Bar-headed geese (Anser indicus) 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, according to new research co-led by Dr Charles Bishop and Dr Lucy Hawkes, both from Bangor University, UK.
Bar-headed geese (Anser indicus) in Bharatpur, Rajasthan, India. Image credit: J.M.Garg / CC BY-SA 3.0.
Bar-headed geese 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 percent at sea level (with oxygen content of 21 percent), to around 50 percent at 5,5 km (equivalent to 10.5 percent oxygen at sea level) and near 33 percent at the top of Mt. Everest (equivalent to 7 percent oxygen at sea level).
“We have developed two independent models to estimate changes in the energy expenditure of birds during flight. One based on changes in heart rate and one based on the vertical movements of the bird’s body. These indicate that, as even horizontal flapping flight is relatively expensive at higher altitudes, it is generally more efficient to reduce the overall costs of flying by seeking higher-density air at lower altitudes,” said Dr Robin Spivey of Bangor University, a co-author of the paper published in the journal Science.
The scientists were surprised to find that, very occasionally, bar-headed geese were flying in relatively strong updrafts of air.
“During these moments, it seems likely that the bar-headed geese are flying on the windward side of a valley wall,” said senior author Prof Pat Butler from the University of Birmingham.
“This would give them the best opportunity to obtaining assistance from wind that is deflected upwards by the ground (known as orographic lift), thus, providing additional rates of ascent with either a reduction in their energetic costs or at least no increase.”
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 and with a typical variation of only 0.6 flaps per second.
Remarkably, 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 plus 5 percent would result in a large elevation in heart rate of 19 percent and a massive 41 percent increase in estimated flight power.
“It seems that geese must keep very fine control over their wingbeat cycles,” Dr Bishop said.
“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 7,000 m, 98 percent of observations show them flying below 6,000 m.
“Our highest single records were of birds flying briefly at 7,290 m and 6,540 m 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,” Dr Hawkes said.
By utilizing a roller coaster flight strategy, along with the occasional benefits of orographic lift and flying at night, these birds can minimize the overall energetic cost of their migrations and adopt a risk averse strategy.
“Bar-headed geese are heavier than most other bird species, yet their average heart rate for the journey from Mongolia to India was only 328 beats per minute, compared to values of around 450 beats per minute recorded in wind tunnels or on rare occasions in the wild. Bar-headed geese have found a way to cross the world’s highest land massif while remaining well within their physiological capabilities,” said co-author Dr Nyambayar Batbayar of Wildlife Science and Conservation Center of Mongolia.
“How is this possible? The physiology of bar-headed geese has evolved in a number of ways to extract oxygen from the thin air at high altitudes. As a result, they are able to accomplish something that is impossible for most other birds,” said co-author Dr Graham Scott of McMaster University in Ontario, Canada.
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C. M. Bishop et al. 2015. The roller coaster flight strategy of bar-headed geese conserves energy during Himalayan migrations. Science, vol. 347, no. 6219, pp. 250-254; doi: 10.1126/science.1258732
