Two Black-capped Chickadees are counter-singing here on a cold morning in the mountains of West Virginia. They are singing at slightly different pitches, so it is easy to tell which is which. Black-capped Chickadees learn their songs, so you might expect them to have several song-types, like Tufted Titmice, Yellow Warblers, and Varied Thrushes. But, alas, they have only one, and you can hear and see it right here. But, they do change pitch, and when they do, the two notes always bear the same musical relationship; the ratio of frequencies of the two notes of the "fee-bee" song averages 1.134 with only 2% variation. As our perception of pitch is based on ratios rather than sums and differences, the fee-bee songs at different pitches all sound like the same tune. Presumably it is the same for the chickadees. This hardly seems to be random, i.e., it must have been produced by natural selection. It has been suggested that fee-bees at different pitches are functionally equivalent to the different song-types we see in most other chickadees, titmice, and other members of the family Paridae. If so, they sing with "eventual variety." For the full story, see this excellent popular article by Ron Weisman and Laurene Ratcliffe.
Now, if they learn their songs, how do they manage to maintain the stereotypy of this simple version in a geographic range that spans the continent. That remains a mystery, but Don Kroodsma and a host of co-authors did show that they deviate from the "normal" pattern on Martha's Vineyard, Massachusetts, and in the coastal Pacific Northwest. Look for an example of that song next February. After living with them for 11 years, I think those Pacific Northwest birds are actually a slightly different species, or a species-in-waiting at least.
My encounter with this fine example of counter-singing in different keys was, like so many, serendipitous. I had attended a National Audubon Society lobbying workshop in Washington, DC for a week, and took the opportunity to visit nearby West Virginia for the only time in my life. People at the workshop said we must visit Canaan Valley State Park, which we did. It's pronounced "ku-NANE" I was told. It was spring in DC, but it snowed at Canaan. I screwed up my courage, though, and headed out at dawn to hear what I could hear. I was richly rewarded. These two birds singing from leafless trees, with the sky slate gray, and the temperature around 25, were perfect harbingers of a mountain spring we all knew was just around the corner.