The roller coaster life of the world's highest flying bird, the bar-headed goose, has been revealed by a new Exeter University study.
It seems the big bird employs an unusual flight strategy when migrating at extreme altitudes across the Himalayas in order to cope in the relatively low-density mountain atmosphere.
Historically, it was commonly assumed the remarkable species would fly to high altitudes fairly easily and then remain there during their flights, possibly benefitting from a tailwind.
But the new Exeter research shows that the geese perform a sort of roller coaster ride through the mountains, essentially tracking the underlying terrain even if this means repeatedly shedding hard-won altitude only to have to regain height later in the same or subsequent flight.
Dr Lucy Hawkes of the University of Exeter said: "Whenever I walk up a hill on a hike, I hate having to descend again and lose all that arduous gain – these geese aren't like that. It seems to be better for them to stay close to the ground, where the air is densest and the oxygen more abundant, at all times. They don't even seem to mind climbing (during flight) that much and they are certainly very good at it."
The birds adopt this roller coaster strategy as flying at progressively higher altitudes becomes more difficult, as the decreasing air density reduces the bird's ability to produce the lift and thrust required to maintain flight.
The new study showed that the wingbeat frequency of bar-headed geese gradually increased with altitude and reduced air density, but was very precisely regulated during each flight.
Dr Hawkes said: "Our highest single records were of birds flying briefly at 7290 m and 6540 m and seven of the highest eight occurred during the night. Interestingly, flying at night means that the air is colder and denser and, again, would reduce the cost of flight compared to the daytime. "
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