Roller coaster geese: Insights into high altitude bird flight physiology and biomechanics

Roller coaster geese: Insights into high altitude bird flight physiology and biomechanics
Bar-headed goose Anser indicus flying from a cliff nest. Credit: Nyambayar Batbayar

Roller Coaster migratory flights of geese give unique insights into bird physiology and biomechanics at high altitudes.

An international team of scientists studying the migratory biology of bar-headed (Anser indicus), during their high altitude flights across the Tibetan plateau and Himalayan Mountains, have revealed how these birds cope with flying in the relatively low-density mountain atmosphere.

Dr. Charles Bishop of Bangor University led the study, along with colleagues Robin Spivey and Dr. Lucy Hawkes (now at University of Exeter), Professor Pat Butler from the 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 study used custom-designed data loggers to monitor pressure-derived altitude, body accelerations and of geese during their southern migration from their breeding grounds in Mongolia to their wintering grounds in South-eastern Tibet or India.

Historically, it was commonly assumed that bar-headed geese would fly to relatively easily and then remain there during their flights, possibly benefitting from a tailwind. Instead, the new study (published in Science 16th January 2015) 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.

Why do they do this?

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).

"We have developed two independent models to estimate changes in the energy expenditure of birds during flight", said Robin Spivey (the Research Officer on the project and developer of the data logging equipment). "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."

The team was 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 Prof. Pat Butler. "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."

Roller coaster geese: Insights into high altitude bird flight physiology and biomechanics
Bar-headed geese Anser indicus on Terkhiin Tsagaan Lake, Mongolia. Credit: Bruce Moffat Photography

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 +5% would result in a large elevation in heart rate of 19% and a massive 41% increase in estimated flight power.

Dr. Charles Bishop 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 6,000 m. Dr. Lucy Hawkes said: "Our highest single records were of birds flying briefly at 7290 m and 6540 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. "

By utilising a 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," said Dr. Nyambayar Batbayar, "compared to values of around 450 bpm 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."

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", said Dr. Graham Scott. "As a result, they are able to accomplish something that is impossible for most other ."

Explore further

Running geese give insight into low oxygen tolerance

More information: The roller coaster flight strategy of bar-headed geese conserves energy during Himalayan migrations, by Charles M Bishop et al. (2015) Science, 16th January. … 1126/science.1258732
Journal information: Science

Provided by Bangor University
Citation: Roller coaster geese: Insights into high altitude bird flight physiology and biomechanics (2015, January 15) retrieved 19 August 2019 from
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User comments

Jan 15, 2015
The secret is simple.
Birds are exquisitely sensitive to air currents.
Each feather is attached to a nerve ending, and the slightest differential of vertical pressure between adjacent feathers registers with the brain, indicating the location and strength of localized air currents (including updrafts, wake turbulence from other birds, etc.) which the bird may then instictively exploit to maximize it's lift/effort ratio.
It is similar to the feeling that us hirsute humans get from air drafts across our hairy forearms which tell us which way the wind is blowing.

Jan 22, 2015
This 'paradox' has troubled biologists for more than 30 years. This problem of the ability of these geese to fly at high altitude with such low oxygen level is related to the
1-The large volume of the geese means lower friction coefficient (drag) at higher altitude because of lower density of air.
2-The geese perform a sort of roller coaster ride through the mountains because on the lower ground they replace the oxygen they consumed at higher altitude. This is due to the transformation from homogeneous ( can take moisture at higher altitude to keep them warm but lost oxygen) to heterogeneous ( an oxygen layer covers the surface) super-hydrophobicity
3-The body of these geese is covered with feathers. Each feather is composed of millions of nano-sized mini-feather. The surface morphology of these mini-feathers is covered with heterogeneous super-hydrophobic layer.

Jan 22, 2015
For example the shark is covered with nano-sized scales. The surface morphology of each scale is composed of heterogeneous superhydrophobic layer. This superhydrophobic layer contains a layer of air. This makes the shark a superior swimmer because the shark feels like swimming in air ( air has much lower viscosity than water and thus much lower drag).
One must understand what is heterogeneous and homogeneous super-hydrophobicty and the transformation from the heterogeneous to the homogeneous super-hydrophobicty.

Jan 22, 2015
The lotus effect refers to self-cleaning properties that are a result of very high water repellence (superhydrophobicity), as exhibited by the leaves of the lotus flower (Nelumbo). Dirt particles are picked up by water droplets due to the micro- and nanoscopic architecture on the surface, which minimizes the droplet's adhesion to that surface. Superhydrophobicity and self-cleaning properties are also found in other plants, such as Tropaeolum (nasturtium), Opuntia (prickly pear), Alchemilla, cane, and also on the wings of certain insects.
A superhydrophobic layer has a self cleaning action. That why docks and geese when swimming the just dive inside the water and go out. That has same effect like taking a shower in soup bath.

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