How Birds Manage in Freezing Cold Temperatures
Half or more of all species of birds migrate to warmer areas when winter arrives. But in the U.S., there are birds like Red Cardinals, Bluejays, Nuthatches, Carolina Wrens, some Robins and many Chickadees, who remain in climates where temperatures often drop below zero – It’s hard not to wonder how they manage to survive such cold.
- Identifying Individuals
- Decadal Oscillation Temperature Patterns
- Avian Basal Metabolic Rate and Hypothermia
- Food Availability
- Avian Survival Advantages
- Social Ecology
Some ornithologists posit that there are migrating waves of same species birds that migrate to different areas as winter cold sets in. Canadian birds are suspected of migrating down to New England replacing a group of the same species that lived and nested there, and those, in turn, migrate south to southern states; less cold-tolerant birds of that species that nested there migrate further south to the Gulf of Mexico and Florida. Researchers can’t be sure that each migration group includes all the resident birds from the warmer latitudes, and without banding them all, there is no way to tell what causes some to leave and some to stay on year round.
Those of us feeder watchers who have special little titmouse visitors such as I do, with distinctive ‘food screams’ aimed at just me, or tiny chickadee friends like mine who come and literally knock on my kitchen or office window for a handful of seed several times a day, are clearly identifiable enough to be both noticed or missed if they didn’t show up fairly often. So even though it’s not as scientific a method as banding, it’s certainly conclusive that they are the same bird(s) when a little guy shows up with this peculiar and identical behavior, summer and winter, 350 days of the year.
I expect the other days he/they might be on vacation, but since they are only missing on the coldest three or four days during storms or cold snaps, it’s fairly certain there wasn’t time to migrate a thousand miles or more south to escape the large stormy weather patterns and return in that time frame. Further, they are not known to give classes to replacement newcomers of their species on such idiosyncratic ‘feed me’ behaviors before they leave for warmer climes.
Decadal Oscillation Temperature Patterns
And during winters such as we have enjoyed in 2010-2011, whose temperatures are like other early years of the decadal oscillation patterns of thirty years ago, where the Jet Stream dips way down to the south from the Great Lakes bringing cold into the mountainous wooded areas of southern states like Tennessee, Virginia, North and South Carolina, and northern Georgia, it brings deep freezes there from 5-20 degrees colder than in the north woods of New England.
Nonetheless, it’s clear that as we study birds to find out how they are able to survive in extreme cold wherever it may be, we can’t help but admire their eloquent design. How can such tiny creatures barely an inch wide that live almost literally on the wing in the freezing air of winter, manage to retain enough body warmth to even stay alive – and sometimes thrive?
Avian Basal Metabolic Rate and Hypothermia
Mammals normally have lower body temperatures than avian body temperatures, which range from about 38º- 42º C. (100.4º- 107.6º F). One factor in avian survival in cold weather may depend on the normal climate in which the species lives, and the bird’s basal metabolic rate (BMR – the faster its heart beats, the more body heat it generates). It is known that the more body mass or larger a bird species is, the lower its average body temperatures as compared to smaller bird species.
Birds living in tropical climates have lower BMRs than those in temperate regions, and birds living in temperate regions tend to have lower BMRs than those in cooler regions. Species studied from temperate and polar latitudes that have high BMRs were found to be generally have higher maximal rates of thermogenesis (heat production) and better cold tolerance.
Winter birds examined also had 23% higher BMRs and about 8% higher summit metabolic or maximum metabolic rates responding to cold exposure) than spring birds examined. Bird species that live normally in colder climates are naturally able to put themselves into a state of hypothermia during cold nights or stormy periods. This is known as “induced torpor” that slows a bird’s heart rate and drops its normal body temperature similar to an animal going into hibernation. The bird achieves a state of sub-consciousness that permits it to begin conserving heat and body energy and endure some perhaps otherwise unendurable cold temperatures.
The greatest challenge for cold weather birds is to locate enough food to build and maintain the fat stores they require to provide enough energy to keep them on the wing and locating needed food. When the jet stream sweeps northern cold down into southern areas where birds have migrated to avoid it, these temperatures can spell disaster for them. Migrating birds unfamiliar with local feeding areas or feeders, combined with dead winter bushes and empty fields, can prevent many insect-eating (protein dependent) birds from finding alternate natural forage like berries, nuts, and seeds. In such cases, only local residential feeders may save them.
Of course, birds that leave one area, may quickly appear in others less affected by a cold weather pattern or where food is more available. Difficult winter weather may require a change in avian behavior even more than a change of location. Birds must feed and rest according to their physical needs, creating a delicate balancing act requiring responses from their governing instincts. Dramatic environmental changes resulting from human expansion can sometimes wipe out entire feeding areas or block avian sensory input, either of which can sometimes cause bird life to get their signals crossed; causing them to make fatal mistakes.
Many bird species acclimated to cold weather such as Nuthatches, Chickadees, woodpeckers, and Bluejays, tend to cache food like squirrels do and those habits can sometimes save them, but in bad weather their stashes may get plundered by others. In blizzards, when those stores may be needed most, anything hidden under the snow is rarely accessible, leaving these birds little better off than the others in bad weather. The smallest birds, like Chickadees and the little Gray Titmice, need to feed throughout the daylight hours, alongside others like Juncos and sparrows, consuming up to 30% of their body weight to keep their fat reserves high enough to stay warm through the cold winter nights.
Finches, Grosbeaks, and game birds like pheasants and grouse, seek to find seed just before dark, digesting them during cold nights for life-sustaining warmth. Smaller birds can die if facing both a cold snap and a night of fasting. This is why bird feeders should be re-filled late in the afternoon, especially in very cold winter temperatures. Finding a feeder for a late afternoon meal may be all that stands between some wild birds and starvation.
Avian Survival Advantages
Watching birds flitting about and feeding in freezing weather might cause observers to assume that winter cold is not a big problem for birds – and that might be true when all their systems remain functional. Cold climate birds have many natural operating systems to build and maintain body heat for oncoming harsh seasonal changes. In addition to finding food, these include special methods of preening, physical heat conservation and loss, feather growth and construction, time of year (for cold acclimatization), species social ecology, body size and metabolic rate, normal species activity levels, and each bird’s ability to fly and protect itself.
• Winter preening - This includes waterproofing their feathers with body oils so that the inside down feathers stay dry and insulate their small bodies against the cold. These inner feathers provide the “R-value” insulation between the bird’s body and its strong outer feathers, which act like roofing shingles to create an impermeable barrier between the warmth inside and cold outside.
And even with an accumulated layer of snow on its back, as can happen when a bird is roosting in a tree during a snowstorm in a state of torpor overnight, the outer feathers prevent heat from escaping from the small body that might melt some of the snow, letting water or cold seep in and jeopardize the life of the creature.
• Heat Loss - Birds do not have sweat glands, so they can only lose heat through breathing. This is why in very hot weather, birds may be seen with their beaks held open as if panting. Seasonal temperature changes decrease or increase metabolism in finches and other small temperate-wintering birds and as temperatures decrease in winter, acclimatize them to the colder weather.
• More Feathers - Birds produce 25-30 percent more feathers in the cold weather than in the summer.
• Fluffing and Natural Insulation - In the coldest weather, birds can also be seen roosting while fluffing themselves up to create air spaces between their feathers to trap heat inside. Particularly species such as Bluejays will periodically take a break from foraging, flying, and screaming to recharge. They may plant themselves on a branch in the sun, fluff their feathers several times, and then appear to get instantly sleepy, possibly because their furnace has just come on. Their eyes close partially and their fluffed under-plumage makes them appear twice their normal size and changes their snow white breasts to pale grey as the fluffy down feathers show through.
Woodpeckers or Flickers may be seen fluffing while hanging on the dry side of tree trunks in rain or snow storms. This activity enables birds to maintain or increase their body temperatures using very little space and weight.
• Shivering - And like humans, birds also use shivering to maintain warmth. Studies have been done comparing certain parts of their bodies and the muscles used at given times (such as torso, wings, “gular shivering” or lungs, etc.) to enable warmth-inducing shivering across various parts of avian muscle systems, while still conserving available body energy and fuel.
• Huddling - Normal loner ecological habits take a backseat in cold weather, and birds seek out communal roosting areas and crowd together to keep warm, especially at night. Birds that will huddle in small spaces for warmth on winter nights or in cold snaps are generally the species that tend to nest in such places, like bluebirds, chickadees, nuthatches, titmice, wrens, sparrows, and doves and pigeons, and small owls. Titmice tend to travel in families or herds, and often roost together at night for group warmth.
• Safe Shelters - Other hospitable shelters are old barns and outbuildings, dead trees, human residences and garages, and summer birdhouses, allowing sparrows, finches and others to cluster together at night for shelter and warmth. Other birds roost in the thick of firs and evergreens, in trunk niches of large Pine trees, and under railroad bridges. Buildings that generate some heat of their own, such as the eaves of human dwellings, church steeples, bus stations, shopping malls, garden stores, apartment buildings, and parking garages are also places of safety out of the wind and cold, and can save lives on cold nights.
If only ground brush is available, sparrows and other ground feeding birds may choose that, but winter predators such as fishers, coyotes, and even squirrels can snatch low-roosting sleeping birds at night. Too close to the ground, in niches of a log pile or a downed tree, and a bird can fall prey to rats and other rodents when taking advantage of what might seem to be a warm and otherwise secure chamber covered in snow.
• Flight - This is a major survival tool for birds in winter, and birders can attest to sudden and dramatic changes in the numbers of birds in an area just before storms and deep freezes. Sudden noticeable reductions in avian populations may be a clue that the local birds have moved on temporarily – or even died, but in other cases, even being a bird biologist may not provide the answers we seek as to where they go.
Some birds tend toward more sociable behavior during cold weather, enabling them to find more food by engaging in ‘herding’ with both their own and other similar species, and can provide critical body warmth at night when roosting in a crowd. Summer nesting boxes may be pressed into service as emergency shelters and larger winter roosting boxes can often protect many of each species.
Co-operative breeders such as wrens maintain ongoing families of 4-8 birds (or more) in given territories, and are adept at ‘forming packs’ of multiple families during inclement weather for finding food or keeping warm. Wren studies in South America have recorded instances of up to 50 wrens roosting in one nest box during cold weather. This gives a whole new meaning to the word ‘commune’. Pigeons and doves also tend to seek out others of similar species and crowd together for warmth on cold nights.
Especially in winter, most backyard feeders that provide sunflower seed, see flurries of birds each morning as they arrive at daybreak to replenish their energy – and heat lost overnight. Most birds visit feeders in cycles, first early and then late morning, early afternoon, and coming and going during the later afternoon. Species like sparrows, Juncos, and Mourning Doves may remain at a feeder most of the day.
Cold weather is also the time when many feeder owners may report small ‘Pigeon Hawks’ of various species dive bombing their feeders and taking a small bird every few days. This food source is easier for them than finding mice under the frozen snow.
Wildbirds, Hummingbirds, Butterflies, and More - http://www.birdfeedersonly.com/articles-how-do-birds-stay-warm
Colvin Run Habitat - - http://crhabitat.blogspot.com/2007/02/how-do-birds-stay-warm.html
Avian Metabolism - http://people.eku.edu/ritchison/birdmetabolism.html
Cornell Lab of Ornithology - http://www.birds.cornell.edu/netcommunity/page.aspx?pid=1670