Hummingbirds in the Andes mountains don’t fight the cold, they embrace it.
New research published this week in the journal Biology Letters shows that some species of hummingbirds let their body temperature plunge to about 38 degrees Fahrenheit, just above freezing, Veronique Greenwood reports for the New York Times. The tiny birds spend their days drinking nectar from hundreds of wildflowers, which provides the energy they need to stay aloft in the thin mountain air. Then at night, they turn their beaks upward, fluff out their feathers and go cold.
“They’re cold as a rock,” says University of New Mexico physiological ecologist Blair Wolf to Science News’ Jonathan Lambert. “If you didn’t know better you’d think they were dead.”
Hummingbird species hold an array of world records, like the smallest bird, the smallest bird egg and the fastest wing-beat of any bird. Now, they can add coldest bird and non-hibernating mammal to the list. Hummingbird species in the Andes face challenges presented by the high altitudes, where the air is thinner and regularly drops below freezing temperatures. But there are benefits: the mountainsides are full of wildflowers and low in predators.
To study how the birds adapted to the chilly nighttime air, the research team captured 26 hummingbirds of six species and placed them in cages overnight. They aimed to answer a key question of hummingbird biology: how do the birds preserve energy overnight?
“They wouldn’t be able to store up enough fat at the end of the day to provide sufficient fuel to last them for the entire night,” study co-author and University of Pretoria zoologist Andrew McKechnie tells Nicola Davis at the Guardian.
The study showed that rather than try to keep their temperature high overnight, the birds dropped into a state of torpor, where their heart rates and body temperature dropped. Hummingbirds spend their days with a 1,200 beat-per-minute heartrate, but at night, their heart rate fell to as low as 40 beats per minute. And the black metaltail hummingbird let its body temperature fall to 38 degrees Fahrenheit.
Each species had a different strategy. Some only entered torpor only briefly, and these lost up to 15 percent of their body weight overnight. Birds who stayed in torpor for 12 hours lost only two percent.
“The extent to which birds can save energy by going into torpor might well affect how well they do at these high altitudes,” McKechnie tells the New York Times. The black metaltail and two related species, the black-breasted hillstar and bronze-tailed comet, stayed in torpor longer and let their body temperatures fall further than the other three species in the study. The group also lives at the higher altitudes than the other three.
In the morning, the hummingbirds warm back up by vibrating their muscles until they reach an active temperature.
“You see the bird quivering there, then all the sudden its eyes pop open and it’s ready to go,” Wolf tells Science News.
McKechnie tells the New York Times that the next step will be to study where the hummingbirds roost each night. This study was performed in captivity, so he says there is more to learn about the birds by observing their behavior in the wild.
There are reports that during cold snaps, hummingbirds will take shelter in caves and emerge several days later. McKechnie tells the Times that this suggests the hummingbirds may enter torpor for several days in a row. In other words, they might hibernate.
“It would be big news if they did,” says McKechnie to the Guardian. “Only one avian hibernator”—the common poorwill—"has ever been reported.”