Over the Himalayas like a roller coaster
Bar-headed geese use a "roller coaster" strategy to save energy on long journeys, according to new research published in Science this week.
These birds can reach altitudes of around 6,000m in their twice-yearly migration across the Himalayas. At these heights the air is so much thinner than at ground level that the force generated by each flap of the wings is drastically reduced, as well as the amount of oxygen available from each breath. This makes flying high very hard work indeed.
Bar-headed geese are known to have a range of physiological adaptations to cope with the challenges presented by the extreme conditions found at high altitudes, including warm downy underfeathers, extra-large lungs and cold-resistant eyeballs. However, it's now becoming clear that behavioural strategies play an important role in helping the geese make this epic journey as well.
Many high-flying birds will climb to their desired height, look for a favourable wind and be off, maintaining a relatively constant altitude until it's time to land and rest. On the bar-headed goose's route the wind is rarely in their favour, so a different strategy is required.
The research team used custom-made sensors, which they implanted in the geese during their summer stopover in Mongolia. These implants recorded pressure, temperature, heart rate and wing beats. When they recovered the implants the following year, they found evidence of rather interesting behaviour.
The team found that the geese were following the jagged contours of the land, up mountains and down valleys, sometimes losing up to a kilometre in height. It sounds absolutely bonkers to be continually climbing up mountains then "throwing away" your progress, but there is method to the madness.
Spending time at lower altitudes allows the geese to take advantage of the thicker air, and all the extra lift and oxygen that comes with it. In addition, the data from the sensors suggests that flying up a mountain may not always be as tough as it sounds.
At times of rapid gains in height the sensors often detected little to no increase in effort, as shown by the geese's heart rate and frequency of wing beats. This is because flying close to the ground lets them take advantage of air currents that sweep up the windward sides of mountains, giving a massive boost to the birds' ascent. The result was that they could sometimes gain several hundred meters in altitude without much effort.
"When we looked at the energetics of what was going on, we determined that the cost of flying increases quite steeply as the altitude increases," explains Dr. Charles Bishop of Bangor University, one of the team that carried out the research. "It looked like it was cheaper, in terms of total energy, for them to keep staying low to the ground, taking advantage of the higher-density air when they could."
Human athletes and mountaineers undergo extensive training and acclimatisation to prepare their bodies for ascents like these. In contrast, bar-headed geese spend weeks without flying while they moult from one season's coat to the next, then, without training, undertake this odyssey across the highest mountain range on Earth.
Further research into how bar-headed geese activate certain genetic pathways to adapt to long migrations and rapid changes in altitude could open up interesting avenues for research into human health and performance at high altitudes.