(From top) A bar-headed goose being released in a Mongolian lake after being tagged; a goose in flight; a bird with the tag; and geese breeding in Mongolia. Pictures by Nyambayar Batbayar and John Takekawa

New Delhi, May 31: Scientists tracking the trans-Himalayan flights of migratory geese between India and Mongolia have found that the birds use their own muscle power rather than take help from winds in the formidable low-oxygen environment.

An international team of scientists has found that bar-headed geese cross the Himalayas through passes between the mountains in just eight hours, climbing to altitudes of up to 6,000 metres under their own aerobic power.

“Now, we know their flight strategy — and it’s a surprise,” said Sivananinthaperumal Balachandran, a team member with the Bombay Natural History Society, Mumbai.

The research findings appeared yesterday in the Proceedings of the National Academy of Sciences.

Wildlife biologists have known for long that bar-headed geese from northern Asia migrate annually to wetlands across India but the routes they choose, the rates at which they climb and the duration of their flights over the mountains were unknown.

Scientists had assumed until now that the geese use daytime tailwinds that can give them some lift up mountain slopes. But the new study has shown that they cross the Himalayas in a single flight and, surprisingly, rather than waiting for favourable winds that could carry them up and over the Himalayas, they wait for the winds to die down — and fly during the night or early morning.

“By flying at night, the geese seem to prefer flying when (wind) conditions are more calm,” Charles Bishop, the principal investigator of the study at Bangor University in the UK, told The Telegraph. “They do not rely on winds to assist them in their migration.”

The scientists tracked the flight paths of the geese every hour using satellite tags gently tied on the backs of 18 birds from three sites — Chilika Lake in Orissa, the Koonthankulam Bird Sanctuary in Tamil Nadu, and the Terkhiin Tsagaan Lake in Mongolia.

The researchers say their findings reveal the remarkable capability of bar-headed geese to sustain flapping flight at heights where even helicopters cannot fly because of thin air and where humans — without high-altitude acclimatisation — would struggle to even walk.

“We were amazed to see geese maintaining the climbs for hours on end,” Lucy Hawkes, a team member at Bangor University, said in a statement issued through the university. At 5,500 metres above sea level, oxygen availability is only half of that at sea level.

Bishop said the calmer winds during the night or early morning may provide the birds a sense of safety. In a relatively calm atmosphere, he said, the birds may find it easier to keep together and fly in formation.

The satellite tags are matchbox-sized transmitters that allow scientists to track the birds with great accuracy. The observations showed that the geese maintained altitudes within 340 metres of the ground and thus avoided climbing any higher than necessary.

Earlier studies have shown that bar-headed geese have proportionally larger lungs than other species of water birds and that their haemoglobin can ferry oxygen more effectively than the haemoglobin of other birds.