There's a saying amongst field geologists, that when you finish a field project and review your samples, you'll find you've failed to collect the single most common rock in the entire field area. I, of course, have been the same way with my bird project. I've documented all manner of birds that I've seen in my travels around California and elsewhere, and I see that I have failed to include any of the most common birds that visit my backyard.
The Northern Mockingbird is an American species. It's northern limit coincides with the Canadian border for the most part. They also range into Mexico, but they are found in all of the lower 48 states. It was part of the title of one of our country's greatest novels (and movie), and Mockingbirds were once kept as pets in the eastern United States, almost to the point of removing them entirely from the wild.