SPECIES ACCOUNT. Black-capped chickadee / Mésange à tête noire (Poecile atricapillus)
Department of Biology, Queen’s University, Kingston, ON Canada K7L 3N6
Taxonomy: Class Aves. Order Passeriformes. Family Paridae. Genus Poecile. Species Poecile atricapillus. While traditionally placed in the genus Parus with other tit species, mtDNA data suggested they be included in a separate genus (Gill et al. 2005) which is now treated as distinct by the American Ornithologists Union (AOU). Originally named Parus atricapillus by Linnaeus in 1766, the translation of the name means “titmouse with a black crown” (Smith 1991). The genus Poecile is from the Greek word poikilos, which means “pied” (Holloway 2003).
Description: Black-capped chickadees (hereafter referred to as “chickadees”) are small songbirds, measuring between 120-150mm long and weighing 9-14g (males are slightly larger, although there is overlap between older females and younger males; Smith 1991). The adult plumage includes the namesake black cap, a triangular throat patch (or bib), and white sides of the face. The back and tail are grey, while the flanks are rusty in colour and the breast and underside are white (Figure 1).
To the human eye, there is no apparent sexual plumage dimorphism in this species, and thus sexing can be quite difficult outside of the breeding season (only males sing, and this can be used to identify them). During the winter sex can be approximated with reasonable accuracy using a multivariate statistical method developed by Desrochers (1990). This function is based on measurements of a bird’s weight, wing length and tail length which, when put into the equation, generate a score for the individual (Figure 2). As males are generally larger for all three measurements, the result is a larger score, while females generally have a smaller score. Males in the Queen’s University Biological Station (QUBS) population generally have scores of >-1.5, while females are generally <-1.5 (personal observation).
Note, however, that the species in not completely monomorphic in plumage, as the sexes differ in the spectral characteristics (although this is invisible to the human eye); males have brighter white patches, larger black patches, and more contrast between adjacent white/black patches than females. There is also significant variation within sexes, as dominant males have darker black caps, bibs with less reflectance, and mantles with more reflectance than males of lower social rank (Mennill et al. 2003).
Distribution: Chickadees are year-round residents across their range, which encompasses most of Canada, and extends from Alaska to California through to Newfoundland and the northeastern USA (Smith 1991). In Ontario (Figure 3), the species occurs in all but the northernmost portions of the province. The chickadee is the state bird of both Maine and Massachusetts, and the provincial bird of New Brunswick.
Across their range, black-capped chickadees co-occur with several other chickadee species. The Carolina chickadee (P. carolinensis) inhabits much of the south-eastern USA and is morphologically very similar to the black-capped chickadee (their plumage appears almost identical). While not each other’s closest genetic relative (Gill et al. 2005), the two species hybridize regularly across a narrow contact zone that runs from Texas to New Jersey (Curry 2005). Black-capped chickadees may also occasionally hybridize with mountain chickadees (P. gambeli) in the mountains of western North America (Hill and Lein 1988). Boreal chickadees (P. hudsonica) occur across most of Canada, but are restricted to the northern belt of coniferous forest. Although their breeding range overlaps with that of the black-capped chickadee, no reported cases of hybridization exist. Extremely rare inter-genus hybridizations have been reported between black-capped chickadees and tufted titmice (Parus bicolor), a species which occurs in south-eastern Canada and much of the eastern USA (reviewed in Cockrum 1952).
Chickadees do not migrate in the usual definition of the word (yearly long-distance movements), but some northern populations do occasionally display what are known as “irruptive” movements or migrations. These generally include only young birds (hatched in the previous summer; e.g. Hussell and Stamp 1965), but can include adults during years of major movements (e.g. Yunick 1981). Irruptions are thought to be triggered by low food levels, particularly reduced seed crops in boreal forests (Bock and Lepthien 1976). In some years and locations, irruptions can be huge, with flocks reaching over 30,000 birds (Bagg 1969). Generally irruptions are characterized by a southward movement in the fall, with only some birds returning northward in the spring (Brooks 1987). Return movements appear to be somewhat aimless (Hussell and Stamp 1965), and many individuals return too late to breed that year (Smith 1991).
Since black-capped chickadees have an extensive range, are non-migratory, and occupy a number of different habitats, they tend to vary in size and plumage traits in different locations. As such, there are several different subspecies, although the total number recognized in North America varies: the 1957 AOU checklist (which was the last to list subspecies) recognized nine, while others recognize fewer (Smith 1991). Across Canada there are five consistently recognized subspecies. The eastern black-capped chickadee (P. a. atricapillus) occurs from the Great Lakes east to the Atlantic, and from James Bay south to Missouri and New Jersey, and is the subspecies which occurs at QUBS. It is somewhat smaller in terms of wing and tail length and has lighter plumage than most other subspecies (Smith 1991).
Habitat: Chickadees are an edge species typical of deciduous or mixed forests, but can be found in a wide variety of habitats, including wetlands, parks, willow thickets, mature forests, and disturbed areas (Smith 1997). The species readily visits feeders during winter months, making them a favourite of backyard birders. The QUBS point property and surrounding areas (running from the lodge to the Opinicon Road) currently encompasses the ranges of several winter flocks, comprised of well over 200 individuals (pers. obs.).
Ecology: The diet of chickadees varies throughout the year, with animal matter making up the majority (80-90%) of their diet in the warmer months. In winter they become increasingly dependent on plant matter, with animal matter declining in important to about 50% of their diet (Smith 1991). The two most common sources of animal matter in the chickadee diet are butterflies and moths (especially caterpillars), and spiders (Robinson and Holmes 1982); however they will also feed on a host of other invertebrates, including slugs, beetles, cicadas, and true bugs (Smith 1991). Interestingly, chickadees will also feed on fat of dead mammals and fish, pecking through the skin to reach the fat stores beneath (Southern 1966; Smith 1991). Plant matter consumed is generally seeds and berries, with small wax-covered berries being a favourite, such as those of poison ivy (Toxicodendron radicans; Baird 1980). Chickadees that occur within the range of the sugar maple (Acer saccharum) often develop a taste for sap that leaks from broken branches of the trees during the early spring (Smith 1991).
The majority of the nonbreeding season is spent in mixed-sex flocks, generally consisting of 2 to 12 individuals that travel and forage together throughout the winter months. In most areas, the initial flock composition has a sex ratio of 1:1 (although subsequent winter mortality can change this; Glase 1973; Smith 1987; Desrochers et al. 1988), with the members of the flock forming mated-pairs during the breeding season (Ficken et al. 1981). Winter flocks are characterized by stable linear dominance hierarchies: in each flock one individual is the top (or alpha) bird, with descending ranks for the each of the other flock members of both sexes (Hartzler 1970; Glase 1973; Ficken et al. 1981). Winter social rank is associated with resource-holding potential (Ficken et al. 1990; Otter et al. 1996), and generally males are dominant to females (Smith 1991), while older birds are dominant to younger birds (Smith 1991; Otter et al. 1999). Social rank can affect multiple areas of a bird’s life history, as high-ranking birds enjoy a number of benefits, including access to more profitable/safer feeding sites (Desrochers 1989), larger territories (Mennill et al. 2004), more mating opportunities (Smith 1991; Otter and Ratcliffe 1996), and higher fledgling success (Otter et al. 1999). Females also prefer high-ranking males as extra-pair partners, and will divorce their social mates if a higher-ranked male becomes available (Otter and Ratcliffe, 1996; Ramsay et al. 2000).
Winter flocks often contain several different species of winter-resident birds, which travel and forage with chickadees for hours, days, or longer (Morse 1970). Mixed-flock species include nuthatches, woodpeckers, warblers and brown creepers (Brewer 1961). The finding of food, foraging efficiency, and protection against predators seem to be the driving forces behind such flocking (Morse 1977). For example, areas that are supplemented with food have significantly fewer mixed-species flocks than unsupplemented areas (Berner and Grubb 1985; Smith 1991), and the anti-predator advantages of flocking are well documented (e.g. Powell 1974). Severe weather may also result in the formation of mixed-species flocks (Klein 1988).
During the winter months chickadees must maintain a high metabolism (due to their small size) in the face of subzero temperatures. At night chickadees solve this problem by decreasing their body temperature by 10-12oC from their daily temperature of 43oC, thereby entering a regulated hypothermia and reducing their metabolic expenditure by >25%. Regulated hypothermia allows a chickadee’s fat reserves to last through the night (Chaplin 1974), and although they are diminished by morning, their reserves can essentially be doubled by mid-afternoon through foraging (Chaplin 1974). Chickadees also generate considerable heat by shivering their pectoral muscles (Chaplin 1976). In addition to these physiological adaptations, chickadees will cache food (mainly seeds) across their winter territory (Sherry 1989; Smith 1991). This can involve hundreds or even thousands of food items being stored per day, whose locations the individuals can correctly recall at least 28 days later (Hitchcock and Sherry 1990). These food stores most likely play an important role in winter survival, as more northerly populations (who experience harsher winter conditions) store more food and are more efficient at cache recovery (Pravosudov and Clayton 2002).
The lifespan of chickadees is generally short, as long-term studies indicate that winter mortality can be high for juvenile birds (26.0% in females, 28.9% in males). Average lifespans have been estimated at less than two years for both sexes (Smith 1994), and only 25% of males and 20% of females survive to a third winter at QUBS (Schubert 2003). A few examples of extreme longevity for birds of their size have been reported; for example, one banded chickadee of unknown sex in New Hampshire was reported to have been living at an age of at least 12 years and five months (Kennard 1975; Clapp et al. 1983).
Most natural predation on adult birds is by avian predators, such as owls, hawks, and shrikes (Smith 1991). Among hawks, the sharp-shinned hawk (Accipiter striatus) is probably the most prominent (Smith 1991; Reynolds and Meslow 1984), while the northern shrike (Lanius excubitor) was probably as prolific a chickadee predator (especially in winter) before their numbers declined (Cade 1962; Ficken and Witkin 1977). As chickadees are cavity nesters, mortality at the nest can be caused by other sources, one of the most common being the house wren (Troglodytes aedon). House wrens will enter chickadee nests and destroy the eggs and remove the nesting material, as well as attack nestlings (Belles-Isles and Picman 1987). In Kluyver’s 1961 study, house wrens were the greatest source of brood mortality in the study area by far, destroying 20% of broods. At QUBS, black rat snakes (Elaphe obsoleta) are a large source of nest predation, taking eggs, nestlings and even incubating parents. Mammals such as squirrels, chipmunks, weasels and raccoons may also be a source of nest predation (pers. obs.). When predators are encountered, chickadees will employ an intra- and interspecific alarm call: the chick-a-dee (CAD) call (actually a complex of calls) which gives them their name. This vocalization serves not only in the attraction of other chickadees, but also a host of other bird species, which will then mob the predator (Hurd 1996). This alarm call is also coded with information: the number of specific notes and the intensity of the call gives information about the size and threat-level of the predator, making it one of the most sophisticated animal signalling systems yet discovered (Templeton et al. 2005).
The CAD call is also used outside of a mob setting by both sexes throughout the year, and has an alternative function of coordinating flock movements (Ficken et al. 1978). Not only are CAD calls distinct among individuals (suggesting a possible source of individual recognition; Mammen and Nowicki 1981), but they also tend to show similar attributes within a flock, suggesting a role in flock identity (Mammen and Nowicki 1981; Nowicki 1983). The call is structured such that it can theoretically generate an open-ended number of unique call types (Hailman and Ficken 1986), and can vary depending on who is calling and the context (energetic state, flight behaviour, feeding behaviour, sex, type of predator; Lucas and Freeberg 2007).
Altogether, chickadees have a complex vocal repertoire with some 15 vocalizations, spanning a number of functions and contexts. Some are sex-specific, while others are age-specific, and their uses are varied, ranging from contact-calls, begging calls, and agonistic encounters (for a comprehensive list see Ficken et al. 1978 and Smith 1991). One of the best-studied aspects of chickadee behavioural ecology is their song, which is a tonal, two-note fee-bee. Male chickadees do not posses song repertoires like many other birds, instead relying on the single songtype and creating a ‘virtual’ repertoire by shifting their songs across a range of 860Hz. Male chickadees shift their song frequencies to match the frequency of rivals’ songs during song contests (Horn et al. 1992). Females, as a general rule, do not sing, but can produce the song on occasion. Male singing behaviour starts in late December with a sharp increase in March, reaching its peak in April and May (Dixon and Stefanski 1970) which corresponds to the breeding season and an increase in testis volume (Phillmore et al. 2006). Daily singing activity during the breeding season reaches its peak at dawn, in what is known as the “dawn chorus”. Singing throughout the rest of the year is rare and sporadic (Dixon and Stefanski 1970).
The mating system of chickadees is one of strong social monogamy, with pairs normally forming in the fall as part of winter flock formation (one reason why there is an initial 1:1 sex ratio within flocks). Some pair formation may happen later due to winter mortality (Smith 1984). Timing of flock breakup generally varies with location and weather; pairs may disengage from the flock with warm temperatures at the start of spring, only to rejoin if there is a particularly cold period. Flock breakup is characterized by an increased amount of inter-pair aggression as the birds start acquiring territories. Breeding territories are generally formed within the area that the winter flock inhabited and are established around five to seven weeks prior to the first eggs being laid (Smith 1991). During a five year study at QUBS, territory sizes were an average of 1.8ha in size, although they tended to be significantly smaller in years with high population densities. In all years, higher-ranked males held larger territories (Mennill et al. 2004).
In the early spring, males will begin feeding the females while the pair forages, the rate of which increases during egg laying and incubation (Orr and Verbeek 1984). The female will produce a begging call (known as a broken dee), and the male will bring her the largest food items he finds (Smith 1991). Copulation generally takes place prior to- and during egg laying (Dixon et al. 1970); both the male and female will begin wing-quivering and producing variable-see vocalizations, after which the male will mount the female, with foraging resuming almost immediately afterwards. Females paired to low-ranking males will actively solicit extra-pair copulations with neighboring males of higher rank. From 1997-2001 33.3% of broods surveyed at QUBS contained extra-pair young (Mennill et al. 2004).
Chickadees at QUBS are primary cavity nesters, meaning they nest in cavities they construct themselves. Prior to- and at the beginning of the breeding season, both the male and the female will excavate several cavities within their territory, eventually choosing one to construct the nest in. These cavities are most commonly in rotten softwood trees (birch, Betula spp., being a favourite; Ramsay et al. 1999), although they will occasionally nest in fence posts, nest boxes, and specially-constructed nest tubes. Old cavities are seldom reused. Most cavity entrances are placed in the side of the tree, although a minority enter straight down from the top. The heights of cavity entrances can range from ground-level to over 20m (Smith 1991), but most occur at around 5m (Ramsay et al. 1999). Nests are lined by the female, using plant and animal substances, including fine strips of bark, pine needles, moss, feathers, and fur/hair.
Clutch sizes can vary from 1 to 13 eggs, with the average number of eggs around 7 (Smith 1991). The eggs are white with fine dots of reddish brown, and are rounded-ovate in shape (Figure 4). The female incubates the eggs exclusively, with incubation generally starting the day before the last egg is laid. Males will often bring food to the female in the nest while she incubates, but she will also leave for short periods of time (usually seven to eight minutes after incubating for 20-25 minutes) to feed herself (Brewer 1961). Once hatched, both parents will feed the young, although the female continues to brood the chicks leaving the male to do the majority of the feeding (up to 2-3 times more than the female until day 12 or 13, when the chicks can thermoregulate themselves; Brewer 1961).
Nestlings are essentially naked and weigh about one gram at hatching, with their first contour (vaned) feathers coming in on their fourth day. Nestling eyes begin to open by day six or seven, but are not fully open until day 12 (Figure 5). It is also by day 12 that their contour feathers have burst through their casings, giving them the capability of true thermoregulation (Smith 1991). By day 15 the nestlings look much like adults (except for shorter tails and yellow corners of their mouths). Nestlings steadily increase in weight from hatching until about day 12, when they reach a weight of about 10-11g (Kluyver 1961). Fledging takes place generally on day 16, with both parents continuing to feed the young. Within a week, most young chickadees can catch food on their own, although they are still fed by their parents (Brewer 1961). Adults will become increasingly aggressive towards their young for a period of about two weeks after fledging, which is thought to encourage independent foraging (Leonard et al. 1991). Territorial boundaries are generally not enforced after fledging, and family groups may wander freely across several territories (Smith 1991). The family group will remain together for two to four weeks, after which the juvenile birds will disperse up to 11km from their natal territories to insert themselves into new winter flocks (Weise and Meyer 1979).
If a pair happen to lose a brood due to predation or disease (and if it is not too late in the season), they will often start a replacement nest within a week, some distance away from the original nest. Second broods (after successfully fledging a first) are rarely observed (Kluyver 1961; Smith 1967). Several factors may affect the likelihood of a second brood, including date of first laying, the age of the female (older females may begin nesting earlier; Glase 1973), latitude (more northerly populations may have a lower propensity for second broods due to the shorter seasons; Rowley and Russell 1993) and the size of the original brood (the larger the original brood, the less likely they may be to start a second; Tinbergen 1987). Second broods may be rare because they are started almost as soon as the original brood has fledged, causing the parents to decrease care to the fledglings, and lowering their chances of survival. Another reason for their rarity may be that raising a second brood potentially decreases their chances of surviving the winter and breeding again the following year (Smith 1991). In July or early August black-capped chickadees enter into a prebasic moult, which may last two to three months, in which every feather is replaced (Smith 1991). Moulting may be another reason why second broods are rare, as parents have to simultaneously provision for their young and replace their feathers late in the breeding season (e.g. Dhondt 1973, 1981).
Divorce in chickadees is uncommon (unless a higher ranking male becomes available, as mentioned above), and pairs will generally continue to breed together each year until the death of one of the individuals (Smith 1991; Otter and Ratcliffe 1996; Ramsay et al. 2000).
Since its beginning in the winter of 1988, chickadee research at QUBS has enjoyed a long and rich history, with the production of many interesting and influential studies. The first large-scale studies at the station on chickadee behavioral ecology using marked birds were started in 1992, and subsequent studies have included the subjects of nest placement strategies (Ramsay et al. 1999), female eavesdropping on male song contests (Mennill et al. 2002), honest advertisement during the dawn chorus (e.g. Otter et al. 1996), dawn chorus function (e.g. Foote et al. 2008), and the importance of social rank on reproductive success (e.g. Otter et al. 1998), among many others. Currently, work at QUBS is being carried out by Cory Toth (Queen’s Univ.) examining the ability of chickadees to display transitive inference, and David Wilson (Univ. of Windsor), examining the relationship between acoustic signaling and reproduction in the species.
Research Needs: While being one of the most-studied songbirds in North America, there remain outstanding research questions about chickadees. One area in particular relates to social rank, specifically how social rank is mediated and how this translates to variability in signalling behaviour towards competitors and potential mates. Another interesting question involves the acquisition of rank by juvenile males; most males enter their new flocks at the lowest ranks, however some are able to attain high rank in their first year. The circumstances that allow some males to rise in rank so quickly remain a mystery.
The general process of flock formation after the breeding season itself is a mystery, and applies not only to juveniles inserting themselves into flocks, but also the formation of new flocks by adults. The formation of completely new flocks by adult birds after several years of association with birds that were still alive has been observed (Scott Ramsay, personal communication). The reasons for such moves are unknown, although they may be rooted in dominance relationships and the possibility of achieving a high rank by the formation of a new flock.
Whether female black-capped chickadees modify their foraging behaviour during the breeding season is unknown, as they have to procure enough calcium in their diet for egg production. Female boreal chickadees (P. hudsonicus) have been reported eating ash high in calcium (Ficken 1989), but these observations were made well after the breeding season. Thus, we do not yet know how female black-capped chickadees receive the necessary nutrients during this time of year.
While there is a large amount of work that has been done on the acoustic signals of this species, relatively little is known about any visual displays used. A possible reason for this may be that such displays are most likely very quick and subtle, and thus difficult to observe (Smith 1996).
Winter flock dynamics and behaviours are also areas which could yield interesting and pertinent research questions. For example, if a male experiences winter mortality, what are the effects on that bird’s mate, who then loses the protection of the male? Smith (1991) also posed an interesting question: if all the adults are removed from a winter flock, how does it affect the survivorship of the remaining juveniles?
For expanded lists of possible research questions see Smith (1991) and Otter (2007).
Literature & Further Reading
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Reviewers: Paul R. Martin (Queen’s Univ.), Scott M. Ramsay (Wilfrid Laurier Univ.)