How Does a Goose Break a World Record?

Most people have never heard of the Bar-headed Goose (Anser indicus). It is a mid-size goose, weighing in at between 1.8-3.2 kg and can live for up to twenty years in the wild. The main distinguishing feature of the Bar-headed Goose is, unsurprisingly, the two black bars on its head. The species is monotypic, meaning males and females look the same, and generally pair for life. The birds have a global range of approximately breeding from Mongolia through China and Tibet to Kyrgyzstan and spend the winter mainly in India and Bangladesh.

A Bar Headed Goose

This requires them to undertake a migration of around 1000 miles. This is not a particularly long migration; some birds such as the Artic Tern (Sterna paradisea) migrate over 20000 miles (40000 for a round trip). What’s remarkable about the migration of the Bar-headed Goose is that it takes them straight over the Himalayas. This means that the geese regularly fly at heights of 6000 metres for sustained periods of time. One radio tagged bird was recorded flying at and altitude of around 7300 metres. Climbers from Edmund Hillary’s famous 1953 ascend of Mount Everest have reported flocks of Bar-headed Geese flying over the mountain, an altitude of at least 8800 metres. Flying at such altitudes means the geese have to cope with Oxygen at a concentration that is less than 10% of that found at sea-level; as well as lower air pressure meaning the act of flying will itself be more strenuous. So how to these birds do it?

It has been known for some time that these geese have evolved several mechanisms that enable them to undertake such an astonishing feat. Firstly Bar-headed geese have a marginally larger wingspan and lower wing loading than other species of goose therefore meaning that a greater lift is produced and less power is needed for flight. Also the lungs of birds are much more efficient at extracting oxygen than mammalian lungs due to a counter exchange system lungs are superior to mammalian lungs. In Bar-headed geese this exchange system is larger than in other species of waterfowl furthermore their flight muscle is better supplied with oxygen than many similar birds.  On top of this the oxygen-carrying pigment haemoglobin is adapted to load more oxygen into the blood for transport to the muscles this coupled with the fact that Bar-headed geese (like some other species of bird) are able to increase their cardiac output to 5 times its usual rate when at altitude allows for the flight muscles to be supplied with plenty of oxygen. Plus they are able to hyperventilate more than 7 times faster than humans and not get dizzy, meaning that the net amount of oxygen reaching the blood increases.

The Migration of the Bar Headed Goose
is an astonishing feat

Recent studies have also found some surprising revelations relating to the birds behaviour and strategies for migration. One major behavioural adaptation these geese exhibit is to fly at night. This is a common migration strategy seen across many birds; in this case it is done because the air is cooler and denser thus reducing the cost of flying compared to daytime. Secondly the geese use an interesting flight pattern, instead of flying constantly at a high altitude they rise and fall following the contours of the mountains. It might seem wasteful to loose attitude only to regain it later but a recent study using satellite trackers on the geese has revealed the reason behind it. The study also monitored the heart rate of the geese and it found that at high altitudes the heart rate is very high; making flying at these heights very costly in terms of energy. Therefore hugging the ground is more efficient. Also the geese may benefit from wind deflections off ridges to gain extra lift. Remarkably the geese do all this without benefiting from any tail winds as they are constantly flapping.

There is still a lot to learn about this fascinating goose that could walk up Mount Everest without suffering any ill effects. Researchers are hoping to be able to look into the genetics of the Bar-headed Goose to understand how they are able to perform such energetic actions at high altitude.