Black-capped chickadee

Black-capped chickadee, Iona Beach Regional Park

The vocalizations of the black-capped chickadee are highly complex.[15] 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.

One of the most recognizable sounds produced, particularly by the males, is the two-note fee-bee song. It is a simple, clear whistle of two notes, identical in rhythm, the first roughly a whole-step above the second.[16] The range of frequencies at which this song starts from varies; the complete frequency range spans roughly 1 kHz. Within this range, male chickadees can sing at various tones. The average starting frequency is around 4000 Hz. A decrease of roughly 200 Hz occurs when the first note (fee) is sung, and then another decrease around 400 Hz takes place between the end of fee and the beginning of bee. In spite of these multiple changes in frequency, though, anybody listening to the song only hears a pure, high-frequency tone.[17] 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. The males sing the song only in relative isolation from other chickadees (including their mates). In late summer, some young birds sing only a single note.[18]

A recent study found that female fee-bee songs have both similarities with and differences from male fee-bee songs.[19][20] Both sexes sometimes make a faint version of the song, and this appears to be used when feeding young.[18] When males are out in the wild, they sing this song to defend their territory or attract a mate.[19] There have also been a few accounts of females singing a version of the fee-bee song when out alone in the wild.[20] The black-capped chickadee is a monomorphic species, so distinguishing males and females based solely on their singing is difficult. A bioacoustic analysis performed on both male and female songs revealed that male fee-bee singing fluctuates more, and the absolute amplitude of both sexes is the same. The fee glissando varies far more in females, which makes identifying each sex easier. The purpose of the female fee-bee song is unknown.[20]

The most familiar call is the chick-a-dee-dee-dee, which gave this bird its name. This simple-sounding call is astonishingly complex.[18] Scientists have been studying it since the mid-1970s. It is produced by both males and females year-round.[17] 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: A, B, C, and D. These four notes only ever appear in this consecutive order[21][18] with each preceding note blending into the next.[17] Not all four notes may appear in the call, though.[21] Like other sounds the chickadee produces, it may be heard in multiple variations. The A and B notes are almost identical to one another in both frequency and duration. The black-capped chickadee possesses the ability to quickly notice the difference between these two notes. As for the C and D notes, no real similarity is seen between them.[17] The C note fluctuates from low to high then back to low, whereas the D note has a constant frequency. While not confirmed, one study found evidence of a function behind the C and D notes. To be able to recognize the difference between a member of the same species and a potential predator, the D note is required. The C note is needed to locate food.[21]

Neither individual notes nor groups of notes have an equal probability of appearing in the chick-a-dee-dee song. Its syntax form may take on several different structures, but the two most commonly heard are [A][D] and [B][C][D]. (The brackets are placed around each note to show that it may be repeated more than once.) Any calls that contain the D note more frequently than others are more commonly heard.[17] A recent study of the call showed that the number of dees indicates the level of threat from nearby predators. In an analysis of over 5,000 alarm calls from chickadees, alarm calls triggered by small, dangerous raptors had a shorter interval between chick and dee and tended to have extra dees, typically 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.[18]

These chickadees make a number of other calls and sounds, 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.[19] Black-capped chickadees develop the gargle noise as a result of learning that starts soon after birth and continues through to adulthood.[17] This noise is among the most complex of the calls, containing two to 9 of 14 distinct notes in one population that was studied.[19] It only lasts for about half a second.

Social learning, in particular, is largely influential to the development of this sound. Beginning 30 to 35 days after birth, strings of low-amplitude precursor or sub-gargles are produced for about a minute. At this time, the young have several close interactions with their family; they learn to produce such sounds by listening to their parents and siblings. Three chickadee populations were observed at three different sites over 8 years, and all of them produced vocalizations that were very similar to one another. Strings of juvenile sub-gargles are almost perfectly continuous and both low and unstable in frequency, yet lacking multiple syllables. When their vocal abilities are fully developed, a stable frequency is produced and a variety of syllables is heard that vary in length.[17]

Chickadees in an environment with ambient noise at the same frequencies as their songs have developed an evolutionary adaptation that enables them to adjust the frequency of their songs much quicker to effectively communicate with the surrounding population. The fee-bee songs of several male black-capped chickadees were monitored to identify their particular frequency. In one particular study, a series of both masking and nonmasking tones was played to multiple male chickadees at various locations to observe how they responded to interfering and noninterfering signals. When interacting with conspecifics close by, the males matched their frequencies, but when the surrounding environment was noisy with other species, the males adapted by increasing the frequency of their songs. The males responded quickly to the masking tones by raising their song frequencies. Another study, though, showed that male chickadees sometimes intentionally match the tones of competing conspecifics as a way of showing aggression. Dominant males in a population often compete with lower-ranked males; one is not at a greater or lesser advantage than the other. Singing contests are a way male chickadees decide who in a population gets to mate. When a male loses a contest, particularly a higher-ranking male in the population, he will often have difficulty finding a mate.[19]

Chickadees will take seeds from feeders and trays over to a tree branch to hammer them open

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 beak 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 family Paridae, black-capped chickadees commonly cache food, mostly seeds, but sometimes insects, also.[22] Items are stored singly in various sites such as bark, dead leaves, clusters of conifer needles, or knothole. Memory for the location of caches can last up to 28 days.[23] Within the first 24 hours, the birds can even remember the relative quality of the stored items.[24]

Foraging behaviour in the winter tends to decrease due to the changing weather. Such behaviour is largely influenced by wind and temperature. When wind speeds are higher, black-capped chickadees avoid exposure to such conditions by flying lower where vegetation offers a degree of protection, and when the temperature decreases, they search for food less frequently.[25]

Chickadee at feeder

In parts of the black-capped chickadee's range with very cold winters, such as Minnesota, survival rates are affected by access to supplemental food. Chickadees with access to bird feeders are twice as likely to survive the winter than those without access to this supplemental food. This difference in survival rates occurs primarily during months with severe weather when the temperature drops below −18 °C (0 °F) for more than five days.[26] In Pennsylvania, with milder winters on the southern edge of their range, differences between populations with and without feeders suggest that feeders influence movements of chickadees rather than actual survival.[27]

At bird feeders, black-capped chickadees tolerate human approach to a much greater degree than other species do. In fact, during the winter, many individuals accustomed to human habitation readily accept seed from a person's hand.

Chickadee feeding from hand

On cold winter nights, these birds can reduce their body temperature by as much as 12 °C (from their normal temperature of about 42 °C) to conserve energy.[28][29] Such a capacity for torpor is not very common in birds. 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 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.[30]

Black-capped chickadees sleep in thick vegetation or in cavities, usually singly, though they may occasionally roost clumped together.[31] Their sleeping posture is with the beak tucked under the scapular (shoulder) feathers.

Their flight is slightly undulating with rapid wing beats. Flight speed is about 20 km/h (12 mph).[32]

Chickadees molt once a year; no prenuptial molt occurs in the spring. The postjuvenal molt at the end of the first summer of life is partial, involving only the body feathers and wing coverts. Thereafter, the postnuptial molts at the end of each reproductive season are always complete, involving all feathers.

A black-capped chickadee excavating a nest cavity in a dead tree

The black-capped chickadee nests in a hole in a tree, 1–7 m (3.3–23.0 ft) above ground. The pair either excavates the hole together, or uses 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 six to eight eggs. Incubation lasts 11–14 days and is by the female only, which is fed by the male. If an unusual disturbance occurs at the nest entrance, the incubating female may utter an explosive hiss, like that of a snake, a probable adaptation to discourage nest predators.[33]

Hatchlings are altricial, naked with their eyes closed. Nestlings are fed by both sexes, but are brooded by the female only (when the male brings food to her, which she passes on to the young). Young leave the nest 12–16 days after hatching, in great part because the parents start presenting food only outside the nest hole. The young are still 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 12 years, but most individuals live only half that long.[34]

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, but 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.[35]

Black-capped chickadees may interbreed with Carolina chickadees or mountain chickadees where their ranges overlap. Interbreeding with boreal chickadees has also been documented, though it is more rare.[36]

During the winter, the species forms 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.[37] 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 ones, and males are dominant over females.[35] 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 neophobia when approaching novel foods and objects, compared to their dominant counterparts. Subordinate individuals are also more likely to enter novel environments than their dominant counterparts. This is similar to subordinate primates, which feed on novel food more readily than the dominant individuals because they are more used to eating suboptimal and unfamiliar food. No difference is observed in ability to learn novel foraging tasks between dominant and subordinate individuals.[37]