The bird that survives by freezing itself during cold nights
Metaltail bird thrive by freezing itself to near-freezing temperatures
For hummingbirds, the high Andes mountains of Peru are a paradise. Filled with flowers and trees, it is a most resourceful habitat for them. But there is one problem: the extreme cold.
At night, the temperature often falls below the freezing point, causing severe distress for the little hummingbirds. How, then, does a six-gram bird that needs nectar from 500 flowers a day just to survive, gets enough extra energy to keep itself warm all night?
The answer is: it doesn’t.
Researched has revealed that, instead, the hummingbirds slip into a state of ‘suspended animation’, or become rock-solid, known as torpor. One species, the black metaltail (Metallura phoebe), chills to 3.26° Celsius, the coldest body temperature ever recorded in a bird or non-hibernating mammal, researchers report September 9 in Biology Letters.
“They’re cold as a rock,” says Blair Wolf, a physiological ecologist at the University of New Mexico in Albuquerque. “If you didn’t know better you’d think they were dead.” By cooling itself to near freezing temperature it preserves precious energy to last whole night and also, to live up to the next day, Wolf says.
The phenomenon of torpor had been observed before. Yet, Wolf wanted certainly detailed pictures of the birds undergoing such stages. To accomplish the task, 26 individuals of six different species were put in a cage and fixed inside their cloacas miniature thermometers.
Extremely steady and straight, the birds stopped their movements altogether. Of all the species observed, the black metaltail cooled the most, dropping from a daytime temperature of about 40° C to just above freezing.
During the day, the hummingbirds’ tiny heart can beat a surprising 1,200 times per minute. But during the night, this count drops as low as 40 times per minute. “It’s an astounding drop,” Wolf says, and it could allow these high-altitude birds to cut their energy use by about 95 percent. By not wasting energy trying to stay warm, these birds can thrive as high as 5,000 meters above sea level. “It’s a remarkable adaptation.”
Around sunrise, the hummingbirds start revving up, warming about a degree a minute by vibrating their muscles. “You see the bird quivering there, then all a sudden its eyes pop open and it’s ready to go,” Wolf says.
