A classic cinema moment: Gregory Peck, in his Academy Award–winning role as lawyer Atticus Finch in a Depression-worn Alabama town, kills a rabid dog stumbling down a dusty street. At supper that night, his children ask how old he was when he got his first gun.“Thirteen or fourteen,” he answers. “I remember when my daddy gave me that gun. He told me that I should never point it at anything in the house; and that he’d rather I’d shoot at tin cans in the backyard. But he said that sooner or later he supposed the temptation to go after birds would be too much, and that I could shoot all the blue jays I wanted—if I could hit ’em; but to remember it was a sin to kill a mockingbird.”Mockingbirds, Peck-Finch explains, “don’t eat people’s gardens. Don’t nest in the corncrib, they don’t do one thing but just sing their hearts out for us.”“I’ve been there,” said Curtis Adkisson, a retired Virginia Tech biology professor, when I read him this discourse from...