Separating black-capped and Carolina chickadees in the field may be one of the most difficult and underappreciated problems bird watchers face. To understand why they are a problem, you need to know something about where each species occurs, what happens where they both occur, and what happens in invasion years.
Black-capped and Carolina chickadees are fairly closely related, which should be clear to anyone who looks at the pictures in the field guide. It is not so obvious that they are not each other’s closest relatives. The latest evidence shows that black-cappeds are more closely related to mountain chickadees than they are to Carolinas, despite the fact that telling black-capped from mountain is not usually difficult.
Black-cappeds are the more northern species and Carolinas are southern. In all but a very few years, the only identification problem exists where the range of the two species overlap — a line running roughly through central New Jersey, southern Pennsylvania, and central Ohio, Indiana, and Illinois, and south in the Appalachian Mountains. Finding the precise area of overlap requires checking local sources, such as a state breeding bird atlas or regional bird lists and books.
The good news is that if you live north of the range of Carolina chickadees, every bird you see is a black-capped. There is virtually no evidence that Carolinas ever wander north, although I am not sure how you would tell for certain if one did. South of the range of black-capped, all the birds are Carolinas. Black-cappeds do move south in some years, at least a few hundred miles into the range of Carolinas. According to data from banders and other sources, it appears that these movements happened once, and only once, per decade in the twentieth century. Otherwise there is no discernible pattern, and the movements do not seem related to the movements of other northern birds. Black-cappeds do what black-cappeds do.
The problem of chickadee identification can be divided into two sections: The range of overlap and invasion years. As you will see, they are very different problems, indeed.
Banding returns show that in invasion years, the black-cappeds that move into the range of Carolinas do not come from the areas close to the overlap zone. They come from the Far North, and that makes it considerably easier. Black-capped chickadees from north of the Canada border are much larger than black-cappeds from farther south, so the other differences between the two species become more obvious. In the region where the two meet during the breeding season, they are most alike in size, plumage, and vocalizations. The reasons are too complicated to go into here. Just be glad that the invaders travel a long way.
Over the past 100 years, a number of characters have been used to separate the black-cappeds and Carolinas. Many of these clues have made it into field guides, and others have become locally established. Many of them are helpful, but only two clues are completely reliable, and one requires a bird in the hand. If you investigate the problem, you will eventually come across the following field marks. Remember, though, these field marks apply only in an invasion year. Why they cannot be used in the range of contact will be explained later.
Black-cappeds are bigger than Carolinas. When birds from the North invade, the size difference can sometimes be striking; black-cappeds often appear closer in size to a titmouse than a chickadee. The sizes of both species are somewhat variable, however, and for many birds it is not a useful character. In the hand, banders can almost always tell which is which by calculating the ratio between the length of the wing and the length of the tail. (The black-capped has a proportionally larger tail.) Forget trying this in the field.
On average, and the word “average” is crucial here, the border between the black bib and the pale gray chest is more ragged on black-cappeds than on Carolinas. On freshly plumaged black-cappeds (and birds in the fall and early winter are in fresh plumage) the black feathers of the bib intrude into the gray in an irregular pattern. The border between the bib and the breast is almost always a neat straight line on Carolinas. This is not always an easy character to assess, and it should be used with caution.
It has been suggested that the size of the white face patch may be useful in separating the two. Although there may be a small difference in some birds, the amount of overlap is so great that this character is essentially useless. Once you have already identified a bird you can check to see if there appears to be a difference, but chances are you will come to no real conclusion. In fact, the size of the face patch may vary even between individual black-cappeds.
It has been suggested that black-capped chickadees tend to feed closer to the ground than Carolinas. Some observers claim that if a flock of chickadees is responding to pishing, the black-capped will come in low while the Carolinas come in high. Could be. I have seen that happen, but I have also seen black-cappeds up high and Carolinas down low.
The song of the black-capped is typically described as a two- noted whistle, fee-bee, and the song of Carolina as a four-noted whistle, fee-bee fee-bay. On the breeding grounds those descriptions hold true most of the time, but they don’t typically sing in winter. Both species have variable songs. Some Carolinas sing two-note songs. Some black- cappeds will sing four-note songs. Some of both sing three-note songs. To make matters worse, they imitate each other. Hearing a typical black-capped two-note song coming from a flock of Carolinas should make you look more closely at the singer, but do not be a bit surprised if it turns out to be a Carolina.
So much for the characters that don’t work, or work only part of the time. Now for the ones that do, more or less.
White in the Wings
In fresh plumage, sitting Carolinas and black-cappeds both have fresh pale feather edges on some of the wing feathers. (We will only be discussing sitting birds. Trying to separate the two in flight is insane.) The result of these pale edges is the appearance of a strong white line running down the wing from the shoulder almost to the rump (see the illustration). Some field guides have suggested that black-cappeds have this white line and Carolinas do not. As a matter of fact, they both do, and it can be as bright on a Carolina as on a black- capped.
The real difference between the two birds shows in the greater coverts, which are the feathers that show on the shoulder of the folded wing. On black-cappeds, the edges of the greater coverts are broad and very white. As you can see in the illustration, these edges look a bit like a miniature hockey stick. The white line on the back that comes up to the shoulder is the handle, and the white on the shoulder, which curves slightly downward, is the blade of the stick. On fresh birds, before the edges wear, this pattern is quite striking. If you see it, you are almost certainly looking at a black-capped. On fresh Carolinas, the greater covert edges are often gray rather than white. They are not as broad, they do not contrast as sharply with the edge of the wing, and they are usually a little darker than the white stripe on the back.
The basic chickadee call, dee-dee-dee, is different from the song, in part because it is less a product of learning and imitation. Both Carolinas and black-cappeds regularly give the dee-dee-dee call, and the number of notes can vary from one or two to a dozen, depending on the bird and the circumstance. In Carolinas, the dee-dee-dee call is usually rapid and somewhat high-pitched. In black-cappeds it is noticeably slower and hoarser, and the difference between the two is easy to recognize with practice. Often, the first hint you get of a black-capped in a flock of Carolinas is hearing a slow, hoarse, dee-dee-dee call obviously different from the other calls emanating from the flock. The one caution is that when birds are agitated, as they are when mobbing an owl or responding to pishing, the calls of both species tend to speed up and sound slightly higher-pitched. However, the difference should still be obvious in almost all cases.
Both species have a variety of other calls, including gargles and contact notes, but there is, as yet, no evidence that any of them can be used in the field to separate the two.
A word about numbers. In invasion years, black-cappeds move into the northern range of Carolina chickadees. It is not unusual to find black-cappeds in groups, almost always with Carolinas, and the Carolinas will outnumber the black-cappeds at least two to one. If you find one definite black-capped in the flock, you can be fairly certain there are others.
Now for the bad news: you cannot identify the chickadees in the range of overlap. If local research establishes that you are within twenty-five miles of the point where the two species come into contact, it is impossible to put an exact name on the birds you are seeing. Many, and perhaps most, of the birds in that zone are the result of hybridization.
Despite the fact that scientists have established that Carolina and black-capped chickadees are separate species, they do hybridize where they come into contact. It was generally thought that the hybridization involved only a few birds and was confined to a very, very, narrow band of the overlap area. The recent evidence indicates that the zone of hybridization is at least fifty miles wide. In that zone, most of the birds show genetic evidence of belonging to both species. It is a bird-watching mess. Even when a bird looks like a Carolina, sounds like a Carolina, and is twenty miles south of where black-cappeds occur, it usually has a few black-capped genes in it — and the same is true in reverse. Audio playback studies have only complicated the problem. A researcher in Virginia went to the contact zone and played the song of a Carolina chickadee. A bird flew in and responded lustily with a typical Carolina song. The researcher changed the tape and played the song of a black-capped chickadee. The bird immediately switched to a typical black-capped song! And it happened over and over. Measurements and other characters don’t help, either. In fact, only genetic testing provides any evidence of hybrid birds’ parentage, and most of the birds tested in the overlap area show that at least somewhere in the past, the two species mingled genetically. Genetic testing is not a useful field technique for most of us.
So if you are in the range of overlap, and a bird looks and sounds like a typical Carolina, what should you call it? There is no good answer. In many areas where the two species overlap, local bird watchers call all birds on one side Carolinas, and all the birds on the other side of the line are black-cappeds. They go with the “if it looks and sounds like one I am going to call it one” technique. Who cares if the bird’s grandfather and grandmother dallied a bit?
This technique is fine as long as you recognize that you will be wrong some of the time, and that much of the time you may be dealing with hybrids. And sometimes, you just need to write some birds off. During ten years of breeding bird atlas work, much of it in areas where these two species come into contact, I ran across birds I simply could not name. First they would look like a black-capped. Then they would look like a Carolina. Then the mate would show up and look like neither one, or a blend of both. I learned that there are some birds I simply cannot put a name on.
The more we learn about the two species, the more complicated the issue becomes and the more fascinating. There is a massive body of literature about Carolinas and black-cappeds, and it grows constantly. Those of you who become intrigued should find a good ornithological library and settle in for some pleasant, contradictory reading. For everyone else, take heart. Once you learn where and when to look, and learn the field marks that separate the two, you can put a name on most of the chickadees you see. Just be willing to occasionally throw up your hands and admit it when you can’t tell if your bird is one species or the other. Apparently the chickadees have the same problem sometimes.