An ascent of Mt. Everest is one of the supreme physical challenges for a human being. Imagine arriving at the summit to be greeted by a flock of geese flying overhead.
If you’ve seen Flight: The Genius of Birds you may remember the data loggers that uncovered the pole-to-pole migration of the Arctic tern. Researchers are increasingly using these devices on migratory animals to uncover their secrets. One of the most recent experiments revealed some astonishing secrets of the Asian banded goose. These geese, heavier than 98 percent of all birds, fly biannually over the Himalayas under conditions that would be lethal to humans. The research, reported in Science, was captured enthusiastically by reporters, because it involves a spectacular "roller coaster" ride over the highest mountains on earth. Now that’s a thrill ride.
An international team of investigators led by C.M. Bishop of Bangor University in the UK outfitted seven banded geese with data loggers that recorded ambient temperature and pressure, acceleration, and heart rate as the birds flew for 391 hours over the mountains. They didn’t know quite what to expect:
Over the years, there has been much debate as to how high these birds might fly and what physiological mechanisms could be involved at the highest altitudes, but, although one goose has been directly tracked as high as 7290 m [23,900′] for a brief period, no measurements of their physiological or biomechanical flight performance have been made in the wild. [Emphasis added.]
You might think that the birds would fly high to conserve energy, but that would put them into exceptionally cold, rarified air that would starve their muscles of oxygen. One might also expect that the birds would try to minimize flapping to save energy. The birds outwitted the scientists by taking advantage of a "roller coaster" strategy that, while requiring more flapping, actually conserves energy. In a short summary of the findings in Science, Sacha Vignieri explains:
Animal migrations provide numerous examples of astonishing feats. Impressive even among these is the migration of bar-headed geese across the Himalayan Mountains, which reach heights of thousands of meters. Bishop et al. remotely monitored birds’ heart rates, movement, and body temperature during migration. The geese "hug" the landforms, taking advantage of drafting and wind patterns. This unexpected strategy conserves energy, even though it means the geese repeatedly lose, and must then regain, altitude.
This "clever behavior" that enables the birds "to conquer altitudes that man will never achieve without a plane" is described further by Elisabeth Pennisi in Science:
For humans, transcontinental flight without jet engines, pressurized cabins, and tens of thousands of kilograms of fuel is almost unthinkable. But each year, bar-headed geese fly from Mongolia to India and back, crossing the world’s highest mountains with just their wings and a little extra body fat. Now researchers know just how these 3-kilogram birds make this journey. Rather than flying high for the whole trip, the geese follow the terrain, taking advantage of updrafts to regain altitude as needed.
One of the birds was measured to drop 1000 meters in just 20 minutes, then climb 2000 meters over the next 1.5 hours. This wild ride, with its screaming climbs and rapid descents, must be incredibly adventurous for the geese. Imagine getting one to wear a GoPro camera helmet on the flight.
But this is not recreation in an amusement park. The birds migrate to survive and reproduce. How do they breathe in such air?
The physiological data explain why. When flying high, the birds flap their wings not only faster, but also more deeply up and down, to stay airborne. The increases in wingbeat frequency exponentially increase the heart rate — which sometimes reaches about 450 beats per minute — and power needed. At lower elevations, where oxygen is more plentiful, the heart rate is much lower — about 300 beats per minute — and the power needed is much less, making descents worthwhile. Even when they were climbing, the birds sometimes didn’t have to work as hard as they do to maintain level flying when high up. They apparently ride wind deflected off the ground to regain lost elevation, Bishop says.
As a result, over the journey the heart rate averages about 330 beats per minute, showing "that most of the time these large birds were flying well within their maximum physiological capacity, despite the extremely difficult conditions," Bishop notes.
The BBC provided photos of the birds in their habitat along with their report of the findings, and also a short video of the geese in action. Other than their markings, these geese resemble the species featured in Flight in the section on flight muscles. Unlike many birds, banded geese don’t soar. Some were noticed flapping constantly for 17 hours in one measurement. "Flapping flight is an energy-intensive activity and, at high altitudes, it is even more challenging to generate lift in very thin, low density air." Another surprise is that "They don’t train or acclimatise," Bishop said. "They could walk on the top of Everest and have no problem at all." He hopes that genetic studies will show how they can do that.
This is but one more spectacular example of animal migration. Around the globe, in almost every habitat, in widely separated phyla, animals amaze with their navigational feats. To do what they do, each needs equipment to gauge its location and bearing, sensory systems to identify landmarks, propulsion systems to move efficiently, and instincts to switch on the appropriate behaviors. These interwoven systems all develop from complex instructions embedded in their genes.
In the Design of Life series, Illustra Media has shown how one-gram butterflies can migrate thousands of miles from Canada to Mexico, and how Arctic terns can fly from pole to pole. Their next film, scheduled for summer 2015 release, will share examples from the ocean that are, in some respects, even more spectacular.