Colorado Springs musician Tom Taylor paid close attention to the bird song in his backyard in the Black Forest for some 30 years. Over time he noticed differences. He compares the variations over the years to blues licks, Beethoven and The Three Stooges theme song.
Ornithologists differentiate between bird calls, used for sounding alarms, and bird songs, which are likely about romance. What Taylor is listening to are bird songs. Ornithologists have noted geographic and other variations in Chickadees' songs, sometimes they’ll sing two, three notes or more notes. Taylor isn't a scientist; his is a musician’s point of view.
At first he most often heard three notes: “B flat, A flat, F," a little like a blues lick he might play on his guitar. That’s what he heard for years.
After some years he noticed that it migrated by a half a step to “B flat, G, F," reminding him of a Motown bass line.
Then the B flat changed, migrating downwards too. It helps to think of these half-step changes by imagining a piano keyboard and the note moving from a black key to an adjacent white key. Now he thinks it sounds like “Three Blind Mice” or, if you are of a certain age, it sounds like The Three Stooges theme song.