The Torpor State
How does this bird manage to survive these extreme temperatures? By entering into a state of competent stillness, the hummingbird manages to weather the cold. This is called the state of torpor. One species known as the black metaltail chills its body to about 37 degrees. If you didn’t know any better, you would think that these birds weren’t even alive. By doing this, the body’s energy is saved and they can survive the cold night while gearing up to feed the next day.
The Research
Scientists wanted to explore this torpor state and conducted the following experiment on six different species: researchers placed 26 individuals in cages overnight with thermometers inserted into their cloacas. As the temperatures were dropped, all of the birds fluffed their feathers and ceased all movement. Phenomenally they were fine. But it was the metaltail hummingbird that cooled the most, going from a temperature of about 104° F to just above freezing. Their heartbeats plummeted to as low as 40 beats a minute from a regular 1,200 times a minute.
What Does It Mean?
During the torpor state, the incredibly low heart rate cuts their energy expenditure by about 95%. By saving energy, these birds can make it through the cold weather. Around sunrise, the hummingbirds warm up and gear for the day. The bird begins to quiver, vibrating its muscles for a few moments as it prepares itself for the rest of the day. It truly is a fantastic adaptation.