Since mockingbirds imitate songs of other birds, how do they find each other during mating season? How can they tell it's a mockingbird singing, instead of other birds? Is there something in their imitations that still identifies them as a Mockingbird?
Wildlife biologist Daniel Edelstein implies as much here: he says that there are in fact variations in tone and context that distinguish mockingbirds from other birds they imitate. (He was writing about the possibility of mockingbirds attracting the birds they are imitating, which doesn't happen. I can't find much about what specifically attracts female mockingbirds, except for the variety he alludes to.)