An orange and brown duck species similar in size to the mallard has been recorded flying at altitudes of up to 6,800 metres (22,000 feet) to cross the Himalayas, scientists have said.

The discovery, which makes the ruddy shelduck the highest known flying duck, was made using satellite data collected from 15 of the birds by researchers from the University of Exeter.

The team wanted to find out how the species migrates from its breeding grounds north of the Himalayan mountain range to south of the Tibetan Plateau, where it is known they spend their winters at sea level.

Their route involves flying over the Himalayas in the spring to get back to their breeding grounds, meaning they have to cross terrain higher than 4,000 metres (13,000 feet), where oxygen levels are halved.

Lead researcher Nicole Parr, from the University's Centre for Ecology and Conservation on the Penryn Campus in Cornwall, said it was "the first evidence of extreme high-altitude flight in a duck".

She said: "This species has probably evolved a range of adaptations to be able to cope with flying so high, where oxygen levels are half those at sea level.

"We don't yet know the nature of these adaptations.

"Our research also shows that the ruddy shelduck has a faster climb rate than the bar-headed goose - the only waterfowl known to fly even higher."

Ruddy shelducks, it was found, avoid mountain peaks but regularly fly above 5,000 metres (16,500 feet) and sometimes go as high as 6,800 metres (22,000 feet).

Supervisor of the current study, Dr Lucy Hawkes previously tracked bar-headed geese to 7,290 metres (24,000 feet) altitude near Mount Everest in 2014.

While some birds soar higher on thermals, the geese were thought to be the world's highest-flying bird based on flapping flight.

Now the team plans to carry out further research to find out if ruddy shelducks reach similar heights to bar-headed geese.

Comparison heights:

:: Passenger jet cruising altitude: 13,000 metres (42,500 feet) (Airbus A380, Boeing 777)

:: Mount Everest: 8,848 metres (29,000 feet)

:: Bar-headed geese: 7,290 metres (24,000 feet)

:: Ruddy shelduck: 6,800 metres (22,000 feet)

:: Burj Khalifa (world's tallest building): 828 metres (2,700 feet)

:: The study, High altitude flights by ruddy shelduck (Tadorna ferruginea) during Trans-Himalayan migration, is published in the Journal of Avian Biology.