Here in the Cleveland area, we live in the land of the Black-capped Chickadee.
When I teach bird song classes or give programs in the greater Cleveland area and surrounding NE Ohio counties, theirs is the first song I present. The Black-capped Chickadee song is a very clear, simple, two-note tune that most people have heard and can easily learn to imitate.
From around Canton south, the Black-capped Chickadee is replaced by its southern cousin, the Carolina Chickadee.
They look very similar, and there is an obvious relationship between their songs. The four-note Carolina Chickadee I typically hear resembles a Black-capped song with an additional high note added before each of the two primary pitches. It's rather like an ornamented version of the simple two-note tune. If I edit out those extra pitches, what's left is essentially...the Black-capped song. Their calls are also similar, but the Carolina Chickadee calls sound like they belong to warmer climate birds - lighter, higher, and faster versions of those belonging to their northern relatives. I laugh with delight when I hear them!
Here is a Carolina Chickadee I recorded singing at the edge of the swamp at Killbuck Marsh Wildlife Area south of Wooster.
South of Cleveland and north of Columbus is an interesting area where the two chickadee species coexist and occasionally hybridize. This is called the contact zone, and the boundary extends from southern New Jersey west into the Great Plains. According to David Sibley in the online version of the Sibley Guide to Birds,
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.
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 that 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 near the contact zone than farther away from it."
(I've noticed that when I can actually see the spectrogram of a song, occasionally a three note song does indeed have a high fourth pitch present before the final lower note and we mere humans simply may not hear it.)
I'd read that when both species are present, hybrids may sing the songs of either one. I did not realize that hybrids might also sing songs that belong to neither species, but I had the opportunity to observe this first hand in the contact zone south of Akron.
I had decided to go to The Wilderness Center in Wilmot, Ohio (southwest of Canton) to try to record Carolina Chickadees. My work schedule does not often allow me to go as far south as Columbus to record them, and my previous Columbus-area recordings had been marred by car, truck, and air traffic noise to the point of being unusable. It seemed that The Wilderness Center would be a good plan. It is a little south of Route 30, which is the unofficial dividing line of the chickadee species. I decided to give it a try.
When I asked at their education center about Carolina Chickadees, I was told that they have Black-cappeds, Carolinas, and hybrids - and that the hybrids could have some unusual songs. Really? Would I be so fortunate as to hear any?
Again from David Sibley:
Many individuals in and near the contact zone sing typical songs of both species, or sing the wrong song or abnormal songs...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. (See the link below to read the entire fascinating article and Sibley's citations.)
The first song I heard may have been one of the Carolina options, though I am still learning their somewhat variable repertoire. It was a four-note song, but the third note was not a high pitch. There was an initial high pitch, a normal chickadee-range pitch, a pitch one step higher, then a return to the second pitch. It was as if the first half of the song was Carolina and the second half Black-capped.
The next unusual song I heard was unlike any chickadee song I'd heard before. It was a three-note song with all three pitches within a very close range. There were no little Carolina high-pitched grace notes, and the song had three clear pitches instead of the two expected notes of the Black-capped song!
I was fascinated, of course - and the next song was even more interesting! I found and recorded another chickadee that sang a three-note song but would occasionally sing two or three additional repetitions of the last note. Sometimes this bird sang just the last note as a single pitch three or four times. His delightful song was my favorite, and you'll hear him here with a Northern Cardinal as a duet partner.
The Black-capped/Carolina hybrid zone is gradually moving north, and Carolina Chickadees have been banded as far north as Geauga County. I love both species and their songs, and now I know to listen for the possibility of unusual hybrid songs as well.
You can read much more about all of them, the contact zone, and see a zone map at http://www.sibleyguides.com/bird-info/black-capped-chickadee/black-capped-carolina-chickadee. As for me, I'll certainly be making additional trips to the contact zone with my microphone!