Bar-headed Goose, near Masinagudi, India. Photo by Mr. Bird Brain, CC BY-NC-ND 2.0
Scientists have long presumed that the five-pound, 30-inch-long Bar-headed Goose, the world's highest-altitude migrant, relies on tailwinds to push it
up and over the world's highest mountains.
The bird winters in India, Burma, and Pakistan and breeds mostly in China and Mongolia, so its migration route leads not only over the Himalayas but also close to -- and perhaps even over -- 29,028-foot-tall Mount Everest.
But researchers who have tracked the goose by satellite say it isn't so.
In a paper published today in the Proceedings of the National Academy of Sciences, biologist Charles M. Bishop of Bangor University and colleagues report that the birds fly from sea level in India over the Himalayas in about eight hours all on their own power.
Rather than climb on afternoon tailwinds, the geese seem to rely on cool morning air. Both north- and southbound birds begin their flight during the night or early morning, when winds are light and the air is cool and dense, qualities that could help increase the birds' oxygen uptake, reduce the energy required for flapping, and make the flight easier to endure.
Further, by forgoing daytime wind-derived uplift, the birds may engage in a canny compromise: Stormy weather is more common in the Himalayas during the afternoon, so early flights might offer more aerodynamic control. Most climbing flights took place before 10 a.m., and the four highest climb rates were recorded before 7 a.m.
Bar-headed Geese fly past the cloud-covered Himalaya Mountains. Photo by Sankara Subramanian, CC BY-NC-ND 2.0
In fact, one bird in the study did not complete its climb in one day "and landed during the afternoon during what were likely to be peak afternoon tailwinds," the authors note. "This goose did not complete its climb until early the next morning. Another bird stopped climbing before midday and completed the climb during the late afternoon, a period when winds should have been slowing."
The birds gained about seven-tenths of a mile (1.1 km) in altitude per hour; one goose topped out at a climb rate of 1.3 miles (2.15 km) in altitude per hour. Amazingly, over three-hour periods, three geese averaged climb rates of 0.86, 0.74, and 0.68 mph (1.4, 1.2, and 1.1 kph). "These observations represent the longest continuous climbing rates ever recorded," the authors write.
And the birds' speeds were equally impressive. At lower altitudes, the geese flew an average of 33.5 mph (54 kph), but as they climbed, they got faster, reaching average speeds of 41.6 mph (67 kph).
Why are Bar-headed Geese able to fly at high altitudes? The authors, citing papers that date back as far as the 1970s, list three distinguishing physical attributes:
• "Their skeletal and cardiac muscles are better supplied with oxygen... than other waterfowl."
• "Bar-headed Goose hemoglobin is also highly effective at oxygen loading, compared with many other bird species, largely as a result of a single amino acid point mutation."
• "Bar-headed Geese also have proportionally larger lungs than those of other species of waterfowl."
Therefore, the birds have a greater ability to take in and hold oxygen at high altitudes and at steep climb rates.
"This feat is particularly impressive," the authors note, "when considering that humans could suffer dizziness, altitude sickness, high-altitude pulmonary edema, and possibly even death when faced with a similarly extreme change in elevation."
High-tech satellite tags were required to tease the migration secrets out of the Bar-headed Goose, but Bishop and his colleagues note that old-fashioned birdwatching from long ago held a good clue to the bird's migration habits. They quote a 1970 article from Natural History by Lawrence Swan, a naturalist who accompanied Sir Edmund Hillary on an expedition to the Himalayas:
"On one such cold and still night in early April, I stood beside the Barun glacier.... Coming from the south the distant hum became a call. Then, as if from the stars above me, I heard the honking of Bar-headed Geese." -- Matt Mendenhall, Associate Editor