How do Birds Stay Warm in the Winter

Short answer

• Birds stay warm in winter by combining insulation, heat production, heat conservation, and behavior. Feathers trap a layer of warm air next to the skin, and fluffing those feathers thickens that layer. When insulation alone is not enough, birds shiver, which burns fuel in large chest muscles and releases heat. A countercurrent arrangement of blood vessels in the legs limits heat loss through bare feet, while fat stored during short winter days fuels long, cold nights. Some species go further and deliberately lower their body temperature for hours at a time, a controlled energy saving state called torpor. Others share roosts, shelter from wind, or avoid the coldest season altogether by migrating. No single trick explains winter survival. It is the combination, used in different proportions by different species, that lets a bird weighing less than an ounce survive a freezing night and feed again at dawn.

Why Winter Is So Expensive for a Warm Blooded Bird

Birds are warm blooded, which means they generate heat inside their bodies and hold their core temperature within a fairly narrow range. Many birds run hotter than people, often above 100 degrees Fahrenheit. That high temperature keeps muscles working and allows flight in cold air, but it comes with a constant cost. Heat always flows from warm to cold, so a bird in freezing air is always losing heat to its surroundings and must always replace it by burning fuel.

Size makes the bill steeper for small birds. A tiny bird has a large surface area relative to its volume, so it loses heat faster for its size than a goose or a swan does. The U.S. Fish and Wildlife Service gives a vivid example: a black-capped chickadee weighing less than half an ounce can maintain a body temperature of 100 degrees Fahrenheit even when the air is 0 degrees Fahrenheit. That feat is possible only because the chickadee feeds almost constantly during the day and burns stored fuel all night.

Winter also squeezes the feeding day. Days are short, nights are long, and food is often hidden under snow, locked in ice, or simply scarce. A small bird must therefore gather enough fuel in a few hours of daylight to pay for many hours of heating through the night. If a storm cuts feeding short, or if the bird enters the night thin or wet, its margin shrinks fast. Winter survival is best understood as an energy budget: income from food, spending on heat, and savings in fat and shelter that carry the bird through the gaps.

Wind and wetness raise the price further. Wind strips away the warm air trapped in feathers, a process similar to wind chill for people. Wet feathers lose much of their insulating power because water replaces trapped air and carries heat away from the body. This is why shelter, dry plumage, and wind protection matter as much as food. A bird that is dry, sheltered, and well fed has a far better chance than a bird that is wet, exposed, and hungry, even at the same air temperature.

Feathers: A Coat, a Windbreaker, and a Blanket in One

The first and most important defense against cold is the feather coat itself. Outer contour feathers form a smooth, weather resistant shell that sheds snow and rain. Underneath lies a layer of soft, downy feathers close to the skin. The down traps air, and that trapped air, warmed by the bird’s body, is what actually insulates. Still air is a poor conductor of heat, so a thick cushion of still air slows heat loss dramatically. People who wear down filled winter coats are borrowing the same principle.

Birds in colder climates may put on a heavier coat of plumage for winter, as the National Audubon Society notes, with a base layer of fine, downy feathers that traps body heat while keeping cold air out. That change comes through molt, the gradual replacement of feathers timed to the season, and it varies by species. A bird does not simply grow a thicker coat overnight. It prepares for winter over weeks, replacing worn feathers and building the plumage it will depend on through the coldest months.

Feather condition matters as much as feather thickness. Preening, the careful drawing of feathers through the beak, realigns the tiny structures that lock outer feathers into a smooth surface. The U.S. Fish and Wildlife Service explains that all birds produce a special oil from a gland near the base of their tails, and some cold tolerant birds use this oil to weatherproof their feathers, while others, such as egrets, herons, and mourning doves, grow special feathers that disintegrate into a powder used to waterproof their plumage. Either way, preening helps maintain a water resistant top layer and a warm inner layer. A bird with damaged, dirty, or waterlogged plumage loses insulation fast, which is why birds spend so much time preening even in cold weather.

Color and pattern play a smaller, supporting role. Dark plumage can absorb more solar heat on a sunny winter day, and birds often orient themselves to catch the sun. But solar gain is a bonus, not the main system. The core work is done by trapped air, dry feathers, and the bird’s own metabolism.

Fluffing Up: Making the Coat Thicker on Demand

Anyone who watches winter birds has seen the classic puffed up shape: a cardinal or sparrow that looks almost round on a cold morning. That posture is an active adjustment called fluffing, and it works by increasing the thickness of the insulating air layer. Birds can raise their feathers using tiny muscles at the base of each feather. When the feathers stand out from the body, the pockets of air between them grow larger and more numerous. Audubon describes this as creating hundreds of air pockets between feathers that trap heat and maximize natural insulation.

The bird looks bigger because, in insulation terms, it really is bigger. The outer surface of its coat is farther from its warm skin, and heat must cross a thicker cushion of still air to escape. Fluffing is often combined with posture changes that reduce exposed surface. Birds tuck their heads, crouch low over their feet, and pull their legs up into belly feathers. The goal is simple: keep warm air next to the body and expose as little bare skin as possible.

Fluffing has a trade off. A puffed up bird is less streamlined, so birds tend to fluff most when perched and resting, not when flying hard. You will often see the roundest birds in the coldest part of the morning, between feeding bouts. Fluffing by itself is normal winter behavior, not a sign of illness. A bird that stays fluffed for hours without feeding, or that looks fluffed and lethargic in mild weather, may be a different matter, but the puffed silhouette on a frosty morning is a healthy bird using its coat well.

Shivering: Making Heat When Insulation Is Not Enough

When insulation alone cannot hold core temperature, birds make extra heat by shivering. Shivering is rapid muscle contraction that burns fuel and releases heat as a byproduct. In birds, the main shivering muscles are the large flight muscles of the chest. They are big, well supplied with blood, and close to the core, so heat made there warms the body rather than escaping from an extremity.

Bird shivering is more controlled than the visible shaking people associate with the word. Research described by Audubon notes that birds shiver by activating opposing muscle groups, producing contractions without the obvious whole body shaking seen in people. That control matters, because violent shaking would waste energy in movement and disturb the insulating air layer in the feathers. A bird can therefore shiver for hours on a cold night, quietly burning fat to hold its temperature steady.

Shivering is effective but expensive. On a cold night, a small bird may shiver for much of the night, fueled largely by fat stored during the day. That is why winter feeding is about banking fuel for the night, not just staying full during the day. A bird that must shiver all night without enough fat will run out of fuel before dawn. This is the central danger of winter for small birds: the night is long, the feeding day is short, and the heating bill comes due every night without exception.

Larger birds shiver too, but their lower surface to volume ratio means they lose heat more slowly and can often get through a cold night with less shivering. Water birds and large songbirds also use behavior to reduce the need for shivering, sheltering from wind and huddling where they can. Shivering is the backup generator: powerful, reliable, and costly to run.

Legs and Feet: The Countercurrent Trick

Few winter sights puzzle people more than a duck standing on ice or a gull perched on a frozen post, apparently unbothered. The answer lies in the legs and feet, which are built very differently from human legs and feet, and in a clever arrangement of blood vessels.

First, bird feet contain mostly bone and tendon, with little muscle and relatively few nerve endings compared with a human foot. Audubon notes that bird feet are made mainly of bones and tendons with little muscle, which helps them withstand the cold. That does not make feet immune to freezing, but it does mean they can safely run much colder than the core. The foot itself may sit barely above freezing while the core stays above 100 degrees Fahrenheit.

Second, birds use a countercurrent heat exchange system in their legs. Arteries carrying warm blood from the body to the feet run very close to veins carrying cooler blood back from the feet toward the body. Heat passes from the warm outgoing blood to the cooler returning blood. The result is that blood arriving at the foot is already cooled, so less heat is lost to the ice, while blood returning to the body is pre warmed, so the core is not chilled by cold blood flowing back from the feet. The Smithsonian Institution quotes Peter Marra, head of the Smithsonian Migratory Bird Center, describing the system in plain terms: veins and arteries in the leg are close to each other, and as warm blood leaves the body, it heats up the cold blood returning to the body.

This arrangement, sometimes called regional heterothermy, lets the core stay warm while the extremities are allowed to run cold. Blood still reaches the foot, and periodic increases in blood flow deliver just enough heat to prevent freezing. It is especially important for water birds, whose feet are in constant contact with cold water or ice. Mute swans, geese, ducks, gulls, and pelicans all use versions of this system.

Third, birds manage their feet behaviorally. Many water birds stand on one leg and tuck the other into belly feathers, then switch legs. Perching birds can lower their bodies to cover their feet, and geese may sit with legs folded underneath them. Canada geese, as the U.S. Fish and Wildlife Service notes, also tuck their bills under back feathers, keeping their bills warm while breathing warmer air. Together, anatomy and behavior keep the unfeathered parts safe without wasting the body’s heat budget.

Fat Reserves and Food Strategies: Banking Fuel for the Night

Fat is the winter fuel tank. During winter, many species increase their insulation and build up energy stores by eating more. Audubon reports that for smaller birds like chickadees and finches, fat can account for more than 10 percent of their winter body weight. That reserve is what pays for shivering and for holding core temperature through the night. A bird that enters the evening with a full fat reserve has a buffer. A bird that enters thin has almost none.

Building that reserve requires a feeding strategy, not just appetite. Many birds switch diets in winter, taking whatever high energy food is available: seeds, nuts, suet, dormant insects, and persistent fruit. They also feed in a daily rhythm shaped by the cold. Morning feeding replaces what the night burned. Midday feeding builds the reserve. Late afternoon feeding tops it up before roosting. Birds that miss the late afternoon feed are at greater risk on a cold night, which is why feeders and natural food sources are busiest in the hour before dusk.

Caching is a second strategy, and it is one of the most impressive feats of winter bird behavior. In fall, chickadees, nuthatches, woodpeckers, and other species carry seeds or peanuts away and hide them, or cache them, for later when resources are scarce. Audubon reports that a single chickadee can store up to 80,000 seeds and remember where they all are. That memory depends on a large hippocampus, the part of the brain responsible for spatial memory, which the U.S. Fish and Wildlife Service notes grows even larger in chickadees in the fall. Caching turns a brief autumn abundance into a winter food supply, and it lets birds feed quickly on a cold day without searching from scratch.

The U.S. Fish and Wildlife Service also reports that chickadees eat more than 35 percent of their weight every day. That figure captures the scale of the winter feeding task. A chickadee is not eating for pleasure. It is eating to stay alive until morning, every single day, for months. Food storage, diet switching, and constant foraging are therefore not side details of winter survival. They are part of the heating system itself, because every calorie burned for heat must first be found, stored, or carried.

This article deliberately keeps feeding brief. Feeding is one survival strategy among several, and it supports the heating system rather than replacing it. A bird with plenty of food but wet feathers, no shelter, and no fat reserve can still fail on a cold night. A bird with good insulation, shelter, and a shared roost can stretch a smaller food supply further. The systems work together.

Torpor: Turning the Thermostat Down on Purpose

Some birds take a more radical approach to a cold night: they let their body temperature fall on purpose. This controlled state is called torpor. In torpor, heart rate, breathing rate, and metabolism slow down, and the bird burns far less fuel than it would if it held its full daytime temperature all night. Torpor is not sleep, and it is not the same as hibernation. It is a regulated lowering of the thermostat, with a controlled rewarming before dawn so the bird can feed again.

Hummingbirds are the best known daily torpor users, and research from Cornell University, reported through Cornell Lab researchers, shows how finely they can control the state. A hummingbird’s normal daytime body temperature is more than 100 degrees Fahrenheit. During shallow torpor, body temperature drops by about 20 degrees Fahrenheit. During deep torpor, it drops by about 50 degrees Fahrenheit below the normal daytime temperature. That is a far larger drop than a human could survive unaided, yet the hummingbird rewarms itself, using shivering and wing muscle vibration, and resumes feeding. Rewarming takes time and energy, often up to about 30 minutes, so torpor is a trade off, not free savings. Deep torpor also carries costs, including reduced immune function and greater vulnerability to predators while the bird is stiff and unresponsive.

At least 42 bird species are reported to use torpor, according to reporting of the Cornell research, but only hummingbirds, nightjars, and one species of mousebird are reported to go into deep torpor. Other familiar birds use shallower versions. Doves, for example, can enter brief torpor to conserve energy, and Audubon notes that hummingbirds, doves, and the whip-poor-will can all enter brief states of torpor.

The common poorwill is the notable exception to the overnight pattern. Audubon, drawing on BirdNote, describes the poorwill as a petite member of the nightjar family that can enter a hibernation-like state and stay in it for hours, or even weeks, as winter deepens. Scientists suspect there may have been an evolutionary advantage for the poorwill if it could stay through the winter instead of migrating, emerging as temperatures rise, days lengthen, and insects become abundant again. The poorwill is widely described as the only bird whose behavior approaches true hibernation, but even its long torpor bouts are best understood as extended torpor rather than the months long hibernation seen in some mammals.

Torpor also explains a sight that alarms many observers: a hummingbird hanging motionless and cold in cool weather. A torpid hummingbird can look lifeless, with slow breathing and a body temperature far below normal. In most cases it is not dead or injured. It is saving energy and will rewarm on its own. The distinction matters because a torpid bird should not be handled as if it were an ordinary sleeping bird, and it should never be warmed rapidly by artificial heat. The safest response for a genuinely torpid hummingbird found in the open is to leave it undisturbed unless it is in immediate danger, and to seek advice from a licensed wildlife rehabilitator if it does not rewarm as the day warms.

Other birds use a milder version of the same idea without full torpor. Many species allow their body temperature to dip slightly overnight, a controlled hypothermia that saves fuel while keeping the bird responsive. Chickadees, for example, are reported to lower their temperature modestly at night, though they do not enter the deep torpor of a hummingbird. The exact depth varies by species, by night temperature, and by body condition. A well fed bird may hold a higher temperature. A thin bird on a bitter night may dip lower to survive until dawn.

Roosting, Huddling, and Shelter: Sharing Heat and Blocking Wind

Behavior is the cheapest heating system a bird has, because shelter and company cost no fat. When winter wind starts, birds seek shelter just as people do. For small birds like titmice and juncos, Audubon notes, the denser the tree or shrub, the better. Tree crooks, cavities, and manmade structures are also popular refuges. Dense evergreen foliage breaks the wind and holds slightly warmer air. A cavity or nest box goes further, enclosing the bird in a small space where its own body heat warms the air around it.

Some birds burrow into snow itself. Grouse and ptarmigan, for example, dive into soft snow and roost in snow burrows, using snow as insulation. Snow is a good insulator because it traps air, and the temperature inside a snow burrow can be far milder than the air above it. Other ground birds shelter behind small rises, rocks, or drifts that block the wind. The principle is the same in every case: reduce wind exposure, reduce the volume of cold air the bird must heat, and let trapped air do the work.

Huddling multiplies the effect. Many small birds, including black-capped chickadees, eastern bluebirds, and house wrens, gather in large groups at night and crowd together in a small, tight space to share body heat, as the Smithsonian Institution reports. Audubon similarly describes bunches of bluebirds and sparrows as a typical winter sight, and tree swallows crowding together in shrubs, vines, and evergreen trees. Cavity nesters like nuthatches, titmice, and downy woodpeckers use tree cavities and nest boxes to stay warm, according to the U.S. Fish and Wildlife Service, and larger birds like American crows and ring-billed gulls are also known to flock together for warmth.

Huddling works because a cluster of birds has a smaller exposed surface, relative to its combined volume, than the same birds roosting alone. Each bird warms its neighbors, and the group loses heat more slowly. The effect is strongest in an enclosed cavity, where the shared heat warms the air itself, but even an open huddle on a branch helps. Flocking during the day serves a different purpose, with more eyes watching for predators and more birds searching for food, but the night roost is where sharing heat matters most.

Where birds go at night therefore shapes how warm they stay. A bird that roosts in a sheltered cavity, a dense evergreen, or a shared huddle starts the night with a major advantage over a bird roosting alone in the open. Readers who want the fuller night roost picture can read the companion article on where birds go at night, which covers roosting behavior in more detail, and the companion article on how birds sleep, which explains how sleep itself fits into the winter night.

Migration: The Alternative to Staying Warm

Not every bird tries to stay warm through winter. Many species avoid the hardest season altogether by migrating to warmer climates where food is available and heating costs are lower. Migration is not a failure of winter survival. It is a different strategy, with its own costs in energy, navigation, and risk, traded against the cost of staying.

The choice between migrating and staying depends on food, body size, and evolutionary history. Species that depend on flying insects, which vanish in northern winters, have little choice but to move. Species that can switch to seeds, fruit, or dormant prey can often stay. Some species do both, with northern populations migrating and southern populations staying, or with some individuals migrating while others remain. Partial migration of this kind shows that staying warm is not a single species trait. It is a population level decision shaped by local conditions.

Staying also has advantages that migration does not. A bird that winters in place keeps its territory, knows its food sources and roosts, and is ready to breed early in spring. A migrant must return, reestablish territory, and rebuild condition. The winter residents people see at feeders and in parks, chickadees, cardinals, finches, woodpeckers, and nuthatches among them, are species whose insulation, shivering, caching, torpor, and roosting systems are good enough to make staying pay. Their presence in January is evidence that the system works, not that winter is easy for them.

The wider context of which species are thriving and which are struggling is covered in the Wave 98 companion article on the 2022 State of the Birds tipping point species list, which tracks birds at a conservation turning point. Winter survival and long term population trends are linked: a species that loses winter habitat, food, or shelter faces harder winters even if its physiology is unchanged.

Do Birds Feel Cold? A Brief Answer

This article focuses on how birds stay warm, not on whether they feel cold, and the distinction is worth keeping clear. The companion article on whether birds get cold covers that question in full. The brief answer is that cold puts real stress on a bird’s body and forces it to spend extra energy, even when the bird looks calm on a snowy branch. Scientists are cautious about equating a bird’s experience with human discomfort, because we cannot ask a bird how cold feels to it. What can be observed is clear: cold changes behavior, raises energy use, and, when severe or prolonged, threatens survival.

That caution matters for how people interpret winter birds. A duck standing on ice is not proof that cold does not affect it. It is proof that its countercurrent system, its feather insulation, and its fat reserves are managing the problem at a cost that must be repaid with food. A puffed up sparrow is not necessarily sick. It is using its coat. A motionless hummingbird in cool weather may be in torpor, not dead. Reading winter behavior through the mechanisms in this article, rather than through human feelings about cold, gives a more accurate picture of what the bird is doing and why.

When the System Fails: Cold Snaps, Wet Cold, and Starvation

Winter survival systems have limits, and cold snaps reveal them. A sudden drop in temperature, combined with wind, ice, or snow that covers food, can overwhelm even well adapted birds. The danger is rarely cold alone. It is cold plus wet feathers, cold plus an empty fat reserve, or cold plus several nights without enough feeding time. A bird that enters a cold snap thin, wet, or ill has less margin than a healthy, well fed bird, and the failure can come quickly.

Wet cold is especially dangerous. Rain followed by freezing temperatures, or a bird that has bathed and cannot dry before dusk, loses insulation at exactly the moment it needs it most. Ice on feathers adds weight and breaks the smooth outer shell, letting cold air reach the downy layer. Birds therefore avoid bathing late in the day in winter, and they preen vigorously after any wetting. A bird that cannot keep its plumage dry and ordered is at a disadvantage no amount of shivering can fully correct.

Starvation is the other common failure. Shivering burns fat fast, and a bird that must shiver all night without enough fuel will run out before dawn. This is why the first cold morning after a storm often brings dead birds, not the storm itself. The birds survived the wind and snow, but they could not feed enough during the short day to pay for the long night. Conservation of energy through torpor, shelter, and huddling is therefore not just comfort. It is what stretches a finite fuel supply across an unpredictable season.

People can help without turning winter into a feeding project. Keeping a reliable water source unfrozen, leaving dense shrubs and evergreen cover unpruned through winter, preserving tree cavities where safe, and avoiding disturbance of night roosts all support the systems described here. Feeding, where people choose to do it, helps most when it is consistent and high in energy, because birds learn to rely on a source and plan their daily budget around it. Irregular feeding is less useful than no feeding at all if birds waste energy visiting an empty source. The main point stands: shelter, water, and undisturbed roosts matter as much as food, and they cost the bird nothing to use.

How to Read Winter Bird Behavior

Watching winter birds with these mechanisms in mind turns a familiar scene into a readable one. A puffed up bird on a branch is fluffing. A bird tucked on one leg is protecting its feet. A group crowded into a cavity at dusk is sharing heat. A chickadee darting between a feeder and a hiding place is caching, not just eating. A hummingbird motionless in cool weather may be in torpor. Each behavior is a piece of the heating system, and each one costs or saves energy in a measurable way.

Identification also helps. Winter flocks mix species that use different strategies, and recognizing them makes the strategies clearer. A bird with a red chest at a winter feeder may be a robin, a finch, or another species entirely, and each one winters differently. The broader picture of birds of North America and the various kinds of birds found across the continent gives context for why some species stay and others go. For readers learning to name winter visitors, the Wave 98 companion articles on the bird with a red head and brown body and the finch with a red head cover two common identification puzzles that often arise at winter feeders.

The key lesson is that winter birds are not passive victims of cold. They are active managers of heat, using feathers, muscle, blood vessels, fat, behavior, and, in some species, a controlled lowering of body temperature. They pay a high price for that management, in food, in time, and in risk, but the system is refined by evolution and it works for millions of birds every winter. Understanding it makes winter birdwatching richer, and it explains why a small bird on a snowy branch deserves more respect than pity.

Frequently Asked Questions

How do birds stay warm in winter without a coat?

Birds carry their own coat in the form of feathers. Outer contour feathers form a weather resistant shell, and soft downy feathers underneath trap a layer of warm air next to the skin. Fluffing thickens that air layer, preening keeps it dry and ordered, and shivering adds heat when insulation alone is not enough.

Why do birds puff up in winter?

Birds puff up to trap more air between their feathers. Raising the feathers with tiny muscles at their bases creates hundreds of air pockets that hold body heat. The puffed shape also usually comes with tucked posture, which reduces exposed bare skin and keeps warm air close to the body.

How do birds keep their feet from freezing on ice?

Bird feet contain mostly bone and tendon with little muscle, so they can safely run much colder than the core. A countercurrent heat exchange system in the legs transfers heat from warm blood leaving the body to cooler blood returning to it, so less heat is lost to ice and the core is not chilled. Birds also stand on one leg, tuck the other into belly feathers, or crouch to cover their feet.

What is torpor in birds?

Torpor is a controlled state in which a bird deliberately lowers its body temperature, heart rate, breathing rate, and metabolism to save energy, usually overnight. Hummingbirds are the best known users, dropping far below their daytime temperature and rewarming before dawn. Torpor is not sleep and it is not hibernation, though the common poorwill can stay in extended torpor for days or weeks.

Do all birds use torpor in winter?

No. At least 42 bird species are reported to use torpor, but deep torpor is reported only in hummingbirds, nightjars, and one species of mousebird. Most winter birds rely on feathers, shivering, fat reserves, shelter, and huddling, with only a modest overnight dip in body temperature, if any.

Why do some birds migrate instead of staying warm?

Migration avoids the coldest season and the food shortage that comes with it, trading the cost of staying warm for the cost of a long journey. Species that depend on flying insects, which vanish in northern winters, usually must move. Species that can switch to seeds, fruit, or dormant prey can often stay and keep their territories.

Do birds need help to stay warm in winter?

Healthy wild birds are well equipped for winter, and they do not need people to keep them warm. People can support them by preserving shelter, leaving dense evergreen cover and tree cavities where safe, keeping water unfrozen, and avoiding disturbance of night roosts. Consistent, high energy feeding helps where it is offered, but shelter and water matter as much as food.

Sources & references

• U.S. Fish and Wildlife Service, "How do birds keep warm in the winter?" Opened and verified 3 November 2026. Chickadee body temperature and daily food intake, feather preening and waterproofing, roosting and huddling, countercurrent heat exchange and bill tucking.
• National Audubon Society, "How Do Birds Cope With Cold Winter Weather?" Published 24 January 2024, updated 27 August 2026. Opened and verified 3 November 2026. Feather layers and heavier winter plumage, fluffing, shelter, huddling, fat reserves above 10 percent of winter body weight in small birds, caching up to 80,000 seeds by a single chickadee, feet of bone and tendon.
• Smithsonian Institution, "Keeping Warm in Winter Is for the Birds," 30 January 2015. Opened and verified 3 November 2026. Feather insulation and waterproofing oil, fluffing to trap air, one leg tucking, countercurrent heat exchange in swans, geese, and ducks, communal roosting in chickadees, bluebirds, and wrens.
• National Audubon Society, "Some Birds Enter A Hibernation-Like State Called Torpor to Survive Winter's Chill," BirdNote transcript, published 25 January 2022, updated 27 August 2026. Opened and verified 3 November 2026. Torpor definition, hummingbirds, doves, and whip-poor-will in brief torpor, common poorwill in prolonged hibernation-like torpor.
• Cornell University research on hummingbird torpor, as reported in coverage of the Journal of Experimental Biology study led by Anusha Shankar of the Cornell Lab of Ornithology. Opened and verified via secondary reporting 3 November 2026. Shallow torpor about 20 degrees Fahrenheit below normal, deep torpor about 50 degrees Fahrenheit below normal, at least 42 species using torpor, rewarming time up to about 30 minutes. Figures are attributed to this reporting in the article and ledger because the primary journal text was not opened directly.