A new study has discovered that the world’s highest flying bird, the bar-headed goose, employs an unusual flight strategy when migrating at extreme altitudes across the Himalayas in order to cope in the relatively low-density mountain atmosphere.
Historically, it was commonly assumed the remarkable species would fly to high altitudes fairly easily and then remain there during their flights, possibly benefitting from a tailwind. But the new research shows that the geese perform a sort of roller coaster ride through the mountains, essentially tracking the underlying terrain even if this means repeatedly shedding hard-won altitude only to have to regain height later in the same or subsequent flight.
Dr Lucy Hawkes of the Centre for Ecology and Conservation at the University of Exeter said: “Whenever I walk up a hill on a hike, I hate having to descend again and lose all that arduous gain – these geese aren’t like that. It seems to be better for them to stay close to the ground, where the air is densest and the oxygen more abundant, at all times. They don’t even seem to mind climbing (during flight) that much and they are certainly very good at it.”
The study used custom-designed data loggers to monitor pressure-derived altitude, body accelerations and heart rate of geese during their southern migration from their breeding grounds in Mongolia to their wintering grounds in South-eastern Tibet or India.
Two independent models were developed by the researchers to estimate changes in the energy expenditure of birds during flight. One is based on changes in heart rate and one is based on the vertical movements of the bird’s body. These indicate that, as even horizontal flapping flight is relatively energy expensive at higher altitudes, it is generally more efficient to reduce the overall costs of flying by seeking higher-density air at lower altitudes.
The birds adopt this roller coaster strategy as flying at progressively higher altitudes becomes more difficult, as the decreasing air density reduces the bird’s ability to produce the lift and thrust required to maintain flight. The birds also face the problem of reduced oxygen availability as the atmospheric pressure falls from 100% at sea level (with oxygen content of 21%), to around 50% at 5500 m (equivalent to 10.5% oxygen at sea level) and near 33% at the top of Mt. Everest (equivalent to 7% oxygen at sea level).
The new study showed that the wingbeat frequency of bar-headed geese gradually increased with altitude and reduced air density, but was very precisely regulated during each flight. The birds’ heart rate was very highly correlated with wingbeat frequency but there is a very steep exponential relationship. For example, a small change in wingbeat frequency of +5% would result in a large elevation in heart rate of 19% and a massive 41% increase in estimated flight power.
Lead author Dr Charles Bishop of Bangor University said: “It seems that geese must keep very fine control over their wingbeat cycles. As they flap faster they also move the wing further, i.e. with bigger amplitude. They are designed with a very high gearing linkage between the movement of the wing and the cardiac output or flow of blood from the heart.
“It is like riding a bike with an increasingly large cog for the pedals as you move faster and a relatively small cog on the back wheel. An increasing effort is required to move the bike pedal (or the bird’s wing) at the same frequency, or even slightly faster, through each revolution but the back wheel (or the bird’s heart) is rapidly increasing its activity and overall speed is increasing.”
While previous studies show that these birds may be capable of flying over 7000 m, 98% of observations show them flying below 6,000m. Dr. Lucy Hawkes said: “Our highest single records were of birds flying briefly at 7290m and 6540m and 7 of the highest 8 occurred during the night. Interestingly, flying at night means that the air is colder and denser and, again, would reduce the cost of flight compared to the daytime.”
By utilising a roller coaster flight strategy, along with the occasional benefits of wind deflected upwards by the ground and flying at night, these birds can minimize the overall energetic cost of their migrations and adopt a risk averse strategy.
Led by Dr Bishop, the research was carried out by Robin Spivey (Bangor University), Dr Lucy Hawkes (University of Exeter), Professor Pat Butler (University of Birmingham), Dr Nyambayar Batbayar (Wildlife Science and Conservation Center of Mongolia), Dr Graham Scott (McMaster University) and an international team from Canada, Australia, Germany and the USA.
‘The roller coaster flight strategy of bar-headed geese conserves energy during Himalayan migrations’ by Charles M Bishop et al. is published in the journal Science.
Source: University of Exeter