Geese that migrate over the Himalayas hug the terrain as they fly, riding the peaks and valleys like the climbs and dips of a roller coaster, researchers have discovered.

This unexpected flight pattern, described in the US journal Science on Thursday, may help the bar-headed geese to conserve energy. It was previously thought that the birds maintained a steady altitude above 7,000 meters.

However, the study found that overall it is more efficient for the birds to fly low to the ground and to lose and regain altitude.