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The black-capped chickadee (Poecile atricapillus) is a small, nonmigratory, North American songbird that lives in deciduous and mixed forests. It is a passerine bird in the tit family Paridae. It is the state bird of both Maine and Massachusetts in the United States, and the provincial bird of New Brunswick in Canada. It is well known for its capacity to lower its body temperature during cold winter nights as well as its good spatial memory to relocate the caches where it stores food, and its boldness near humans (sometimes feeding from the hand). It is almost universally considered 'cute' due to its oversized round head, tiny body, and curiosity about everything, including humans. 
Though often placed in the genus Parus with most other tits, mtDNA cytochrome b sequence data and morphology suggest that separating Poecile more adequately expresses these birds' relationships. The American Ornithologists' Union has treated Poecile as a distinct genus for some time.
The genus name Poecile has often been treated as feminine (giving the species name ending atricapilla); however, this was not specified by the original genus author Johann Jakob Kaup, and under the ICZN, the genus name must therefore be treated by default as masculine, giving the name ending atricapillus.
At one time the black-capped chickadee was considered by some to be conspecific with the willow tit of Eurasia, due to their very similar appearance. This is reflected in an older version of the Peterson Field Guide for the Birds of Britain and Europe, which states "N Am. Black-Capped Chickadee" as an alternate name for the willow tit.
The black-capped chickadee has a black cap and "bib" with white sides to the face. Its underparts are white with rusty brown on the flanks. Its back is gray and the tail is normally slate-gray. This bird has a short dark bill of 8–9.5 mm (0.31–0.37 in), short rounded wings 63.5–67.5 mm (2.50–2.66 in), a tarsus of 16–17 mm (0.63–0.67 in) and a long tail at 58–63 mm (2.3–2.5 in). Total body length is 12–15 cm (4.7–5.9 in), wingspan is 16–21 cm (6.3–8.3 in) and body mass is 9–14 g (0.32–0.49 oz). Sexes look alike, but males are slightly larger and longer than females.
Although range can generally be used to separate them, the black-capped chickadee is very similar in appearance to the Carolina chickadee. The black-capped is larger on average but this cannot be used reliably for identification. The most obvious difference between the two is in the wing feathers. In the black-capped chickadee, the wing feathers have white edges that are larger and more conspicuous than those of the Carolina chickadee. The latter is often mistaken for black-capped chickadees with feather dystrophy which sometimes affects the appearance of the primary feathers making them look slimmer, a phenomenon caused by illnesses such as fatty liver disease in malnourished birds. Overall, the Carolina appears slightly paler colored whereas the flanks of the Black-capped can appear to have a trace of off-yellow or rusty coloration. Also, the black-capped generally has a more "ragged" looking black bib, whereas the bib of the Carolina has a more smooth-edged look. These subtle features are often even more vague in populations around where the black-capped and Carolina overlap in range (possibly the result of hybrids) and the two cannot always be distinguished as two species. The two species were formerly thought to be easily distinguished by call, but they often learn each other's vocalizations where their ranges overlap (their point of overlap is a narrow band that runs along the east-central United States, with the black-capped chickadee to the north). A bird located near the zone of overlap that sings both songs, or sings "odd-sounding" songs, cannot be positively identified solely by voice in the field.
The black-capped chickadee is found from coast to coast, from the northern half of the United States in the south, to James Bay, the southern edge of the Northwest Territories and the Yukon, and the southern half of Alaska in the north. In winter it may wander outside this range, both to the north and south.
Its preferred habitat is deciduous woods or mixed (deciduous/coniferous) woods. It is also found in open woods, parks, and suburban areas. Habitat segregation is the principal factor that separates the black-capped chickadee from both the boreal chickadee in the north and the chestnut-backed chickadee in the Pacific northwest (these two species prefer strictly coniferous forests). Altitude also separates the black-capped chickadee from the (higher) mountain chickadee in the western mountains and the (lower) Carolina chickadee in the Great Smokey Mountains.
Insects (especially caterpillars) form a large part of their diet in summer. The birds hop along tree branches searching for food, sometimes hanging upside down or hovering; they may make short flights to catch insects in the air. Seeds and berries become more important in winter, though insect eggs and pupae remain on the menu. Black oil sunflower seeds are readily taken from bird feeders. The birds take a seed in their bill and commonly fly from the feeder to a tree, where they proceed to hammer the seed on a branch to open it.
Like many other species in the Paridae family, black-capped chickadees commonly cache food, mostly seeds but sometimes insects also. Items are stored singly in various sites such as bark, dead leaves, clusters of conifer needles, or knotholes. Memory for the location of caches can last up to 28 days. Within the first 24 hours, the birds can even remember the relative quality of the stored items.
At bird feeders, black-capped chickadees tolerate human approach to a much greater degree than do other species. In fact, during the winter, many individuals accustomed to human habitation will readily accept seed from a person's hand.
On cold winter nights, these birds reduce their body temperature by as much as 10-12 °C (18-22 °F) (from their normal temperature of about 42 °C (108 °F)) to conserve energy. Such a capacity for torpor is rare in birds (or at least, rarely studied). Other bird species capable of torpor include the common swift Apus apus, the common poor-will Phalaenoptilus nuttallii, the lesser nighthawk Chordeiles acutipennis, and various species of hummingbirds.
These birds are permanent residents, but sometimes they move south within their range, and even outside of it, in the fall or winter.
During the fall migration and in winter, chickadees often flock together. Many other species of birds – including titmice, nuthatches, and warblers – can often be found foraging in these flocks. Mixed flocks stay together because the chickadees call out whenever they find a good source of food. This calling-out forms cohesion for the group, allowing the other birds to find food more efficiently. When flocking, black-capped chickadees soon establish a rigid social hierarchy. In such hierarchies, males usually rank over females, and older birds over juveniles.
Black-capped chickadees sleep in thick vegetation or in cavities, usually singly, though there have been suggestions that they may occasionally roost clumped together. The sleeping posture is with the bill tucked under the scapular (shoulder) feathers.
This bird scratches its head with its foot over the wing. It can bathe in water, dew, or snow; young chickadees have been observed dust-bathing.
Their flight is slightly undulating with rapid wing beats. Flight speed is about 20 km/h (12 mph).
The vocalizations of the black-capped chickadee are highly complex. Thirteen distinct types of vocalizations have been classified, many of which are complex and can communicate different types of information. Chickadees' complex vocalizations are likely an evolutionary adaptation to their habitat: they live and feed in dense vegetation, and even when the flock is close together, individual birds tend to be out of each other's visual range.
The song of the black-capped is a simple, clear whistle of two notes, identical in rhythm, the first roughly a whole-step above the second. This is distinguished from the Carolina chickadee's four-note call fee-bee fee-bay; the lower notes are nearly identical but the higher fee notes are omitted, making the Black-capped song like bee bay.
Some 'gargles', then a minute of singing.
NOTE: American Robin singing in background.
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The males sing the song only in relative isolation from other chickadees (including their mates). In late summer, some young birds will sing only a single note. Both sexes sometimes make a faint version of the song, and this appears to be used when feeding young.
The most familiar call is the chick-a-dee-dee-dee which gave this bird its name. This simple-sounding call is astonishingly complex. It has been observed to consist of up to four distinct units which can be arranged in different patterns to communicate information about threats from predators and coordination of group movement. Recent study of the call shows that the number of dees indicates the level of threat from nearby predators. In an analysis of over 5,000 alarm calls from chickadees, it was found that alarm calls triggered by small, dangerous raptors had a shorter interval between chick and dee and tended to have extra dees, usually averaging four instead of two. In one case, a warning call about a pygmy owl – a prime threat to chickadees – contained 23 dees. The Carolina chickadee makes a similar call which is faster and higher-pitched.
There are a number of other calls and sounds that these chickadees make, such as a gargle noise usually used by males to indicate a threat of attacking another male, often when feeding. This call is also used in sexual contexts. This noise is among the most complex of the calls, containing 2 to 9 of 14 distinct notes in one population that was studied.
Recent studies have shown that chickadees in an environment with ambient noise at the same frequencies as their songs have developed an evolutionary adaptation which enables them to adjust the frequency of their songs much quicker in order to effectively communicate with the surrounding population. 
The black-capped chickadee nests in a hole in a tree, 1–7 m (3.3–23.0 ft) above ground. The pair either excavate the hole together, or use a natural cavity, or sometimes an old woodpecker nest. This species will also nest in a nesting box. The nesting season is from late April through June. The nest is built by the female only. It consists of a base of coarse material such as moss or bark strips, and lining of finer material such as mammal hair. Eggs are white with fine dots of reddish brown concentrated at the larger end. On average, eggs are 1.52 cm × 1.22 cm (0.60 in × 0.48 in). Clutch size is 6–8 eggs. Incubation lasts 11–14 days and is by the female only, who is fed by the male. If there is an unusual disturbance at the nest entrance, the incubating female may utter an explosive hiss, like that of a snake, a probable adaptation to discourage nest predators.
Hatchlings are altricial, naked with their eyes closed. Nestlings are fed by both sexes but are brooded by the female only (at which time the male brings food to her, which she passes on to the young). Young leave the nest 12–16 days post-hatching, in great part because the parents start presenting food only outside the nest hole. The young will still be fed by the parents for several weeks but are capable of catching food on their own within a week after leaving the nest.
Black-capped chickadees usually breed only once a year, but second broods are possible if the first one is lost. First breeding is at one year of age. Maximum recorded lifespan is twelve years, but most individuals live only half that long.
Black-capped chickadees are socially monogamous, and males contribute greatly to reproduction. During the laying and incubation periods, males feed their partners extensively. When the nestlings hatch, males are the primary providers. However, as the nestlings grow, females become the main caretakers. Females prefer dominant males, and greater reproductive success is closely related to the higher ranking of the male.
Black-capped chickadees may interbreed with Carolina chickadees or mountain chickadees where their ranges overlap. It appears to be more rare, but interbreeding with boreal chickadees has also been documented.
During the winter, Poecile atricapillus form flocks through which dominance hierarchies can be easily observed. Dominance hierarchies play an important role in determining the social behaviors among the birds in these flocks. Positive correlates to higher social rankings include territory size, body condition, singing rate, and reproductive success. The hierarchies are linear and stable; once a relationship is established between two birds, it stays the same for many years. In general, older and more experienced birds are dominant over younger individuals, and males are dominant over females. Dominant and subordinate members differ in their foraging strategies and risk taking behaviors. Dominant individuals control access to preferred resources and restrict subordinates to foraging in novel, riskier or suboptimal environments. Subordinate individuals are often observed foraging in the outermost tree parts that are more prone to predators, while dominant individuals forage low and close to the tree trunk. In experiments, subordinate individuals display less neophobic when approaching novel foods and objects, compared to their dominant counterparts. Subordinate individuals are also more likely to enter novel environment than their dominant counterparts. This is similar to subordinate primates who feed on novel food more readily than the dominant individuals because they are more used to eating suboptimal and unfamiliar food. There is no difference observed in ability to learn novel foraging tasks between dominant and subordinate individuals.
In the states of Alaska and Washington, and in parts of western Canada, black-capped chickadees are among a number of bird species affected by an unknown agent that is causing beak deformities which may cause stress for affected species by inhibiting feeding ability, mating, and grooming. black-capped chickadees were the first affected bird species, with reports of the deformity beginning in Alaska in the late 1990s, but more recently the deformity has been observed in close to 30 bird species in the affected areas.
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