Merlin generally gets Mockingbirds correct which is part of it's magic.

Mockingbirds do a minimum of three repeats of any particular sound before moving onto the next. So the songs are done more times in a row than the real species would and then immediately it turns into a Robin (or something else). I'm sure there's subtle audio differences between a mockingbird and the real thing too that Merlin can detect.

A feature I'd love would be if Merlin told you the call the bird was making rather than just the bird ID. eg. "Blue Jay mimicking a Red Shouldered Hawk" would be a cool bit of info it could give since it really does catch the mimics out really well. It currently just says "Blue Jay" when it hears a blue jay trying to be a red shouldered hawk.