(Pat Leonard, February 23, 2017)
On top of Mt. Everest, at 29,000 feet, a lungful of air provides less than one-third as much oxygen as at sea level. To understand how birds cope with that lack of oxygen, or hypoxia, Cornell Ph.D. student Sahas Barve turned to the steep Himalayan valleys of his native India.
Over five years, he studied the evolutionary solutions these avian mountaineers had come up with. He and his colleagues published their findings in December 2016 in the journal Proceedings of the Royal Society B. Though he was working in the world’s tallest mountain range, Barve’s study focused on moderate elevations (up to 10,500 feet), meaning his findings are applicable to mountain species around the world—especially as it warms.
“One of the most common predictions of climate change is that species are going to shift upslope to get out of warmer temperatures,” Barve explains. But while moving upward may sound like a straightforward way to avoid warming, it ignores the problem of thin air. “If hypoxia is a major hurdle and birds cannot make their oxygen transport any better than they already have,” Barve says, “then it might severely limit their ability to adapt and shift their ranges higher.”
First, Barve and his hardy field assistants had to figure how the birds managed to compensate for thinner air. The researchers used mist nets to catch 15 species of birds at elevations ranging from 3,280–10,500 feet (1,000–3,200 meters). At these elevations, air has between 89 percent and 69 percent as much oxygen as at sea level.
They collected a drop of blood from each bird, allowing them to study the birds’ hemoglobin—the molecule in red blood cells that carries oxygen from the lungs to the muscles. The blood sample gave them two key measurements: the volume of the blood made up of red blood cells (hematocrit) and the hemoglobin concentration in the blood, measured using a handheld monitor.
The researchers tested resident species—ones that live at the same elevations year-round, such as the Green-backed Tit and Gray-winged Blackbird—and migrants, which breed at high elevations and spend winters lower down, including the Variegated Laughingthrush and the Blue-fronted Redstart. As it turned out, the two types of species solved the hypoxia problem in different ways.