They were once reported to fly over Mount Everest itself, almost 9 kilometres up.
Now it turns out that bar-headed geese are a bit more level-headed about how they make their gruelling trip each year from Mongolia to spend winters in warmer India. Instead of climbing to dizzying altitudes and flying in a straight line over the mountain tops, they follow a “roller-coaster” route, closely tracking the contours of the mountains and valleys below.
They do this to save energy – which is paramount considering that the geese flap their wings for the entire journey, without taking a break to glide.
“It worked out that it was eight per cent cheaper for them energy-wise if instead, they followed the undulating pattern of the landscape,” says Charles Bishop of Bangor University in Wales, UK, whose team tracked a flock of geese across the mountains.
They implanted small monitors that capture a range of data into the abdomens of 29 wild geese caught at the Terkhiin Tsagaan Lake in Mongolia. The monitors allowed them to calculate how high the geese flew and how much energy they burned.
They measured the birds’ heart rates, worked out how many times a second they flapped their wings with data from an accelerometer and monitored each bird’s altitude from measurements of atmospheric pressure, which diminishes with altitude.
They obtained enough data for analysis from seven of 14 birds recaptured a year later when they returned to the same lake.
Surprisingly, they found the geese closely hugged the contours of the route, rising and falling as if on a fairground roller-coaster. While flying, the birds averaged 328 heartbeats a minute – compared with around 70 beats a minute when resting – and 4 wingbeats a second.
Into thin air
From the heart and wing-flap data, the researchers worked out the birds’ energy expenditure. Despite repeatedly sacrificing hard-won altitude, the roller-coaster geese spent less energy overall. The predominant gain came from higher oxygen content and air pressures at the lower altitudes.
Extra oxygen gives the birds more energy with each breath, while the extra air density enables each wingbeat to generate more forward thrust. “It’s a double-win for them,” says Bishop.
Sometimes, the birds also appeared to benefit from updraughts around the mountains, gaining altitude for as much as half an hour without flapping more or increasing their heart rate.
Bar-headed geese are the only large birds known to cross the Himalayas without gliding, even when descending. Others, such as demoiselle cranes, make the journey too but have broader wings, flap less vigorously and do spend time gliding.
A feather in their cap
Bishop says that previous studies have shown that bar-headed geese have lungs that are a quarter larger than those of other geese. Their haemoglobin also captures oxygen more efficiently from thin atmospheres, and they have extra blood vessels that feed their hearts and muscles.
“We know already they’re adapted to flying high, but our study shows they don’t fly as high as previously thought,” says Bishop.
“This is a species that has received lots of attention for its supposed feats of endurance, but actual data have been lacking,” says Steve Portugal of Royal Holloway, University of London, whose research last year showed how ibises benefit from flying in a “V” formation. “This paper finally takes the geese from anecdote to data, and what’s novel is their use of topography to reduce flight costs.”
“Geese are not renowned for flight manoeuvrability or sophistication, just flying fast and straight,” says Portugal. “But here, they’re using a sophisticated mechanism using updraughts and wind-assisted flight that’s more akin to how albatrosses use wind from waves, or a vulture searching for thermals, to reduce the energetic costs of flight.”
Journal Reference: Science, DOI: 10.1126/science.1258732
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