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How Do Geese Cross the Himalayas?

Bar-headed geese do not maintain a stable altitude when crossing the Himalayas, as previously thought, but rather fly up and down like a rollercoaster, tracking the terrain below.

A new study published in Science used small custom-engineered data loggers to monitor the altitude, body accelerations and heart rates of the geese during their southern migration from breeding grounds in Mongolia to wintering grounds in south-eastern Tibet or India.

While it was previously assumed the geese would fly to altitudes in excess of 7000 metres and then remain there during their flights, the study confirmed that they actually conserved energy by adopting a roller coaster approach.

“This means the geese were repeatedly shedding hard-won altitude only to have to regain that height later in the journey,” said Prof Peter Frappell of the University of Tasmania. “The birds adopt this rollercoaster strategy because the higher they fly, the more difficult flight becomes.”

The decrease in air density associated with high altitude reduces the bird’s ability to produce the lift and thrust required to maintain flight. Thus wing beat frequency and amplitude must increase with altitude, leading to an exponential rise in the estimated energy required for flight.

It’s generally more efficient to reduce the overall cost of flying by seeking higher-density air at lower altitudes. To aid their ascent, the birds also occasionally obtained assistance from updrafts of air deflected by the ground, and often flew at night when the air was colder and denser.

Frappell said that the bar-headed geese had a number of physiological characteristics that made such a high altitude migration possible. Their lungs favour a high diffusion of oxygen, their haemoglobin has a high affinity for oxygen, their relatively large heart increases their cardiac output, and their muscles perform very efficiently.