Hummingbirds live fast. By day, a hummingbird's heart can beat up to 1,200 times a minute, and its body runs at about 40°C. That speed has a cost: these tiny birds burn energy so quickly that they often cannot store enough during the day to last through the night.
Shutting down on purpose
To survive cold nights, hummingbirds enter a state called torpor. Their body temperature and metabolism drop far below normal, and they stop moving almost completely. It is the most effective way that birds and mammals have to save energy.
During torpor, a hummingbird's heart can slow to as low as 50 to 80 beats a minute.
A record from the Andes
In 2020, scientists published a study in the journal Biology Letters about six hummingbird species living at about 4,000 meters in the Andes of Peru. The researchers caught 26 birds and measured their body temperature through the night.
All six species dropped below 10°C at some point. One species, the black metaltail, cooled to just 3.3°C. According to the team, it is the lowest body temperature reported so far in any bird or non-hibernating mammal. The previous bird record was 4.3°C, in the common poorwill.
Waking up
Some birds stayed in torpor for about three hours, others for up to 13 hours. To wake up, a hummingbird shivers: its muscles contract rapidly and generate heat. Its body warms by about 1.5°C a minute, which can take up to half an hour. Then its eyes open and it flies away.
Why it matters
Torpor has a risk: a bird that cannot move is easy prey. Many hummingbirds of the high Andes roost in caves, clinging to the walls through the night. The study also found that species living under the same weather used torpor differently, which suggests the habit has evolved differently in each species.
This article accompanies episode 4 of our series Weird Animal Bodies.