A squirrel feeds with the help of volunteer Marla Saville at Greenwood Wildlife Rehabilitation Center in Longmont. Tiny claw marks crisscross Bob Nightwalker’s arms and hands. He doesn’t mind, though.

But Squirrel Nutkin, whose cautionary tale of impertinence (and maybe working-class rebellion) was published in 1903, nonetheless endeared generations of Brits to the reddish rodents native to their ...