How a hummingbird survives the night

A hummingbird lives at full speed. It hovers in front of flowers on wings that move too fast to see, and its heart can beat around 1,200 times a minute during the day. Such a life burns energy quickly. During the day, a hummingbird can refuel at every flower it visits. At night, it cannot feed at all.

So what does a hummingbird do when the sun goes down — especially high in the Andes, where nights are cold?

The problem of the night

Small animals lose heat much faster than large ones, because they have a lot of surface compared with their volume. For a bird that weighs only a few grams, staying at its normal daytime temperature through a long, cold night would cost a great deal of energy — energy it collected during the day and cannot top up until morning.

Hummingbirds solve this with a state called torpor.

Turning down the heat

In torpor, a hummingbird lets its body temperature fall and slows its heart rate dramatically. Instead of spending energy to stay warm, it spends very little.

A study published in the scientific journal Biology Letters, reported by Smithsonian Magazine, measured how far Andean hummingbirds take this. Researchers held 26 hummingbirds of six species in cages overnight and monitored them. The results were extreme. The heart rate of the birds fell from around 1,200 beats per minute in the day to as low as 40 beats per minute at night.

One species stood out. A black metaltail cooled to about 38 °F — around 3.3 °C. For a warm-blooded animal, that is astonishingly cold.

The birds even look different in this state. Smithsonian Magazine describes how they point their beaks upward and fluff up their feathers. In the morning, they warm themselves up again by vibrating their muscles.

Not all hummingbirds go equally deep

The study also found differences between species. Three of them — the black metaltail, the black-breasted hillstar and the bronze-tailed comet — stayed in torpor longer and cooled down further than the other three. These three species also live at higher altitudes, where nights are colder.

Why torpor pays off

The researchers could also see how much torpor helped. Birds that entered torpor only briefly lost up to 15 percent of their body weight overnight. Birds that stayed in torpor for around 12 hours lost only about 2 percent.

For an animal that has to eat constantly to stay alive, that difference is huge. It can decide whether a bird starts the morning with enough reserves to go out and find its first meal.

Torpor is not sleep

It is tempting to call torpor a deep sleep, but it is something different. In torpor, an animal deliberately lowers its body temperature and metabolism far below normal levels. Sleep alone does not do that. Torpor is closer to a short, nightly version of the dormancy some animals use to get through a whole winter.

Why it matters

The hummingbird’s night is a lesson in energy budgets. A creature that seems to have endless energy by day survives by saving it at night — so effectively that its heart slows by a factor of thirty and its body cools almost to the temperature of a refrigerator. Living fast, it turns out, depends on knowing when to slow down.