The mockingbird image appears at least four times.
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The first time the mockingbird appears is when Atticus gives the children air guns for Christmas and instructs them not to kill mockingbirds. The second time is when Underwood writes about Tom's death in his newspaper column; the third time comes just prior to Ewell's attack on Jem and Scout, and the fourth when Scout agrees with Atticus that prosecuting Boo for Ewell's death would be like killing a mockingbird.
To Kill a Mockingbird