At Penguin Ranch in Antarctica, Meir investigated emperor penguins, the world's best bird divers. These penguins can dive deeper and longer than any other bird and can tolerate extremely low levels of oxygen in their blood—far below those that would render a human unconscious. Meir observed the birds diving for fish from an underwater viewing chamber. "They look like different animals underwater," she says, "like ballet dancers." The penguins routinely dive for 5 to 12 minutes at a time. One penguin made a 27-minute dive on a single breath. Meir wanted to understand how these animals can stay underwater for so long. "They're air breathers just like we are," she says. "They take a breath before they dive and then use the oxygen in that breath for the entire time they're down there." One of their secrets: They slow their heart rate from 175 beats per minute to around 57 beats per minute, which allows them to slow the use of their oxygen stores.
Later, Meir turned to a bird famous for one of the most extreme migrations on Earth. The bar-headed goose crosses the Himalayas twice a year on its migratory route from sea level in southern Asia up over the enormous mountain range to its summer breeding grounds in the central Asian highlands.
One cold April night in the high Himalayas, naturalist Lawrence Swan stood listening to the silence. From the south a distant sound came, a quiet hum that became a call, the honking of bar-headed geese. Swan followed their movement directly over the summit of Makalu. "At 16,000 feet, where I breathed heavily with every exertion," he writes, "I had witnessed birds flying more than two miles above me, where the oxygen tension is incapable of sustaining human life—and they were calling. It was as if they were ignoring the normal rules of physiology and defying the impossibility of respiration at that height by wasting their breath with honking conversation."
Flapping flight consumes ten to fifteen times more oxygen than resting. Most of these geese reach altitudes of 16,000 to 20,000 feet. One bird was recorded at almost 24,000 feet. At this altitude, oxygen levels are roughly a half to a third what they are at sea level. Bar-headed geese sustain the high oxygen demands of flight in air that is so thin that even the most elite human athletes can barely walk in it.
Meir wondered, do the birds use thermals, those upward currents of warm air, to save energy? "No, they actually fly at night and early in the morning, when there's a strong headwind and the temperature is lower," she says. Moreover, they're flapping fliers and almost never glide or soar. So how do they do it?
To find out, Meir decided to train them to fly in a wind tunnel. And to do so, she became Mother Goose, raising a gaggle of twelve goslings from birth so that they would imprint on her. "We would go for walks together, take naps together," she says. "It's true what they say about children, they grow up fast." She started the geese flying by riding a bike so that they would fly right next to her, almost beak to cheek. That worked for a day, but they were too fast, so she took to riding a motorcycle up and down small roads, the birds by her side, wingtips brushing her shoulders. "Looking into the eye of a bird like that is really special," she says. Eventually, Meir and her colleague Julia York of the University of Texas readied the geese for flight in the wind tunnel, fitting them with tiny backpacks that recorded their vital signs and with special custom-made masks that changed the oxygen content of the air they breathed to mimic those they might experience through the passes of the Himalayas and at the summit of Mount Everest. Then they set the birds flying in the tunnel to measure their heart rate, metabolic rate, blood oxygen levels, and temperature under different conditions.
Scientists knew that these geese have several adaptations that help them at high altitudes: larger lungs than other birds, more efficient breathing (deeper and less frequent), a kind of hemoglobin that grabs oxygen more effectively (allowing them to extract more of the gas from each breath of air than can other birds), and blood capillaries that are especially densely distributed throughout their muscles to deliver the oxygen. What Meir and York learned through their experiments was that the geese have yet another superbird mechanism: a unique response to temperature. In their bodies, the temperature difference between their cold lungs and their warm muscles can increase the delivery of oxygen by twofold during sustained flapping flight at high altitudes. The geese also minimize their metabolic rate, reducing the amount of oxygen they need to fly.
"But this isn't the whole story," says Meir. "We still don't know how these birds cope with the low barometric pressure at extremely high altitudes, which would do in other species."
It's what I love about so many aspects of bird biology and behavior.
They're still layered with mystery.