Biologists have obtained new insight into the migratory habits of bar-headed geese – namely, that the birds treat mountains like roller coasters during migration, according to a study published today in Science.
Each autumn, bar-headed geese migrate south from their breeding grounds in Mongolia to Southeastern Tibet or India, Christian Science Monitor reported. In 2011, before their annual trek, an international team of researchers surgically implanted 30 geese with equipment that measures body movement, heart rate, and altitude. Only 17 geese could be recaptured after the migration, and only seven of those provided reliable data. The results, however, were astounding in that they seemed to contradict long-held assumptions about the geese’s migratory strategy.
“At the beginning we were just puzzled that there were so few reports of very high flight over the last 60 years,” Charles Bishop, the University of Bangor researcher who led the study, says. “We knew that these birds were especially well-adapted to fly in air containing very low oxygen, so we certainly expected to record some high flights. The question was, how high did they normally fly? For how long? And how difficult were such flights? How close to the limit did they have to go?”
Scientists used to think that bar-headed geese spent their entire migration at high altitudes, where only a favorable tailwind might aid their flight. But Bishop’s study, which follows up on previous research, shows that these birds change altitude as they fly over the Himalayas, hugging the mountainous terrain. By flying close to the ground as the mountainside rises and falls, their flight pattern resembles that of a roller coaster ride.
The geese do this for several reasons: first, flying 20,000 feet in the air for over 1,000 miles is hard work. Bar-headed geese are known as one of the world’s highest-flying birds – a title biologists previously attributed to their impressive flapping strength and maximally efficient respiratory systems. As it turns out, even these “superbirds” grow tired at extreme altitudes, where oxygen is scarcer. Bishop’s research shows that the geese actually prefer flying at lower altitudes, where more abundant oxygen makes for a more economical flight.
“I think it is the most economical way to travel through the mountains,” Bishop said. “It might be justified for a bird to choose to fly a bit higher if there was a tailwind helping it cover the ground quicker. In this way it would reduce the travel time as well as reduce the overall energy cost for the journey. However, it appears that the winds were seldom blowing in the right direction to help the geese with their migrations, so the best thing to do is stay as low as possible and keep in the most dense air available.”
Dropping altitudes doesn’t expend much energy either – it’s just gravity in action. On the contrary, if you considered how much energy they expend flying upward, this sporadic and thrill-seeking flight technique could seem wasteful. While this seems likely, these birds are aided by a phenomenon known as orographic lift.
“If a wind hits a mountain ridge in the ground or a large wave at sea, then some of it will be deflected upwards and create an updraft,” Bishop said. “A bird can use this to help it when climbing – it increases [the bird’s] rate of climb, reduces the energy required to climb at some rate, or a bit of both. When flying horizontally, it decreases the energy that needs to be supplied to the air to fly along.”