Many people have told me that chickadees are at or near the top of their list of favorite birds, and they are one of my very favorites too. What many people don’t realize is that we have two species of chickadees in this area and that the common one in Berks County has changed over the last several decades.
So writing about chickadees as a favorite bird brings up a very complicated topic that takes us beyond simply admiring these cheerful little birds! They are a bird that always brings a smile to my face when I see one, but trying to explain what the story with them here in the Berks County area has been daunting since I started writing for Berks Country six-and-a-half years ago.
Here in southeastern Pennsylvania, we have an east-west boundary line (actually a boundary zone maybe 20 or 30 miles wide) with Carolina chickadees to the south, and black-capped chickadees to the north. Just to confuse things, both species have black caps and subtle differences of plumage, and while their songs and calls can help separate them, location is an easier way to tell which species is coming to our bird feeders.
Dr. Robert Curry of Villanova University and his students have been studying the genetics, behavior and ecology of this phenomenon of the Carolina/black-capped chickadees’ contact zone since the 1990s, with four study sites making a transect across the area from south to north. The sites are a park in central Chester County, Nolde Forest Environmental Education Center in Cumru Township, Hawk Mountain Sanctuary on the Berks/Schuylkill County line and a state park in the Poconos. When they started, the birds in Chester County were pure Carolinas and the ones at Hawk Mountain were mostly black caps and Nolde Forest was in the contact zone with many hybrids. But they soon realized they didn’t have 100 percent black caps at Hawk Mountain even then, so they added the Pocono site to get completely beyond the Carolina range. Their research has found the interspecies hybrid pairs have low fertility, producing only a fraction of the number of offspring that leave the nest as same-species pairs produce!
They found that the contact zone is moving northward and not getting wider. The populations of these two species aren’t merging. The Carolinas are replacing the black caps over the years. Now in Berks County we have basically no black-capped chickadees breeding except at the very northern edge of the county, and those probably will be gone in a few years as the contact zone continues to move north. Sometimes in winter black caps will irrupt to the south looking for food, so they still may occasionally show up at our feeders, but their visits are decreasing in frequency. At my house in southwestern Berks County, we have seen two black caps over the last five years.
Telling the two kinds apart is possible but requires careful study. Black caps are a bit bigger and a little more colorful, with definite white wing-marks that have an upside-down hockey stick shape. They also have big, bright-white cheeks. The black throat-patch or bib is supposed to be bigger with a ragged lower-edge, but I have a hard time seeing this feature in living birds because they don’t sit still long enough!
Carolinas are a little smaller and drabber, more grayish, and the pale lines in the wings are not bright white. The white of their cheeks looks smaller and blends into the gray of their back feathers rather smoothly. Most bird ID books and apps nowadays do an excellent job os showing the differences between the two.
The two species both do the “Chick-a-dee-dee-dee” call, but they sing different songs (except in the hybrid zone). Black caps sing “FEE-bee,” and Carolinas sing a four-note song “FEE-be FEE-bay,” but songs are learned by youngsters; they aren’t instinctive, so youngsters in the hybrid zone hearing their parents and also singing neighbor may sing either song or even a three-note combo. Fortunately for those of us who like to ID birds, hybrids have become much less common in the Reading/Berks area now, and the chance is that almost any chickadee you see from Berks south is a Carolina.
I’m just glad I can see chickadees most anywhere I go, and I don’t care which species, because both of them always bring a smile to my face.
Mike Slater is a member of the Mengel Natural History Society of Berks County and the Muhlenberg Botanic Society of Lancaster. He lives in Brecknock Township. Reach him at paplantings@gmail.com.
