The world’s champion high-altitude migratory bird uses a unique “roller-coaster” flight strategy to save energy.
How geese fly high. I’m Bob Hirshon and this is Science Update.
Bar-headed geese migrate across the Himalayas twice a year. To find out how they accomplish this high-altitude journey, University of Bangor biologist Charles Bishop and his team outfitted the birds with heart rate monitors and accelerometers.
The problem with flying high is that the air has become less dense and it’s becoming more and more difficult to fly. If they were to go up and just fly along at that level, they’re spending an awful lot of time at a very difficult, high-energy flight. What we actually discovered was that instead, these birds are regularly going up and then down again, and up and then down again, within the same flight. And we called this the roller coaster strategy, and this was kind of unexpected.
Bishop’s team reports in the journal Science that even factoring in the costs of climbing back up again, this flight strategy saves the birds energy overall. I’m Bob Hirshon, for AAAS, the science society.
Making Sense of the Research
The bar-headed goose, a pale grey bird with an orange beak and legs and two striking black bars on its head, lives in central Asia. Living as it does in an area that includes three very high mountain ranges, the bar-headed goose is widely believed to make the highest altitude migration on earth.
At high elevations, the atmosphere has less oxygen and the thin air provides less lift to aid bird flight. So, how does the bar-headed goose accomplish this difficult migration? That’s what Charles Bishop and his team of researchers are trying to find out.
“We wanted to know how high they flew, what were their flight paths and strategy with respect to the weather conditions, how difficult did they find these journeys, and how much energy did it require,” says Charles Bishop, a zoologist at Bangor University in the United Kingdom.
The old assumption was that bar-headed geese would fly to high altitudes relatively easily and then remain there during their flights, possibly benefitting from a tailwind. But by tracking a flock of bar-headed geese from Mongolia to India, researchers found that the birds descend to a lower altitude before flying to new heights. Their study, published in Science 16th January 2015, shows that the geese perform a sort of roller coaster ride through the mountains, essentially tracking the underlying terrain even if this means repeatedly giving up altitude only to have to regain it later.
How does this up and down flying strategy help the birds? Flying at progressively higher altitudes is very difficult as the decreasing air density reduces the bird's ability to produce the lift and thrust required to maintain flight. The birds also face the problem of reduced oxygen availability as the atmospheric pressure and oxygen levels drop in the higher elevations.
Professor Pat Butler from the University of Birmingham said the geese would seek out the side of valleys in order to swerve higher—in the way a roller coaster might swoop around a corner and upwards. The roller coaster pattern helps birds conserve energy as they migrate over the Himalayas.
“During these moments,” says Butler, “it seems likely that the bar-headed geese are flying on the windward side of a valley wall. This would give them the best opportunity of obtaining assistance from wind that is deflected upwards by the ground, providing additional rates of ascent with either a reduction in their energetic costs or at least no increase.”
How is this possible? "The physiology of bar-headed geese has evolved in a number of ways to extract oxygen from the thin air at high altitudes," said Dr. Graham Scott, another member of the study team. "As a result, they are able to accomplish something that is impossible for most other birds."
Now try and answer these questions:
- Why is flying at high altitudes more difficult than flying at low altitudes?
- What did the scientists do to track the birds’ flight pattern?
- Before this research, how did scientists think that bar-headed geese were able to fly at such altitudes?
- Why is the strategy used by the birds described as a roller coaster maneuver?
- How does this strategy help the birds save energy for the strenuous flight?
- In addition to this unique flight strategy, what else helps the geese fly such long distances at high altitudes?
Read Feather Biology to learn more about how birds fly.
Check out the How Do Birds Fly animation to learn more about different techniques birds use for flight.
In addition to the resources mentioned, you can extend the concepts in this lesson by helping your students explore the role that variation within a species plays in Feathers: The Evolution of a Natural Miracle.
To learn more about bird migratory patterns and the methods researchers use to study them, see Bird Populations.
Birds of Prey
Feathers: The Evolution of a Natural Miracle
9-12 | Hands-On
Organisms in Motion: Practical Applications of Biological Research
9-12 | Video
6-12 | Audio