When bird watchers observe cardinals, bluejays, nuthatches, wrens, even robins who may have spent their summers nesting locally, remaining in areas where the winter temperatures often go below zero, one can’t help but wonder how they survive in such cold.
Some ornithologists even say that there are geographical waves of the same species migrating to and from different areas as temperatures drop, such as chickadees from Ontario migrating to New England, as New England chickadees migrate down to the Carolinas, and Carolina chickadees migrate further south to the Gulf of Mexico and Florida. Researchers can’t be sure each migration includes all the resident birds from the warmer seasons, 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 window for a handful of sunflower seed several times a day, are identifiable enough to be noticed–or missed if they don’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 few 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 to N. Carolina and return. Further, they are not known to give classes to replacement newcomers of their species on such idiosyncratic behaviors before they leave for warmer climes.
And, of course, during winters like 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 some southern states like Tennessee, Virginia, North and South Carolina, and northern Georgia, it freezes those areas to 5-20 degrees colder than in the woods of New England. Nonetheless, it’s clear that as we study birds to find out how they are able to survive during 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 stay alive?
Avian Basal Metabolic Rate and Hypothermia
Animals have normally lower body temperatures than avian body temperatures, which range from about 38º- 42º C. (100.4º- 107.6º F). Two factors in a bird’s survival in cold weather is 1) the normal climate in which the species lives, and 2) the bird’s basal metabolic rate (BMR – the faster its heart beats, the more body heat it generates). The more body mass or larger the bird species, the lower the average body temperatures compared to smaller bird species.
Birds inhabiting tropical climate ranges have lower BMRs than those inhabiting temperate regions, and birds in temperate regions have lower BMRs than those inhabiting cooler to polar regions. Various species studied from temperate and polar latitudes having high BMRs were found to be associated with high maximal rates of thermogenesis (heat production) and increased cold tolerance.
Also, winter birds examined had 23% higher BMRs and 8% higher summit metabolic rates (maximum metabolism rate in response to cold exposure) than spring birds. Cold climate bird species are able to put themselves into a state of hypothermia during cold nights or during stormy periods. This induced torpor slows a bird’s heart rate and drops its normal body temperature like an animal in hibernation. The bird goes into a state of subconsciousness that enables it to conserve heat and body energy to sometimes endure otherwise unendurable cold temperatures.
The greatest test for birds in cold weather is to find enough food to build and maintain adequate fat supplies for their body stores to provide enough energy to keep them flying and looking for yet more food. When the jet stream sweeps Canadian air down into southern areas where birds have migrated to avoid the northern cold, it can spell disaster for them. Unfamiliar areas where they may not know of local feeding areas or feeders, combined with dead bushes and empty fields, can prevent birds like thrushes, robins, wrens, and other protein eaters from finding their natural forage like berries, nuts, and seeds. In such cases, backyard feeders can save countless little lives.
Of course, the birds that leave one area may soon appear in others that are less affected by the weather or where food is more available. Hard winter weather may require a change in avian behavior rather than just a change of location. Birds have to feed and rest according to their physical needs, creating a delicate balancing act that must be responded to by their governing instincts. With dramatic environmental changes resulting from human expansion that can wipe out entire feeding areas or block avian sensory input from just one year to the next, mankind can sometimes cause bird life to get their signals crossed – possibly resulting in fatal mistakes.
Nuthatches, chickadees, many woodpeckers, and bluejays cache food like squirrels do and those habits can sometimes save them, but in bad weather their stashes may get discovered and plundered by other hungry hunters. Further, anything hidden under snow is often inaccessible, so they may be little better off than others in bad weather. The smallest birds, like chickadees and the little gray titmice, have to feed throughout the daylight hours, alongside the continual grazers like juncos and sparrows, consuming up to 30% of their body weight to keep their fat reserves high enough so they can stay warm through cold winter nights.
Finches, grosbeaks, and ground birds like pheasants and grouse may need to eat seed just before dark, and digest them through a cold night for life-sustaining heat. Smaller birds can die if facing both a cold snap and a night of fasting. That’s why bird feeders should be filled on late afternoons, particularly when the weather is very cold. Occasionally finding an open field with high-energy food such as thistle plants or wild sunflowers still containing seeds may be all that stands between wild birds and starvation.
Avian Survival Advantages
Watching them fly about and feed in the freezing weather, one would quickly assume that cold is actually not a big problem for birds – and that would be true if all their systems remain functional. Birds that live in cold climates have a number of natural operating systems that build and maintain their body heat to see them through oncoming harsh seasonal changes. In addition to finding food sources, these include special methods of preening, standard heat conservation and loss, feather growth and construction, time of year (for cold acclimatization), their social ecology, body size, and metabolic rates, 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 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, doves and pigeons, and small owls. Titmice and wrens 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.
- Ground Roosting – 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 become more sociable during winter, and that enables them to find more food by hanging with a crowd of both their own and other similar species, and provides more critical body warmth at night when roosting in a crowd. Summer nesting boxes that are pressed into service as emergency shelters and larger winter roosting boxes can often protect many of each species.
Wrens, who are co-operative breeders, maintain ongoing families of 4-8 or more birds in any given territory, but they are adept at ‘forming packs’ of multiple families during inclement weather for purposes of finding food or keeping warm. Wren studies have recorded instances in South America of up to 50 wrens at a time roosting in one nest box during cold weather. It 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 feeders, especially those that provide sunflower seed, see a flurry of birds each morning as they 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, with some retiring for the night as early as 4:30 p.m. on winter days. Other species like sparrows, juncos, and mourning doves may remain at a feeder most of the day.
And cold weather is also the time most feeder owners may report the small Sharp-shinned hawk and the slightly larger Cooper’s Hawk dive bombing their feeders and taking a junco or sparrow every few days. This food source is sometimes all that is available to them in the dead of winter, and easier for them than finding mice under the frozen snow.
Wildbirds, Hummingbirds, Butterflies, and More – 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/ritchisong/birdmetabolism.html
Cornell Lab or Ornithology – http://www.birds.cornell.edu/netcommunity/page.aspx?pid=1670