It’s called the ‘death zone’.

The area above 8,000 meters where humans can no longer acclimatise to the effects of altitude; the oxygen is too low, the air too thin. We can only venture there briefly if we want to survive*. Yet despite the thin air and low oxygen there are reports of an unlikely visitor: geese.

The geese in question are bar-headed geese. Normally residents of areas in China and Mongolia that migrate to over-winter in India. Unfortunately, slap-bang in the middle of their migration route lies the home of Everest, the highest mountain range in the world: the Himalaya.

How bar-headed geese travel at such altitudes is something of a puzzle for physiologists. Flying for any distance in the death zone is remarkable; it takes 50% more power to fly and with only 40% of the oxygen available (60% reduction in the partial pressure) than at sea level the metabolic costs are magnified. However, the only evidence for this is from eye witnesses, which always carries a risk of error and/or exaggeration. Now a new study has gathered real data on just how high, and how far, these geese really do travel.

A bar-headed goose. Photo by Noel Reynolds http://www.flickr.com/photos/29237715@N05/

Other than their plumage, to the untrained eye, there’s not a lot that distinguishes bar-headed geese from other species. They do have a relatively large wing area for their size—helping flight in thin air—but it’s not a lot larger than lower flying relatives.

There are at least two adaptations that have been found that could help. Firstly, their haemoglobin is different; it binds oxygen more easily than that found in other species, helping to grab more from the thin air. Secondly, under low oxygen conditions bar-headed geese have been shown to breath far more deeply and efficiently than other species. Both adaptations that extract more oxygen from the high altitude air, helping them cope with the higher metabolic cost of flying at altitude.

But the question still remains: Do they really fly that high?

To try and answer this question a global research team collaborated to capture bar-headed geese, tag them with a small GPS device and analyse the data. The device would give accurate readings of both location and altitude, for the first time, as the geese migrated to and from their wintering grounds.

The data showed that not all geese migrated to the same places—there was a split. Some travelled to winter at sea level in India, while others opted for the lesser schlep of wintering in eastern Tibet at altitudes of 3–4,000 meters above sea level.

Those that did migrate to India appeared to take the lowest, shortest route they could through the mountains; following a 3,152 km route taking them through the eastern Himalaya, where the mountain range was narrowest, and crucially, the valleys were deep. The geese would not necessarily have to climb mountains to get to India.

The Himalaya mountain range

Just how high did they go though? On their southbound journey, to India, the highest recorded altitude—from one individual goose—was 7,290 m. Short of the death zone, but still impressive. The majority of geese were recorded at lower than 6,000 m for most of the time. Again, not in the death zone, as previously thought, but this is still a remarkable and impressive feat; air pressure would still be half that at sea level, as will the partial pressure of oxygen, compared to sea level. Most humans still struggle to function normally at that height.

One further insight the data revealed was that migrating geese tended to travel at night, and in the early morning. Despite the darkness, there may be a good reason for this; the air would be colder and therefore more dense—less thin—making it slightly easier and less energetically costly to fly.

While flying at any of these heights would be hard work, it is possible that there are strategies that the geese used to reduce the burden, and this is something the team were not able to accurately rule out from this study. It’s possible that geese made use of back winds and the uplift generated by winds meeting the mountain faces to ease their journey and offset some of the labour required for the journey.

Though data was only gathered from a small number of geese, it’s clear they do travel at impressive altitudes, and still possible that some geese might venture as high as the 8,000m death zone, though it’s unlikely to happen often. Unfortunately, despite having to traverse the Himalaya, bar-headed geese were never in the running for the award of highest flying bird. That has already been won by the Rüppell’s Vulture, with evidence of them as high as 11,000 m… (though, sadly, that evidence is that one collided with an airplane over the Côte d’Ivoire…)

Reference: Hawkes, L.A. et al., 2012. The paradox of extreme high-altitude migration in bar-headed geese Anser indicus. Proceedings of the Royal Society B: Biological Sciences . Available at: http://rspb.royalsocietypublishing.org/content/early/2012/10/25/rspb.2012.2114.abstract.

*To stay at that height, un-aided, is certain death. Blood vessels close due to lack of oxygen. In turn your body ramps up its blood pressure to get oxygen through. The blood hits this bottleneck and the increased pressure causes fluid to leak from your blood vessels into other parts of your body. Your hands and face swell. Your lungs start to fill, making it even harder to get vital oxygen. And, worst of all, fluid starts to build up in and around your brain; you become disoriented, uncoordinated, eventually falling into a coma. Once this happens death can come quick, unless treated or moved to lower ground.
The closest anything comes to living that high permanently is a tiny jumping spider called Euophrys omnisuperstes which ekes out a living at 6,700 m.

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