Before Jessica Meir became a NASA astronaut who blasted into space, she made a career of studying animals that live at extremes. She followed elephant seals and emperor penguins as they swam through frigid waters. She raised a dozen bar-headed geese, capable of flying three miles high, that treated her as their mother.

Few creatures dare to fly over the tallest mountains on Earth. Above the Himalayas, where the atmosphere is so thin that human exertion is nearly impossible, bar-headed geese beat their wings as they migrate from India to Mongolia.

Meir led the geese — they followed her anywhere — into a wind tunnel designed to test submarines and sports equipment. A year later, Julia York, then a student at the University of British Columbia, led another group of geese into the machine. When scientists lowered the oxygen, the geese chilled their blood and slowed their metabolism, Meir, York and their colleagues reported in the journal eLife.

The geese would be fine in conditions that would “probably kill us,” said biologist Graham Scott.

To get birds to fly in a wind tunnel, the study authors took advantage of the avian behavior known as imprinting — freshly hatched birds will bond with the first large shape they see. But few researchers use imprinting.

Meir met her goslings in 2010 in Scotland Neck, North Carolina. She was the first thing the animals saw. “It was one of the most amazing things I’ve ever experienced,” Meir said.

When the birds were old enough to travel, Meir took them to Seattle. They drove to the University of British Columbia, where she taught them to fly. She began by biking away from the geese. Eager to stay with her, they started to run.

“And then they realize that they can keep up better if they fly,” she said. After two days, the birds flew faster than she could bike, so Meir switched to a motor scooter.

The devices required to study the geese were Rube Goldberg-like contraptions: masks fitted to the geese bills, connected by tubes to tanks of oxygen and nitrogen.

Meir and her colleagues tested the birds at three oxygen concentrations: normal oxygen, equivalent to sea level; diminished oxygen, comparable to 3.4 miles up; and very little oxygen, equal to about 5.6 miles high.

When humans exercise, our muscles warm up and our blood heats with it. “The fact that their blood cooled during flight has really important implications for how birds can fly in low-oxygen conditions,” Scott said. In lower temperatures, hemoglobin molecules bind more tightly to oxygen — meaning each red blood cell can carry more oxygen.

Despite the demands of flight, the birds’ metabolism slowed as the external oxygen dropped. “They proved quite conclusively that it’s through a reduction in metabolic rate” that these animals can fly in oxygen-poor conditions, said Douglas Altshuler, who studies flight behavior at the University of British Columbia. What Meir and her colleagues accomplished in the wind tunnel “tells us more about the physiology of bar-headed geese and flight than almost all of the research that came before it, combined,” he said.