High-flying goose raises the bar

ABAR-HEADED goose visits our local estuary. It hangs out with swans, gorging itself on stale bread offered by local children.

This goose doesn’t occur in Ireland; our visitor is an escapee from a wildfowl collection. It’s of no interest, therefore, to the bird-watching fraternity, which is a pity because the bar-head is one of the world’s most extraordinary birds.

This ornamental fowl is a familiar resident of city parks and ponds. The plumage is grey and white, with a black bar across the crown and another around the back of the neck. Bar-heads hit the headlines during the last H5N1 bird flu scare; they allegedly carried the lethal flu virus from the crowded farmsteads of south-east Asia to eastern Europe.

Each spring, flocks of these geese head northwards from the Indian lowlands to Russia. On the way, they encounter the world’s most formidable mountain barrier, the Himalayas. To get past this, they must lift their heavy bodies to an altitude of almost 9,000 metres and fly for hours in rarefied air.

How they manage to do so had been a biological mystery until now. Oxygen levels up there are so low that an ordinary farmyard goose would die even if it remained at rest. In 1978, Reinhold Messner and Peter Habeler reached the summit of Everest without artificial oxygen but only after years of rigorous training. That they didn’t suffer permanent brain damage is a medical enigma.

It was thought that the bar-heads ascend the mountains in daylight, using updrafts to help them climb and that the birds fly by day. However, according to a report in the November edition of Scientific American, new research has turned this theory on its head.

Charles Bishop of the University of Wales in Bangor is an expert on the oxygen requirements of mammals and birds. He and his team fitted backpacks to bar-heads prior to their migration. The packs held instruments to determine the speed, altitude and locations of the birds during their journey. Miniature transmitters relayed the data to the scientists over a satellite link. As the birds crossed the mountains, researchers tracked their progress.

The results were intriguing; almost everything previously believed about the famous migration turned out to be false. The sun warms the mountain slopes during the day, generating updrafts which can reach speeds of 20km per hour. To make use of these for their arduous seven-hour climb, the birds would have to set out in the early afternoon. However, to everyone’s surprise, they did not do so until nightfall. The up-draughts had ceased by then. Indeed, downdraughts had begun.

Did the geese get it wrong? Bishop believes that the crucial issue for the bar-heads is oxygen demand. A bird requires 10 to 20 times more oxygen when flying than at rest. This gas accounts for a fifth of the mixture we call air, which is perfectly adequate at sea level. However, with increasing height, air density falls and there is progressively less oxygen for a given volume. At Himalayan altitudes, getting sufficient oxygen is a major problem. But there’s another factor at play; air temperatures fall at night. Cool air is denser than warm air, so more oxygen is available to birds flying in darkness. There are also aerodynamic benefits. It’s harder for wings to generate lift and thrust when the air is thin. Flying is more efficient in the denser night air. Also, the colder air helps dissipate the heat generated by hard-working flight muscles.

There was yet another surprise in store for the scientists; the geese managed to cross the huge mountain range in a single day. Bar heads have extra large wings and lungs. An elaborate system of blood vessels feeds their massive flight muscles. It’s no surprise, therefore, that these super-efficient oxygen users are among the world’s high-flying specialists. Pilots claim to have seen them over Everest itself. If these reports are true, the bar-headed goose, not Messner and Habeler, hold the animal high altitude record.