The old adage “free as a bird” doesn’t quite apply in the world’s tallest mountain ranges. Instead, songbird species are confined to specific elevations, where they have evolved to fit that particular climate.
The crimson sunbird, for instance, lives from the foot of the Himalayas up to about 1,600 feet. The green-tailed sunbird, its evolutionary cousin, lives between about 5,000 feet to 10,000 feet of elevation, while another close relative, the fire-tailed sunbird, rules the roost from about 11,000 feet to 13,000 feet.
Scientists who study birds are still unraveling the factors that keep each bird in its elevational niche. Research published this week in the journal Ecography adds a new piece to the puzzle: the higher a songbird species lives in the Himalayas—and the colder temperatures it faces, because of the altitude—the thicker its downy feather layer. The finding could help researchers predict how songbirds will adapt to a changing climate.
Insulation is pretty important,” says vertebrate zoologist Sahas Barve, a Peter Buck fellow at the Smithsonian’s National Museum of Natural History and the first author on the new study. Temperatures in the Himalayas regularly drop below freezing at night. Birds, however, need to keep their bodies at about 104 degrees Fahrenheit. “The straight-line distance between the outside air and the bird's heart is less than an inch. So, it has to maintain that temperature difference across that little barrier,” says Barve.
Feathers provide key insulation. To investigate the ways that feathers evolved to keep birds warm, Barve measured the downy feathers of more than 200 species of Himalayan songbird specimens held in the museum’s vast collections, where rows and rows of file-like cabinets hold taxidermied examples of the world’s avian species.
“Irrespective of body size, birds that live at the bottom of the mountain have less downy feathers than birds that live at the top of the mountain,” says Barve.
Birds are warm-blooded, like humans, so they use a familiar strategy to heat themselves up when the temperature falls—shivering. But at an elevation of 12,000 feet, nighttime temperatures can drop to between 0- and 20-degrees Fahrenheit. Birds in the Himalayas have to shiver so much to stay warm that they can lose a fifth of their body weight in one night. Birds sometimes starve to death because they burn so many calories simply because they are shivering.
As climate change continues to alter weather patterns, extreme cold events could occur more often and last longer each time. That could put a lot of stress on mountain-dwelling bird populations. “To fully understand how birds will deal with changing temperatures, we need to understand this basic, fundamental concept of how birds use their feathers to stay warm,” says Barve.
Barve analyzed 1,715 specimens from the museum’s collections representing 249 Himalayan songbird species. The species were gathered from a 1,000-mile span of the Himalayas, and at elevations from 246 feet to 16,150 feet. The species were as small as the black-face warbler, which weighs just a fraction of an ounce, to the half-pound blue whistling thrush. The oldest specimen included in the study was a blue rock thrush collected in 1876.