This is indeed a Northern Mockingbird, whose scientific name is Mimus polyglottos. In Latin, that means "many tongued mimic". They are indeed excellent mimics. Many bird calls are territorial calls, so by imitating the calls of other birds, the Mockingbird is claiming the territory in multiple "languages".

Each call is usually repeated three or four times before changing to the call of a different bird. Some individuals have been known to mimic over 400 songs.

Part of the reason it can do this is because of its syrinx, the bird's equivalent of our vocal cords. In an earlier Science Photo, we learned that the Turkey Vulture does not have a syrinx. Many birds have one pair of syrinx. Parrots have two pair, and Mockingbirds have four pair.