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Lee uses the "senseless slaughter of songbirds" in connection to Tom's death in order to tie everything to the novel's major theme. Namely, the novel's title.
To Kill A Mockingbird
Underwood's editorial denigrates the action of a town that would somehow drive a man like Tom Robinson to his death. This metaphor shows that songbirds (a comparison to the literal Tom Robinson) are often senselessly slaughtered as was Tom when he dies.