The geese that can conquer Mount Everest

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

The geese have been recorded at heights of more than 7,000m (23,000 ft) and mountaineers have claimed they have seen the birds fly over Mount Everest.

Their ability to fly in such extreme conditions has fascinated scientists for decades, as the BBC's science reporter Victoria Gill reports.