They have apparently been heard flying across the summit of Mount Makalu – the fifth highest mountain on earth at 8,481m (27,825ft). And George Lowe, who supported Edmund Hillary and Tenzing Norgay in the first ascent of Mount Everest in 1953, reported seeing geese fly over the top of the world’s highest mountain at around 8,840m (29,000ft).
It was long thought that Bar-headed Geese reached these high altitude by catching a ride on the jet stream across the mountains. However, a recent study found that they prefer to fly early in the morning when there is less wind, spurning the updraughts or tailwinds that most other migrating birds would use.
So how do these birds manage to attain a height at which the air is so thin that it provides less than half the oxygen available at sea level? Not surprisingly, studies have shown that they have to flap much harder, putting in around 30 per cent more effort at altitude than at lower altitudes. But research has also shown that, in common with Rüppell’s Vulture, the Bar-head’s blood cells contain a special type of haemoglobin — the blood protein responsible for transporting oxygen around the body — that absorbs oxygen more quickly at high altitudes.
Another factor is that the bird’s wing muscle fibres are particularly dense in small blood vessels (capillaries), which extend especially deeply into the muscles. And each muscle cell’s mitochondrion — the “powerhouse” that generates cellular energy — is found close to the cell membrane adjacent to capillaries, which reduces the oxygen’s diffusion distance within the cell.
All these adaptations show that, compared with low-altitude geese and ducks, Bar-heads have evolved beautifully to cope with thin air by enhancing the oxygen supply to their flight muscle.