Identification: Black-capped vs. Carolina Chickadee
Across most of eastern North America, chickadee identification is simple. Carolina Chickadee occurs in the south and Black-capped in the north ((Black-capped overlaps with Boreal Chickadee in the far north, but those species are easily separated)). In a narrow band from northern New Jersey to Kansas, however, Black-capped and Carolina Chickadee meet and hybridize. In that area, chickadees are essentially unidentifiable, and observers can only say that an individual bird “shows the characteristics of” one species or the other. Luckily, these populations are essentially sedentary, so hybrids are not a big concern away from the contact zone.
The normal range of Black-capped Chickadee is north of the hybrid zone and the normal range of Carolina south, but you can expect each species to wander a bit and cross over a short distance into the range of the other species. Black-capped is more strongly migratory, and therefore more likely to show up south of the hybrid zone (and farther south) than Carolina is to show up north of the zone. Carolina Chickadees should be expected to show up just north of the current contact zone, and Black-cappeds can be found in winter (especially in irruption years) well south of the contact zone. Identification in these cases is extremely tricky and an observer claiming a record of either species out of range will have a difficult time proving the claim.
If you are lucky enough to live in or near the contact zone, the best way to familiarize yourself with each species is to take a field trip north or south into pure Black-capped or Carolina country. Studying birds of known species – and the variation within each species – will give you more confidence for identifying the chickadees in the zone of contact.
In order of importance the things to look (and listen) for are:
- color of the neck sides
- contrast of tertial edges and back
- speed of call
- color of the wing covert edges
- overall color
- overall size and shape
- The differences described here are for birds in fresh plumage, and are more difficult to assess when birds are worn in May-Sep. There may be other differences that become apparent when birds are worn, but I have little experience with summer chickadees.
- Hybrids may be fairly common in the contact zone (up to 2/3 of individuals in some areas) and may show any combination of features. Many chickadees in that area are simply not identifiable.
- Confident identification requires a very detailed view. Chickadees seen briefly as they flit through the treetops are usually not identifiable, and distant views can be misleading, Just let those birds go and concentrate on identifying the ones you see at close range.
- No single characteristic is entirely diagnostic by itself. Look for several different features and consider the whole package.
- Rear part of white cheek patch, technically the sides of the neck, is whiter and more extensive on Black-capped (vs. grayish and less extensive)
The white cheek patch is prominent on both species, but pure white extends much farther back across the sides of the neck and even onto the upper back on Black-capped. This is often obvious in a close view, as Black-cappeds have a long sweep of clean white on the nape contrasting directly with the greenish back, while Carolinas have a shorter extension of distinctly grayish-white feathers, and less contrast with the back. To use this field mark more objectively, look for a faint shadow line at the rear edge of the auriculars (ear coverts), and study the light-colored feathers behind this line. On Black-capped Chickadee these feathers are all white, and the long fluffy nape feathers create a larger area of white behind the auriculars than on them. Carolina Chickadee has these feathers behind the auriculars faintly washed with gray, and the area of grayish-white space behind the auriculars is equivalent to the area of white on the auriculars. Obviously this is most easily seen at close range, and takes some practice, but it should provide a useful identification clue all year. Note: There is considerable variation in cheek color, and Mennill et al (2003) found that some Black-cappeds (especially high-ranking males) have whiter cheeks than others.
- Tertials darker, contrasting strongly with white edges on Black-capped (vs. tertials paler and upperparts overall drabber and more uniform grayish on Carolina)
All wing feathers are darker with more contrasting white edges on Black-capped. This fact combined with the greener back of Black-capped creates a more contrasting upperside. Fall and winter Black-cappeds show a crisp green-black-white pattern on the back and tertials, while Carolinas show a less contrasting pattern in shades of gray. This feature seems quite reliable in fresh plumage, but is probably less so in summer, when the back of both species has faded to grayish-green and much of the white tertial edging has worn off.
- Greater coverts (and secondaries) white-edged on Black-capped (vs. silvery gray on Carolina)
This feature has often been stressed as a field mark, but in fact it is variable and can be difficult to judge. The greater covert edges are a more reliable mark than the secondary edges, but beware of brief or distant views. In addition to whiter edges Black-capped has darker feather centers, for an overall more contrasting wing pattern (as mentioned above) and this is the more reliable thing to compare. Pitfall: In practice these differences are very hard to interpret. The silvery quality of the pale edges is strongly affected by light, and a Carolina seen in sunlight can appear just as white-edged as a Black-capped. Furthermore, a very close view of a Black-capped sometimes reveals a grayish tinge on the covert edges. Pale edges wear quickly, and both species will be darker in spring and summer than in fall.
- Overall Black-capped Chickadees appear brighter and more contrasting than Carolina
The back is greener, the flanks are cleaner and brighter buff, the breast is whiter, the white cheek patch is whiter and more extensive, the tertials are darker gray, and the edgings on wing and tail feathers are bright white. Carolina Chickadees appear overall more grayish; all contrast between the back, tertials, wing coverts, flanks, and breast is reduced. Most of these characters are subtle and variable, but together they create a different appearance that should alert an experienced observer.
- Black bib of Black-capped more uneven or “ragged” and more extensive (vs. Carolina’s bib generally neater, more triangular)
Typically, Black-cappeds show a slight downward bulge of black on each side of the bib creating a “double-rounded” lower edge, while Carolina tends to show a neat triangle of black. Less reliably, on Black-capped the black bib extends slightly farther towards the shoulder, and this extension often takes the form of a ragged smudge. Bib shape changes constantly as the birds move, however, so this feature is of limited use. Bib pattern is at least partly determined by white feather tips that conceal black; as these wear, more black is exposed, and overall bib shape changes, so summer birds show more black than winter.
- Black-cappeds appear larger-headed, fluffier, shaggy-naped, and longer-tailed.
This is subjective and variable, but the overall difference in shape can be obvious when the two species are together.
- Call with slower series of dee notes in Black-capped (vs. faster in Carolina)
about 4 dees/second in Black-capped and 6-7 dees/second in Carolina. See voice description below.
- Black-capped tends to be more inquisitive and bold than Carolina
It is not unusual for Black-capped Chickadees to be attracted within a few feet of an observer, while Carolinas rarely approach that close and seem less attracted to pishing and other lures. This fearlessness may be strongest in Black-cappeds from the far north, and the two species may have similar behavior where their ranges meet, but I have noticed a difference in birds as close as southern New Jersey (Carolina Chickadees) and southern Connecticut (Black-capped). Obviously this is a subtle and unreliable difference, but interesting nonetheless.
Song and calls
Both Black-capped and Carolina Chickadees have a song comprised of simple clear whistles. Black-capped’s song is lower-pitched and virtually always two-noted, with the second note lower and usually in two pulses fee beeyeee. Carolina’s song is higher-pitched and more variable from two- to six-noted, often a three-noted fee bee bee very similar to Black-capped. The so-called classic Carolina Chickadee song fee bee fee bay is, year-round, no more frequent than other variations, although Frank Gill (pers. comm.) reports that it is the most frequent song in spring, and Ward (1966) reports that the song is less variable in and near the contact zone than farther away from it.
Laboratory experiments by Donald Kroodsma (Smith 1991) have shown that the song is learned, not innate, and field observations show that chickadees in the contact zone respond to playback of recorded songs of both species (Merritt 1978). Many individuals in and near the contact zone sing typical songs of both species, or sing the wrong song or abnormal songs (Merritt 1978; Wade Wander pers. comm.. Frank Gill pers. comm., Curry). For example, at a study site in Pennsylvania all birds sing Black-capped songs, and about 60% of those same birds also sing Carolina songs, even though genetic tests indicate all of these birds are hybrids and Carolinas (Curry). Thus, contrary to many published reports, song is of little value for identification within the contact zone, since a young chickadee there has the opportunity to learn both songs, or to incorporate elements of both songs into a “hybrid” song.
Calls are also learned, but apparently not in the same way that song is learned, e.g. at that same study site in Pennsylvania, calls are intermediate between Black-capped and Carolina, but birds giving more Carolina-like calls do not necessarily use Carolina-like songs. Individual birds give only a single call-type, none are “bilingual” with calls as they are with songs.
Calls are complex and varied, but the entire repertoire is shared by both species. Both species give almost constant high tsik notes while foraging calmly. When disturbed they give some variation of the characteristic loud tsi-tsi-dee dee or chika-chika-dee dee dee dee. Interacting with other members of their flock they give a complex “gargle” call – a descending jumble of harsh and liquid notes. The differences between the two species are in the details of these shared calls. In general the voice of Black-capped Chickadee is lower and fuller than Carolina, while Carolina sounds a little softer and more nasal, but variation between individuals of the same species obscures much of the subtle difference between species.
The easiest call to listen for and compare is the series of harsh dee notes. In Black-capped the dee notes are slightly lower-pitched and each note is longer, making the series slower than Carolina. If you have a watch it is possible to measure the rate of “dee” notes. When you hear a chickadee call, repeat the dee-dee-dee... series to yourself, count at the same rhythm, and count the notes while watching the seconds on the watch. I find that Black-cappeds call at about 4 dees/second, while Carolinas call at 6-7 dees/second. There is variation depending on the “mood” of the bird and other factors. Listen for several calls, and use this character in conjunction with others.
Hybrids of these two species have been found wherever the contact zone is studied. Recent research including DNA analysis reveals that, in fact, hybrids are common and many birds with hybrid ancestry look and sound like typical individuals of one species or the other. In other words, some hybrids give no outward indication of their mixed ancestry! Hybrids are almost impossible to confirm by field observations, although within the contact zone any bird mustbe suspected of being a hybrid, and those showing combinations of features are very likely hybrids.
Research suggests that, while hybridization occurs regularly, the hybrids are less fit than pure birds, raise fewer young, and therefore the hybrid population remains small and stable (Frank Gill, pers. comm.). Based on measurements Menill (1978) found no more than 15% intermediate among banded birds in Indiana, and at Hopewell, New Jersey, Hannah Suthers (pers. comm.) has banded only “a couple” of possible hybrids in 15 years. In contrast, recent DNA studies in Pennsylvania (Curry) found over 60% hybrids at some sites.
The contact zone is slowly shifting north. For example, near Hopewell, New Jersey, Carolina Chickadees arrived in the 1980s and now coexist with Black-cappeds (Hannah Suthers, pers. comm.}; and at Princeton, New Jersey (Institute Woods), both species nested as recently as 1980, but only Carolina has been recorded since then (Laurie Larson, pers. comm.). There is also evidence of a northward shift in Ohio and Pennsylvania (Curry)
The first Carolina Chickadees pioneering north of their range are mostly females, so hybridization can be occurring even in areas where all songs are still typical Black-capped (e.g. at Hawk Mountain, PA as of 2007). Black-capped song seems to persist a long time after hybridization begins.
Curry finds that in the contact zone, females are more likely to engage in extra-pair copulations if their mate is more Black-capped like, and they seek out more Carolina-like males for these excursions. This could be a mechanism for the slow northward shift of the contact zone.
The actual isolating mechanism that prevents hybrids from spreading beyond the narrow contact zone could be as simple as the reduced fitness of hybrids. Curry reports that in Pennsylvania eggs of mixed pairs have a much lower hatching rate than those of pure pairs. But other factors might also come into play. Brewer (1963) suggested that reproductive isolation was achieved through different habitat tolerances and timing of nesting, rather than visual or vocal characters. One possible isolating mechanism is the brief pre-copulatory display (Smith 1991). In Black-capped Chickadee the pre-copulatory sequence is initiated by either the male or the female giving high, thin, variable “see” notes. The male may give a gargle call. In Carolina Chickadee it is reported that the male initiates the sequence by giving typical whistled song, and the female responds with soft nasal “dee” notes similar to those given during courtship feeding (similar to the begging calls of fledglings). If these differences are true and consistent, they could inhibit interbreeding.
Distribution and Movements
In general these two species are sedentary and are easily distinguished by range. Black-cappeds occur to the north and at higher elevations, Carolinas to the south and at lower elevations.
Black-capped Chickadees from the northern parts of the range stage periodic irruptive flights in fall, when hundreds or even thousands can be seen in a day flying past traditional migratory concentration points along the shores of the Great Lakes or the coast of New England. Most of these birds seem to evaporate just before they reach the range of Carolina Chickadee, but Black-capped Chickadees can occasionally be found in winter a short distance south of the contact zone, and very rarely even farther south. Carolina Chickadees seem to be essentially sedentary, and have almost never been recorded north of their normal range.
Several subspecies have been named within each of these two species, based on very slight and clinal differences in size and plumage. In general both species become slightly larger and paler to the north, so that relatively large pale Carolina Chickadees occur in the contact zone together with relatively small, gray Black-capped Chickadees. Think of these species forming a continuous cline from small and gray in Florida to large and pale in Alaska, with a distinct step – a small but abrupt change – along the line from New Jersey to Kansas.
When northern Black-capped Chickadees appear at the southern edge of their range during the periodic fall-winter irruptions, they should appear paler and fluffier than the resident southern Black-cappeds, and therefore may stand out more from the Carolina Chickadees.
Subtle but distinct differences exist in the plumage of these two species, but when looking for these differences it is important to remember the season. Both species molt into fresh plumage in late summer, and appear relatively clean and bright from then until early spring, Summer adults (after thousands of trips in and out of a small nest hole) become very worn, and look darker and scruffier than in winter, although differences between the species can still be seen. Juveniles appear as early as April and as late as September, and the contrast of a fresh-plumaged juvenile next to a worn adult could lead to misidentification of the drabber adult as Carolina and the brighter juvenile as Black-capped.
I thank Bill Boyle, Frank Gill, Rich Kane, Laurie Larson, Hannah Suthers, and Wade Wander for information and comments on chickadees for an earlier version of this paper published in Records of New Jersey Birds.
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