New Delhi, Jan. 16: Bar-headed geese from Mongolia use the Himalayan mountains as roller-coasters during their annual migratory journey into India, according to scientists who have for the first time monitored these birds while they made the challenging trans-Himalayan crossing.

The scientists who implanted tiny devices into the bar-headed geese to record their heart rates, wing flaps, oxygen consumption, and abdominal temperatures, have found the birds repeatedly climb and descend rather than fly at a steady altitude during the Himalayan crossing.

Their study of the data retrieved from the implanted devices has shown that these geese - among the heaviest in the bird kingdom - fly close to the terrain, climbing with the mountains and dropping into valleys before climbing over the next mountain.

'This ground-hugging flying pattern was a surprise,' Charles Bishop, a biologist who specialises in flight mechanics at the University of Bangor in the UK and who led the study told The Telegraph. 'The birds are smart enough to adopt a flying strategy that takes up the least energy expenditure.'

Their study's findings will be published in the US journal Science on Friday.

During winters, the geese leave the cold steppes of Mongolia and fly past the mountains into India. Bird watchers have spotted bar-headed geese at multiple locations across the country, including Bengal, Rajasthan and sites in southern peninsular India.

Scientists have long assumed that the birds, which can perform exceptionally well in low-oxygen, cross the mountains flying steady at about 6,000 metres, an altitude at which there is little terrain even in the Himalayas. Such a strategy of steady altitude would mean a single climb and a single descent. Instead, the researchers have found that the birds adopt what they call a 'roller-coaster strategy' of climbing and descending as they pass the mountains.

Bishop and his colleagues from Australia, Canada, Germany, Mongolia, the UK and the US implanted the devices in 30 birds near a lake in Mongolia. They waited for the birds to return and, using telescopes and canoes, retrieved the devices from 17 birds. Seven of these 17 had logged sufficient data for analysis.

Their study suggests that the ground-hugging strategy is economical, allowing the birds to expend less energy during the crossing. 'It helps the birds avoid headwinds that might pose some resistance to their flight as well as exploit the upward air flow in valleys,' Bishop said. The scientists estimate that birds can cross the mountains in six to 10 hours.

Bishop said he was struck by the strong relationship between tiny changes in the wing flaps, the birds' heart rates and their oxygen consumption. A 5 per cent increase in the wing flapping rate translated into a 19 per cent jump in the heart rate and corresponded to a 40 per cent rise in estimated oxygen consumption.

While some previous studies have shown that the birds are capable of flying at altitudes above 7,000 metres, most observations have found them flying below 6,000. The research team, however, did observe birds flying briefly at 7,290 metres and 6,540 metres.

Seven of the eight highest altitude flying sessions occurred during the night. 'Flying at night means the air is colder and denser and would reduce the cost of flight compared to daytime,' Lucy Hawkes, a team member now at the University of Exeter said in a release.

Bar-headed geese are heavier than most other bird species, Nyambayar Batbayar, a team member at the Mongolian Academy of Sciences said in a statement issued through the release. 'Bar-headed geese have found a way to cross the world's highest land massif remaining well within their physiological capabilities.'