Most birds migrate out of the snowy regions of temperate deciduous forests at the end of the breeding season (Figure 1).  However, a few species are permanent residents, remaining throughout the year in their temperate breeding range.  Other species are winter visitors, migrating into temperate regions from subarctic nesting ranges.  Some winter visitors appear regularly in temperate regions, such as tree, song, white-throated sparrows and juncos.

Figure 1. Pie graphs showing the proportion of North American bird individuals which migrate (black sectors). Stippled area represents forested regions (Taken from MacArthur 1959)

Other winter visitors are irregular, migrating in large numbers in some years, or not at all in others.  The movements of such birds are referred to as irruptive migrations, and are related to the food supply in the boreal forest breeding range.  Irruptive winter visitors -- including red-breasted nuthatch, evening grosbeak, redpoll and pine siskin -- may occasionally be abundant at feeders.  Finally, each year a few individuals of migrant species decide not to migrate.  These stragglers may include robins, catbirds, brown thrashers and myrtle warblers, some of which are poorly adapted to snowy environments and often perish before spring.

Birds are easily attracted to a feeder.  Winter feeding as a recreational and aesthetic activity has increased in popularity recently.  In fact, one home in five nationwide, and one in three in New England, have bird feeders.  Furthermore, about six million pounds of bird seed -- an amount equal to a train of box cars 1 mile long -- was sold in 1972 in Maine alone.  It is no wonder that provisioned winter feeders are thought to have an influence on both the numbers and the winter distribution patterns of some wintering birds!

Because birds are readily attracted to feeders and can be observed there close range, winter is an excellent time in which to introduce students to bird natural history, behavior, and ecology.  In addition, it is easier to identify birds in winter because there are fewer species for the beginner to learn.

Kinds of Birds at Feeders

The kinds of birds which are attracted in a particular year to any given winter feeder in North America depends upon the vegetational region or biome, the specific habitat within the biome, the kinds of foods provisioned at the feeder and, of course, the chance presence of irruptive migrants and stragglers.

Biomes.  Table 1 lists the birds which commonly come to winter feeders in snowy portions of North America, their breeding and wintering range, feeding location at the feeder, and some of the commercial seeds they eat.  In general, as one proceeds south at any longitude, one may expect both a greater variety of species and greater number of individual birds at a winter feeder.  In northern boreal forests, the few wintering species that visit a feeder may include gray jays, boreal and black-capped chickadees, white and red-breasted nuthatches, purple finches, redpolls, siskins, evening grosbeaks, and downy woodpeckers.  In north temperate regions, common visitors to feeders include downy woodpeckers, black-capped chickadees, white-breasted nuthatch, mourning doves, blue jays, and a variety of sparrow and finch species.  In south temperate regions, tufted titmice, red-bellied woodpeckers, Carolina wrens, and various blackbirds, are added to those occurring in the north temperate region.  In prairies, sparrows of various types predominate, including song, tree, Harris', white-crowned, field and swamp sparrows.  Finally, Steller's jay, Cassin's house finch, purple and rosy finches, and mountain chickadees may be visitors to an established feeder in western mountain regions.

Table 1. Birds which may commonly feed at seed-provisioned feeders. (Symbolism: PR = Permanent Resident; WW = Winter Visitor; S = Straggler; SF = Sun Flower seeds. Feeding location refers to a platform feeder, F, or ground, G).

Habitat.  Table 2 lists the kinds of birds attracted to feeders according to the general habitat in which the feeder is located.  Feeders in urban habitats, which lack trees, will be visited by few winter birds.  The species which have adapted to urban environments include rock doves (pigeons), house sparrows, and starlings.  The presence of shrubs and trees increases the likelihood of attracting a variety of birds in a city.  As Table 2 indicates, some species have a generalized habitat niche and will occur in several kinds of environments, while others with specialized habitat niches are found in only one.  Feeders located near forests will regularly attract titmice, chickadees, nuthatches, and woodpeckers; these species are permanent residents which glean dormant insect from trunks and branches of trees in winter.  Feeders near fields will attract mainly seed-eating birds, such as sparrows, finches, and blackbirds.

Table 2. Birds Commonly Attracted to a Seed-Provisioned Winter Feeder According to Habitat.

*Refers to Southern portion of snow regions.

Foods Provided.  Although Table 1 is not an exhaustive list of commercial seeds eaten by winter birds, it nonetheless illustrates that some species are picky eaters (i.e. specialized food niche) while others eat a variety of seeds (i.e. generalized food niche).  The kinds of seeds a bird will eat depends upon seed size and nutritional content.   For example, the larger-beaked tufted titmouse and white-throated sparrow take larger sunflower seeds than do the smaller-billed black-capped chickadee and dark-eyed junco.  In a controlled laboratory study, Willson (1971) found that long and medium-billed finches ate more large seeds, and thus ingested more calories per unit time than small-billed finches.  The advantage of eating smaller seeds, however, is in quicker handling time, which reduces the risk of predation while feeding and a larger supply of seeds.  It is possible, however, for a small-billed bird to learn to feed on large seeds.  For example, the beaks of chickadees, titmice and nuthatches are too small to crack whole sunflower seeds, which they nonetheless are able to eat by holding the seed with their feet or wedging the seed in a crevice and hacking it open.  Cardinals and grosbeaks, in contrast, crack open the sunflower seed coat with their large bills, while mourning doves and blue jays ingest the seed whole.  In the latter two species the seed coat is then abraded away in the gizzard prior to digestion in the stomach. 

Birds which eat insects in the warm months will choose seeds at a winter feeder that are high in proteins and fats, such as peanuts and sunflower seeds.  Adding these seeds to the feeder will attract chickadees, titmice, nuthatches, blue jays and woodpeckers.  Year around seed-eaters, such as sparrows and finches, tend to choose seeds rich in carbohydrates such as corn, wheat and millet.  As shown in Table 1, they also prefer feeding on or near the ground, the natural location of seeds in fields.

Irruptive Migrations.  The mix of birds which come to a feeder in any given year is also dependent upon movements of seed-eating birds migrating south from northern boreal forests.  These irruptive migrations throughout the northern regions of the world appear to be cyclical and, in turn, are caused by variation in birch and cone seed production in boreal regions.  When these seed crops fail, or when baby production has been especially successful, nutcrackers, red-breasted nuthatches, crossbills, siskins, redpolls, purple finches, and evening and pine grosbeaks will be found in more southern temperate regions in large numbers.  Crossbills and pine grosbeaks, incidentally, do not come readily to feeders.

Adaptations to Winter Conditions

Birds that overwinter in the north have evolved several mechanisms for surviving inhospitable winter climates.  These mechanisms can be behavioral, morphological or physiological.

Behavioral Adaptations to Winter.  The main behavioral adaptations of wintering birds involve flocking, feeding, and roosting.

Flocking -- A general adaptation of nearly all birds in winter is the social behavior of flocking.  In the summer many birds defend territories in pairs; in the winter they may associate in a flock.  Birds benefit from flocking in two major ways.  First, flocks may locate food and feed more efficiently than single birds, and second, birds within a flock are less vulnerable to predation.  Sparrows within a flock spend less time looking for predators and more time feeding; the larger the flock, the more this effect is enhanced.  Laboratory studies by Krebs et al. (1972) have shown that titmice foraging in a group have a greater chance of finding hidden food than if they foraged singly, because they learn the location of food from one another.  Furthermore, many wintering birds forage in mixed species flocks.  For example, different finch species often associate (e.g., goldfinches, redpolls, siskins), as do sparrows (e.g. tree, white-throated, song).  Various chickadee, nuthatch, and titmice species commonly forage in mixed flocks.  Laboratory experiments by Krebs (1973) have shown that different species of chickadees are able to learn the location of hidden food from one another.

In the simpler winter environment, birds are more easily located by predators.  Flocking, however, seems to offer individual birds some protection from predation.  The constant motion of individuals in a flock provide multiple targets, confusing the predator. Flocking birds enhance this confusion effect by flying in a tight flock in the presence of the predator.  Each individual of a flock may use other members as cover; peripheral flock members, which are more susceptible to predation, continuously move toward the center, reducing the chance of being caught, in what Hamilton (1971) has termed a "selfish-herd" effect.  Although each flock member spends less time in vigilance, a flock is less apt to be surprised by a predator than is an individual.  Trained goshawks are able to capture single wood pigeons on 80% of attacks, but pigeons in flocks of 11-50 are caught less than 20% of the time.  On the average, the larger flocks take flight when the hawk approaches within 30 m, whereas the flight distance of individual pigeons is less than 5 m.

While there are benefits in locating food and avoiding predation, being a member of a flock also has some associated costs.  As more birds join a flock, competition for food, roosting, and hiding places increases.  Competition is expressed as aggression.  As the temperature decreases, or flock size increases, more time is spent fighting and feeding and vigilance time decreases.  The specific form in which aggression is expressed varies among different species.  The intensity of aggression also varies from overt chasing and pecking, to threat displays, to simple approach and avoidance.  Because birds at a winter feeder are forced together by the highly clumped distribution of the food supply, aggression is likely to be observed in all of its forms and intensities.  In general, threat postures indicating an attack involve facing the rival with a horizontal body posture, erect nape (back of head), raised wings, and fanned tail.  The form of these aggressive postures are shown in Figure 2 a-d and h-k for the great tit, an old world relative of the chickadee.  Postures which indicate a tendency to attack include erect body

and crest, fluffed feathers and open beak (Figure 2 e-i).  Birds other than chickadees show attack/escape postures similar to those in Figure 2.

Figure 2. Various threat displays of the great tit, showing attack tendencies a-d and j-k and escape tendencies e-i.

The result of agonistic interactions (attack or flee behaviors) within a flock is that some individuals come to dominate others.  Over time, most winter flocks develop a hierarchy in which the most dominant individual wins all encounters, the second dominant wins all encounters except against the most dominant, the third dominant wins against all except the first and second, and so on until the most subdominant bird wins no encounters.  In such a hierarchy, dominant individuals in the flock gain first access to food; in severe winters, subdominants are the first to starve.  In white-throated sparrow flocks, where the birds prefer to forage near cover, the dominant individuals feed more often near shelter.  This forces the subdominants to feed where they are exposed to predators.

The most common outcome of a competitive encounter at a feeder is more subtle, and may involve no overt agonistic behavior at all.  Instead what is observed is one bird leaving as another arrives.  Supplanting is considered a form of approach/avoidance (or attack/flee) encounter.  The bird which occupies the feeder is the dominant individual, the one that leaves is the subdominant.  Supplantings at a feeder can occur between members of the same or different species.  Table 3 shows the results of student observations at a platform feeder located outside a classroom.  The largest species, blue jays and purple finches, were infrequently supplanted by other species; chickadees, the smallest species, were supplanted by nearly all other species.  Furthermore, the larger species supplanted on most of their visits, the smaller on few of their visits.

Table 3. Supplanting Attacks at a Winter Feeder in Upstate New York.

Feeding -- Behavioral adaptations to winter also involve feeding strategies.  Wintering birds survive in northern latitudes because they are able to utilize the foods available in winter, largely seeds and fruits.  Seed and fruit eaters that overwinter in temperate regions may be somewhat insectivorous in the summer months, but seeds or fruits are the bulk of their diet for most of the year.  Thus they do not change their diet very much from summer to winter.  In contrast, some overwintering insectivorous species such as chickadees, titmice and nuthatches are able to change their diet in cold weather to take advantage of seed sources at a feeder.  Before bird feeders became popular, the winter diet of black-capped chickadees was largely invertebrates (58%) and to a lesser degree plant materials (21%), such as buds and the berries of poison ivy and sumac.  It is likely that chickadees maintain the protein and fat quality of their diet by feeding on sunflower seeds or peanuts at a feeder.

Since food becomes increasingly scarce as the winter progresses, many wintering bird species have evolved the habit of caching, or hiding food for later use.  Away from artificial food sources, caching occurs largely during the food-rich late summer and early autumn period. Birds which cache include nutcrackers, jays, titmice, nuthatches and chickadees (Figure 3).  You may observe that the chickadee at your feeder flies off with food.  Chickadees may store seeds in several hundred cache sites each day within their permanent winter home range.  Laboratory studies of black-capped chickadees have shown that they are able to remember not only the cache locations, but the kind of food stored in each, which cache site they previously emptied, and which sites they found empty as a result of another animal.

Figure 3. Willow tit, a close relative of chickadees, in several seed caching postures: left - bark crevice at base of main branch, right - under lichen at base of main branch, (Taken from Haftorn 1956).

Roosting -- Night is usually the coldest time of the 24 hour period, and it is the time during which birds do not keep warm by feeding or movement.  As winter approaches, locating the proper microhabitat in which to spend the night becomes critical to survival.  Birds occasionally are found dead in their winter sleeping locations, still clinging tightly to the perch on which they slept.  Roosting is sleeping, often on a perch, by a bird. When roosting a bird usually locates a microhabitat in which the loss of body heat is minimized, and, in some species, grouping together as well. 

Hole-nesting birds such as titmice, woodpeckers, nuthatches, and chickadees tend to roost singly in cavities or nest boxes, although chickadees have been observed roosting under eaves, in clumps of dead oaks, and commonly in the tips of snow-covered spruce branches.  A detailed study by Kendeigh (1961) showed that while the night temperature outside an experimental nest box continues to drop, the temperature within the box, on typical nights, remained at a constant warmer temperature once the bird entered (Figure 4).  Furthermore, the colder the outside temperature the more energy is saved relative to a bird roosting outside. Choice roosting cavities are aggressively defended against other individuals of the same or different species.  Dominance behavior seems to be involved in obtaining and retaining a roost box, with the larger, more dominant birds obtaining the better roosts.  In European titmice the larger birds are males, so it is the females that perish in harsh winters for lack of a suitable roosting site.  For species like great tits, the availability of suitable winter roosting cavities could be a density-dependent factor (one which depends upon the number of individuals) influencing population size.  In boreal forest regions where the snow is soft and fluffy, snow buntings and redpolls are known to roost within the insulating snowpack and the larger grouse actually dive into the snow for insulation. 

Figure 4. Temperature recorded inside and outside of a nestbox occupied by a house sparrow on an average night (Taken from Kendeigh 1961).

Many wintering birds roost socially.  In cities, the chattering of house sparrows as they settle down for the night under eaves or in vines around buildings, is a common evening sound.  Some communal roosters, such as eastern bluebirds, European tree creepers, and some nuthatches, conserve heat by huddling tightly beneath bark or in cavities.  Other communal roosters, such as crows, starlings, robins, finches, and various species of blackbirds, choose sheltered habitats, mainly dense deciduous trees or conifer stands.  Meteorological studies have shown that these sites offer shelter from wind and a slightly slower rate of nightly cooling.  Studies have shown that starling and blackbirds have not evolved special physiological adaptations to survive the cold.  Instead, these species depend upon the behavioral adaptation of roosting in large flock -- sometimes as many as several thousand birds -- in suitable microhabitats.  In starlings the energy saved by such communal roosting in a sheltered spot like pine woods has been estimated to vary between 12-38% compared to single birds in the open.

Morphological Adaptations to Winter.  Birds must maintain a high internal body temperature of around 30-40 C^o even in winter.  The loss of heat from all of the body surface, except bill and legs, is slowed by plumage.  On a per weight basis, feathers are the most effective insulating materials found among animals. 

Some wintering birds may adapt to the cold by insulative acclimation -- growing more feathers.  Larger permanent residents such as ptarmigan, grouse, jays, and owls, and the medium-sized house and tree sparrows, tend to possess thicker plumage in the winter than in summer, a trait not found in migrant birds.  Unfeathered body parts like beak, feet and legs possess few blood vessels, but still lose more heat from their surface than any other body part.  The legs and feet of larger northern residents are somewhat feathered.  In contrast to the larger birds, the smaller finches, sparrows and chickadees, have a much greater surface area relative to their volume.  This means that their heat loss through the body surface is proportionately much greater.  Furthermore, because of their small size they are physically incapable of adding feathers to increase their winter insulation.

As outside temperatures drop, birds fluff their feathers (ptiloerection) to increase the insulative dead air spaces within the plumage (Figure 5).  The degree of ptiloerection increases with decreased environmental temperature.  In addition, as the outside temperature drops, birds withdraw their feet into the plumage and tuck their head under the back feathers.  Since the head is the warmest part of the bird and heat is lost through respiratory evaporation, the head is implicated as the major route of heat loss in birds.  Thus, tucking the head no doubt affords considerable energy savings.  As measured by Hill et al. (1980) in the black-capped chickadee, as the air temperature is lowered from 10^oC to -20^oC the plumage surface temperature drops accordingly, but the skin and core temperature remain constant at around 40 ^oC as a result of ptiloerection.

Figure 5. As the outside temperature decreases, birds first fluff their feathers, then at further temperature decrease, withdraw their feet and tuck their head under the back feathers.

Physiological Adaptations to Winter.  Winter poses a set of severe problems for permanent northern resident birds.  Not only must they cope with low temperatures and high winds, they must be able to survive long northern nights without feeding.  Storms may also disrupt foraging in an environment where food is already scarce. Fasting is a common adaptation.  Small birds such as chickadees, redpolls, and goldfinches are not only relatively less insulated but also are unable to fast for as long a time as are larger residents.  As a result, smaller birds have evolved other physiological ways to cope with winter.  Physiological adaptations to cold by small northern birds may include fattening, hypothermia, shivering, and acclimatization. 

Winter fattening -- The fat stores of small northern birds increase in the winter, and for most vary between 9.5 and 13% of the total body weight.  Among species that range over a wide latitude, northern residents have significantly higher fat reserves than their southern counterparts.  The increased fat reserve may serve as added insulation, resulting in cooler skin temperatures.  This, in turn, results in a lower temperature gradient between the skin and the environment, thus reducing heat loss and yielding an energy savings to the bird. Fat reserves also serve as an important source of energy, enabling the bird to survive fasting through the long northern nights and occasional storms.

Hypothermia -- Birds of the family Paridae (titmice and chickadees) are among the smallest permanent residents in subarctic regions.  Small birds, because of their size, are unable to increase feathered insulation in winter and have small fat reserves.  They consequently suffer greater thermal stress in the cold.  The fat reserves in the 10-14 g black-capped chickadee, the smallest subarctic resident, amounts to 7.5% of its total body weight.  Yet this species winters north into Nova Scotia and Alaska where winter temperatures commonly reach -50^oC and the amount of daylight available for foraging may be reduced to only three or four hours.  According to studies by Chaplin (1974, 1976) black-capped chickadees, like some other northern Paridae enter a nightly hypothermia.  That is, chickadees reduce their nightly metabolic energy loss as they roost by decreasing their body temperature 10-12^oC below their daily temperature of 42^oC.  At an ambient temperature of O^oC, this nightly hypothermia represents an energy savings of about 23% per hour.  As the night temperature continues to drop, chickadees do not enter a deeper hypothermia, but maintain their hypothermic temperature at around 30-32^oC by shivering.  Hypothermia in humans progressively results in shivering, nervous discoordination, unconsciousness, and, if not treated, will lead to death. However, in the small northern Paridae it appears to be a part of a normal suite of adaptations to combat winter cold.  In contrast to chickadees, redpolls do not enter a nightly hypothermia.  Yet redpolls are able to survive colder temperatures than any other small bird. Redpolls feed nearly exclusively on fat-rich birch seeds in winter, and possess the highest winter fat reserves (11.8% of body weight) of any small northern bird.  This fat reserve is used to maintain a high body temperature through cold winter nights.

More importantly, they possess a two-parted esophageal pocket (diverticulum) in which they can store added seeds to tide them through the long northern nights.

Shivering -- In contrast to mammals, birds in winter do not usually adapt to cold by increasing their metabolic rate (nonshivering thermogenesis).  While moving around and feeding, a bird's body is warmed by the action of its muscles.  While sitting or roosting, however, most small birds throughout the winter maintain their high body temperature by continual bursts of shivering (shivering thermogenesis).  As the outside temperature falls below 3^oC, shivering increases.  Shivering is effective enough in the cold to increase the metabolic rate by a factor of five above the basal metabolic rate.

Acclimatization -- Birds in summer and winter differ in their heat-generating and heat-saving abilities, and therefore in their tolerance of cold.  For example, house sparrows are able to withstand a temperature of -25^oC in January, but they would die if the temperature dropped to 0^oC in August (Figure 6).  In laboratory experiments, small northern birds were able to withstand colder temperatures in the winter than the same species could in the summer.  This gradual adjustment in plumage and physiology which allows survival at colder temperatures, is called cold acclimatization.  Cold acclimatization includes changes in insulation, as previously discussed, as well as physiological changes.  Physiological acclimatization in birds may include increased fat storage and a greater ability to mobilize energy stores, as well as lower limits of temperature tolerance.  These changes may be mediated by seasonal changes in thyroid and adrenal hormones.

Figure 6. Lower limit of temperature tolerance for house sparrows (closed circles), and lowest minimum temperature for each month in central Illinois (open circles) (taken from Barnett 1970).

In conclusion, perhaps the most important point that should be kept in mind about adaptations of wintering birds is that each species has evolved its own unique constellation of behavioral, morphological and physical traits enabling winter survival.  Thus the temperate woodland-dwelling chickadees forage within a fixed winter home range, on an energy-poor insect food source in small flocks together with titmice, nuthatches and woodpeckers.  Chickadees have low fat reserves, but survive the cold by caching food, roosting in sheltered spots and entering a nightly hypothermia.  The redpoll, a native of boreal regions, prefers field and scrub habitat where it forages on energy-rich birch and alder seeds in large mixed flocks with goldfinches and siskins.  Redpolls may migrate irruptively and depend upon large fat reserves while roosting singly in sheltered spots, sometimes within the snow, to survive cold.  Finally, consider the larger house sparrow which, unlike the previous species, has become a city dweller.  Like the redpoll, it prefers open areas such as farms and fields where it often forages with other house sparrows in large flocks of up to 100 birds.  To survive cold, the house sparrow depends upon a thick plumage, roosting in sheltered spots in cavities, barns, buildings, where it utilizes food stored in the gut to maintain body temperature overnight.

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