How Do Hummingbirds Sleep? Torpor on Cold Nights

Why hummingbirds need torpor

Hummingbirds are tiny, and tiny bodies lose heat fast. During the day a hummingbird refuels every few minutes with nectar, but at night it cannot feed at all. A bird weighing 3 g (0.1 oz), like the ruby-throated hummingbird, simply cannot store enough fat to keep its body at full temperature through a long, cool night.

Torpor is the solution. By turning down its internal thermostat, a hummingbird cuts its energy use dramatically, in deep torpor to a small fraction of normal resting levels. It can then make it to morning on the fat it has stored.

What happens to a hummingbird in torpor?

As torpor begins, the bird fluffs its feathers, often points its bill upward and goes very still. Its body cools toward the temperature of the air around it, but it does not cool forever: each species defends a minimum body temperature, often somewhere between about 10 and 20 °C (50 and 68 °F).

• Body temperature: falls from about 40 °C (104 °F) to as low as 18 °C (64 °F) in some Caribbean hummingbirds, and far lower in Andean species.
• Heart rate: drops from over 1,000 beats a minute in the day to roughly 50–180 beats a minute.
• Breathing: slows sharply and may pause for periods.
• Kidneys: almost stop filtering, saving water while the bird cannot drink.

A study of hummingbirds in the high Andes of Peru found that a black metaltail’s body temperature fell to about 3.3 °C (38 °F), the lowest ever recorded in a bird. Andean birds stayed in torpor for up to 13 hours on cold nights and still lost around 6% of their body weight overnight.

Do hummingbirds go into torpor every night?

Not always. Torpor is used when the bird needs it. A well-fed hummingbird with good fat reserves may simply sleep normally, keeping its body warm. Thin birds, birds facing cold nights, and those in the high mountains are much more likely to enter torpor.

Social status matters too. Studies have found that dominant birds that defend rich feeding territories enter torpor less often or for shorter periods, while subordinate birds without a good territory rely on it more. Before migration, birds building up fat may use torpor to save every bit of energy for the journey.

How do hummingbirds wake up?

Waking from torpor is not instant. The bird generates heat by shivering its large flight muscles, warming itself by roughly a degree or more per minute. Depending on how cold it got, rewarming usually takes roughly 20 minutes to an hour.

Hummingbirds often begin rewarming before dawn, apparently guided by an internal clock and by hormones such as corticosterone, so they are ready to feed at first light. A bird in deep torpor that is disturbed cannot respond quickly, which is one reason they choose sheltered, hidden perches.

Why a sleeping hummingbird can look dead

People who find a hummingbird in torpor are often alarmed. The bird sits motionless with its feathers puffed up, does not react to movement and may feel cool to the touch. Some even tip backward and hang upside down from a branch or feeder perch, still gripping with their feet.

This is normal. A torpid hummingbird is alive and healthy, simply saving energy. The best thing to do is leave it alone; it will usually wake on its own as it warms in the morning and fly off to feed.

Torpor vs hibernation

Torpor and hibernation work in similar ways, but they last very different lengths of time. Hummingbird torpor lasts for one night or part of a night, and the bird is active again the next day. Hibernation, in animals such as ground squirrels and some bats, involves long periods of torpor lasting days or weeks through the winter.

Hummingbirds do not hibernate. The only bird known to spend long periods in hibernation-like torpor is the common poorwill, a nightjar of western North America. Hummingbirds survive winter by migrating, as described in where do hummingbirds go in winter?, or, in species such as Anna’s hummingbird, by combining nightly torpor with winter food.

Where do hummingbirds sleep?

Hummingbirds sleep on thin twigs and branches, usually in shrubs or trees out of the wind and rain. They do not use nests as bedrooms, except females, which sit on their eggs and small chicks at night. Many other birds behave the same way, as explained in do birds sleep in their nests?