Obviously, Harper Lee has given Atticus the surname of Finch for a reason since the finch is a quick, intelligent bird that adapts well. In fact, Charles Darwin studied them on the Galapagos Islands as examples of niche adaptation. For instance, some of the finches on the island had developed a longer bill in order to feed on cacti, flowers, and fruits, as well as seeds. The Woodpecker Finch, on the other hand, had learned to use a cactus spine or a twig in order to dislodge insects that it could eat. One remarkable finch, the Sharp-billed Finch, had learned that if it pecked at the undersides of wings and tail feathers of molting seabirds, it could drink their blood.
Like the finch, Atticus is intelligent, resourceful, and quick to react when necessary. He devises a plan to get Jem to learn about Mrs. Dubose, he reacts in tense situations by shooting a mad dog, or by placing himself before the jailhouse door with a bright light beaming overhead. In court, he adapts his cross-examination to expose the prevarications of Mayella and Bob Ewell. Although he loses the case, Atticus does discredit the Ewells and by actually providing Tom with a viable defense, Atticus wins a small victory and "soars" in the opinion of the black community, who demonstrate their respect by standing as he leaves the courtroom. However, much like a songbird that only brings joy, Atticus is no predator himself and, therefore, does not comprehend the true nature of the predatory Bob Ewell who truly intends to harm the Finches at the end of the narrative of To Kill a Mockingbird.
I think it would be a mockingbird because more towards the begining of the story he is innocent and is watching out for his kids and helping out other people in the town. Then farther on in the story when the trial comes around he loses a bit of innocence because he is fighting for a black man's innocence and the people of Maycomb don't understand why he is doing it and they don't like it.