A tracking study has revealed the secrets of the world’s highest bird migration - the Himalayan flight of the bar-headed goose.

The geese have been recorded at heights of more than 7,000m (23,000ft). Now, a team led by researchers from Bangor University has tracked the flight and revealed the basis for the birds’ ‘rollercoaster flight’ pattern. The findings, published in Science, show how the birds hug the mountainous terrain, and that this saves energy. Bar-headed geese have fascinated biologists for decades.  They achieve physiological feats that seem impossible - flying at extreme altitude, where there is less than 10% the oxygen found at sea level. George Lowe, the New Zealand born climber who supported Edmund Hillary and Tenzing Norgay’s ascent in 1953, said he had seen the geese flying over the top of Mount Everest - the peak is approximately 29,000ft.