Bar-headed geese and the physiology of high-altitude flight

Abstract: How geese can beat their wings where angels fear to fly…

Imagine you’re a mountaineer of the highest order. After years of training and planning you’re on Mount Everest; and after weeks of acclimatisation, you’ve left base camp far behind and are on the last stretch to the summit.

The air is so thin, the temperature so low and the winds so strong that without the specially designed clothing you’re wearing and the oxygen you’re breathing you’d be dead within minutes. You’re clawing your way upward, bit by bit, every breath belaboured even with the precious oxygen you’re using.

Eventually, almost unbelievably, you make it to the summit. You have only minutes before you have to descend; but for those few, brief moments in time, you’re on top of the world. You gaze around knowing that nothing else lives at this height. You slowly, painfully raise your arms in silent cheer…and then you watch a group of geese fly by, energetically flapping their wings, without even batting an eyelid, honking happily and dressed in nothing but a thin layer of feathers.

Meet the world’s highest flying birds…bar-headed geese, which beat their wings where even angels fear to fly.

Hawkes on geese

So what on earth are bar-headed geese doing flying across the Himalayas, and how do they seem to relish flying so high?

Dr Lucy Hawkes is a post-doctoral researcher at the Department of Biological Sciences at the University of Bangor in Wales. Since 2008 Dr Hawkes has been working with Dr Charles Bishop, the principal investigator of a study of migration patterns of bar-headed geese as they cross the Himalayas – twice every year – between Tibet and India. She has some insight into how they perform at such heights.

First of all, she points out, birds, in general, have a different physiology to humans, one that has adapted to operating at altitude.

“Birds don’t get the problem of altitude sickness because they have a much stronger barrier between their blood and alveoli. Likewise, they don’t change their circulation to their brain, and therefore are not susceptible to a swelling of their brain tissue,” she said.

Furthermore, because birds generally have a higher body temperature to humans, and generate a tremendous amount of heat when they fly, they can operate in cold environments. In fact, according to Dr Hawkes, studies of geese flying in wind tunnels have shown that they fly better when the air is colder.

At the edge of ornithological endurance

But even birds have a limit to how high they can fly, and they all dip their wings in respect to bar-headed geese, which skirt at the very edge of ornithological endurance.

They are the extreme athletes of the bird world, and part of their capability lies in their muscles. Bar-headed geese have a greater capillary density in their muscles.

“This means,” Dr Hawkes explained, “that they have more blood working through their muscles, which means a greater quantity of ‘fuel’ making its way to their muscles”.

This is especially important when we consider that the heart is, of course, also a muscle. In studies comparing bar-headed geese with pink-footed geese, the former were shown to have 25-35 per cent more blood going to their heart than the latter, meaning their hearts can beat faster and more efficiently.

Their wings are also proportionally larger than those of other geese. This means that each bit of surface area is being asked to carry less mass, thereby reducing the amount of work necessary.

Finally, they have a secret tool hidden in their blood. According to Dr Hawkes, “bar-headed geese haemoglobin is effectively ‘hungrier’ for oxygen. Even if there’s not much oxygen about, these geese can get more of it into their haemoglobin than other geese”.

Part of this is to do with their ability to hyperventilate faster, almost twice as fast as other waterfowl.

The fact is, bar-headed geese need all these remarkable physical characteristics to fly the extreme altitudes necessary to cross the Himalayas.

Why fly so high?

But why even cross the Himalayas? Dr Hawkes has a possible answer for that as well, and it’s quite simple: because the Himalayas have popped up in the way of their migration route.

Whilst the Himalayas are the tallest mountain range in the world, they are also amongst the youngest – a mere 50 million years old, and still growing at about 5 millimetres a year. Bar-headed geese are not all that much younger. They belong to the family Anatidae, a very old order of birds, possibly tens of millions of years old.

“This means”, says Dr Hawkes, “that when the progenitor of the bar-headed geese was making its migration over the Himalayas, the mountains weren’t anywhere near as high as they are today”. As a result, over the millions of years that the Himalayas have grown, the birds have physiologically evolved to deal with the demands of flying over them.

And ‘flying’ is the operative word. What has become clear from tracking bar-headed geese is that there is no evidence that they glide at all. They may use prevailing headwinds to give them lift, but it’s at the expense of forward movement. They cross the Himalayas in one go, flapping their wings all they way. It’s an exhausting procedure. “They’re honest little hard workers”, says Dr Hawkes. “They just get the job done”.

But precisely how they get the job done is still an active area of research. For instance, while no one denies the incredible endurance bar-headed geese display, questions have been raised about how high they actually fly, and whether or not in crossing the Himalayas the birds actually do fly right over them, or whether they take a route through them.

Dr Bishop, Dr Hawkes and their team have used GPS tracking, accelerometry, heart rate logging and respirometry to not only measure the geese’s performance but also the exact paths they follow.

Their research, to be published later this year, promises to lift the lid on how high bar-headed geese really do fly, and whether if you’re at the top of Mt Everest you’ll ever see them go by.

Originally published in Elements, 29 February 2012