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

How bar-headed geese cope with flying in relatively low-density mountain atmosphere as they migrate across the Tibetan Plateau and Himalaya Mountains

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 geese (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 heart rate 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 high altitudes 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."

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.

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