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.
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| 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.
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| 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.
