Do Birds Hibernate? Torpor, the Poorwill and Winter Myths

No, birds do not really hibernate. Almost all birds stay awake and active all winter or migrate to warmer places. The one exception is the Common Poorwill, a nightjar of western North America that can stay in deep torpor for weeks. Many other birds use shorter torpor or let their body temperature drop a few degrees at night.

So the honest answer has two parts. True hibernation, the months-long shutdown that hedgehogs, dormice and bats use, is almost unknown in birds. But the tools behind it, slowing the heart, cooling the body and cutting energy use, are widespread. Birds just use them for hours instead of months.

Quick answer: how birds get through winter

Do any birds hibernate?

Only one bird is known to come close: the Common Poorwill (Phalaenoptilus nuttallii). Audubon describes its winter state as hibernation-like, with heart rate, breathing and body temperature all slowing, and notes that a poorwill can stay in torpor for hours or even weeks.

The poorwill is a small nightjar of about 18 cm and 36 to 58 grams that lives from British Columbia through the western United States to northern Mexico. On the southern edge of its range, some birds spend much of the winter inactive, tucked into rock piles. They wake as temperatures rise, days lengthen and the flying insects they eat return.

Scientists were slow to accept this. Meriwether Lewis recorded a torpid poorwill in 1804, but nobody recognised what it meant. In 1946 a torpid poorwill was found in the Chuckwalla Mountains of California, and Edmund Jaeger described the behaviour in 1948. It remains the only known case of a bird staying torpid for weeks or months at a time.

What is torpor, and how is it different from hibernation?

Torpor is a controlled drop in body temperature and metabolism that saves energy. Hibernation is essentially long torpor: weeks or months of it, broken by short arousals. The difference is mainly about duration.

Birds normally run hot. NABU puts their body temperature at about 38 to 42 °C, several degrees warmer than ours. Keeping that furnace going on a frosty night costs a lot of fuel, which is why lowering the setting, even a little, pays off.

Which birds use torpor?

Torpor is more common than many people think, but it is short-lived in almost every species that uses it.

• Hummingbirds enter torpor on many nights. In a 2020 study in the Peruvian Andes, published in Biology Letters, 24 of 26 hummingbirds from six species went into torpor. One Black Metaltail cooled to 3.3 °C, the lowest body temperature ever recorded in a bird, according to the Cornell Lab of Ornithology. Our guide on whether hummingbirds sleep explains what a torpid hummingbird looks like.
• Nightjars, the poorwill’s family, are the group most strongly linked with long torpor. Audubon notes that the Eastern Whip-poor-will, a close relative, also uses brief torpor.
• Doves can enter brief torpor too, according to Audubon.

In all of these, torpor is a way to survive a cold night or a short spell without food. The bird rewarms, usually by shivering, and gets back to feeding.

Do small garden birds cool down at night?

Yes, many do, but they stay well short of torpor. Biologists call this regulated or nocturnal hypothermia: the bird lets its body temperature fall a few degrees and holds it there.

The National Wildlife Federation reports that chickadees lower their body temperature by about 12 to 15 °F (about 7 to 8 °C) at night from a daytime level of around 108 °F (about 42 °C). At freezing air temperatures, that saves almost 25 percent of the energy the bird would otherwise burn each hour. In Alaska, chickadees gain 10 percent or more of their body weight each day in fat, then burn it through 18-hour winter nights.

European birds do the same. A 2020 study of great tits near Lund in southern Sweden, published in Biology Letters by Nilsson and colleagues, found that females given an unpredictable food supply let their body temperature drop further at night than birds with a reliable supply. The authors estimate the saving at about 2.7 percent of energy per night at the temperatures in their study, and cite about 12 percent for mountain chickadees at 0 °C. In other words, birds that are unsure of breakfast turn the heating down further.

This is also why your feeders matter most late in the day. A small bird needs to go to roost with enough fat to fuel the night, and a reliable feeding station helps it do that. Our guide on how birds stay warm in winter covers feathers, shivering and roosting in more detail.

Why don’t more birds hibernate?

Hibernation works best for animals that can hide somewhere safe and live off stored fat for months. Most birds are built the other way around.

• Flight limits fat. Carrying months of fuel would make a bird too heavy to fly well, and flight is how birds escape predators and find food.
• Migration is an option. Many birds can simply leave. Swallows and swifts, which feed on flying insects, fly to Africa instead of waiting out the winter.
• Many winter foods stay available. NABU points out that insect-eaters switch to fat-rich seeds and nuts, and still find insects, spiders and their eggs under bark and in leaf litter. Jays bury acorns, and marsh, willow and crested tits hide seeds in bark crevices for later.
• Waking up is slow. A deeply torpid bird cannot fly away. Short torpor at night, when the bird is hidden on a roost, keeps that risk small.

The poorwill is the exception that fits the rule: it eats flying insects, which vanish in cold weather, and it can hide in rock crevices where few predators find it.

The old belief that swallows slept in mud

For centuries, many Europeans believed swallows hibernated. According to History Today, Aristotle thought some swallows wintered in hollow trees. In 1555 the Swedish writer Olaus Magnus claimed that swallows gathered over lakes in autumn, sank to the bottom and stayed in the mud until spring. Linnaeus repeated the idea in his Systema Naturae in 1758, and Samuel Johnson declared that swallows certainly slept all winter in a river bed. Even Gilbert White, writing about Selborne in 1789, struggled to believe they left England.

Doubters pushed back. Francis Willughby argued in 1676 that swallows migrate to warmer regions. In the late 18th century the surgeon John Hunter released 20 swallows into an icehouse with an artificial pond, and none survived, which undercut the mud theory. A famous piece of evidence arrived in 1822, when a white stork landed in Klütz on Germany’s Baltic coast with an African spear through its neck. Stuffed with the spear still in place, it is kept in the zoological collection of the University of Rostock and is known as the Pfeilstorch, or arrow stork.

Today ringing and tracking have shown exactly where swallows go. If you are curious about the timing, see when birds migrate and where birds go in winter.

Myths vs facts about birds in winter

What this means for your garden

Because garden birds do not hibernate, they need food every single day of winter, and cold nights drain their reserves fast. A few simple habits help:

• Feed high-energy food. Sunflower hearts, peanuts and fat balls give a lot of energy per bite. Our guide on what to feed birds in winter lists the best options by bird.
• Top up before dusk. Birds feed hard in the last hours of daylight to build fat for the night.
• Keep feeding steady. The great tit study suggests that birds with a predictable supply do not need to cool as much at night.
• Offer unfrozen water. Birds need to drink and keep feathers clean in winter too.
• Leave shelter. Dense hedges, ivy, evergreens and empty nest boxes give roosting spots out of the wind. Small birds sometimes roost in nest boxes on cold nights.

Sources

• Audubon: Some birds enter a hibernation-like state called torpor to survive winter’s chill
• Cornell Lab of Ornithology, All About Birds: The coolest hummingbird, as measured by an actual thermometer
• National Wildlife Federation: How chickadees weather winter
• Nilsson et al. 2020, Biology Letters: Predictability of food supply modulates nocturnal hypothermia in a small passerine
• NABU: So überleben Vögel im Winter
• History Today: The great migration mystery