There are a lot of mockingbirds in my neighborhood.
Older mockingbirds sing.
The point, though, is that they don't sing a standard mockingbird song. They sing the songs of whatever birds they've been exposed to. If I hear a robin, or a cardinal, or a bluejay, there's always the chance that I'm really hearing a mockingbird that heard one of those birds, or that learned the robin song from another mockingbird. That looks a lot like meme transmittal.