“Atticus said to Jem one day, “I’d rather you shot at tin cans in the backyard, but I know you’ll go after birds. Shoot all the blue jays you want, if you can hit ‘em, but remember it’s a sin to kill a mockingbird.” That was the only time I ever heard Atticus say it was a sin to do something, and I asked Miss Maudie about it. “Your father’s right,” she said. “Mockingbirds don’t do one thing except make music for us to enjoy. They don’t eat up people’s gardens, don’t nest in corn cribs, they don’t do one thing but sing their hearts out for us. That’s why it’s a sin to kill a mockingbird.” – Harper Lee, To Kill A Mockingbird
Mocking Birds, like Brown Thrashers, are in the family Mimidae. Although many species of bird imitate the vocalizations of other birds, the northern mockingbird is the best known in North America for doing so. It imitates not only birds, but also other animals such as cats, dogs, frogs, crickets and sounds from artificial items such as un-oiled wheels and even car alarms.  These birds sing endlessly and will even sing into the night. Most nighttime singers have not yet found a mate. They also sing more during a full moon. 
Mockingbirds are mostly gray but have white patches on each wing. When they fly the patches appear as flashes of white creating an unmistakable visual image which is easy to associate with the Mockingbird.
Mockingbirds perch on high spots to sing their songs: power wires, phone poles and high branches of trees. They are very territorial.