Bart accidentally kills a mother bird with Nelson Muntz's bee-bee gun, and tries his best to raise the mother's eggs on his own. But when the hatchlings turn out to be a pair of destructive lizards that harvest nests for their own survival, the town's bird population is placed at great risk.
When Marge takes the family to the Family Fun Center, she notices that Nelson is acting like a madman, especially when he knocks Milhouse off the track and wins a BB gun. Bart and Nelson attempt to be friends, but Marge insists that she does not want him to be with Nelson. Bart uses Nelson’s BB gun anyway, and when he does, he accidentally kills a bird, and leaves two eggs behind after the bird dies. Marge is mad that Bart killed an animal, but instead of punishing him, she refuses to have anything to do with his destructive ways. Bart secretly keeps the eggs warm, and the act is praised by Marge. With her help the eggs eventually hatch into lizards, which he names “Chirpy Boy” and “Bart Junior.”
Bart and Marge take the lizards to the Springfield Birdwatching Society, and when they are there, Principal Skinner, a member of the Society, explains they are Bolivian Tree Lizards, a type of lizard that steals a bird’s eggs and leaves their own eggs to be watched after by the mother bird. Skinner wants the lizards killed by law, because they have killed many bird species including the dodo, the cuckoo, and the ne-ne. Bart escapes and runs away with the lizards, which later decimate the entire pigeon population of Springfield. Much to Marge’s surprise, the rest of the town considered the pigeons to be a nuisance, so they are in fact delighted. As a result, Bart was thanked for this and was honored with a loganberry candle by Mayor Quimby. Lisa is worried that the town will be infested when Skinner reinforces they will send in Chinese snakes, then gorillas and then winter will take care of the rest. In the end, while the Simpsons head to their car, a pigeon gets trapped and mauled by several lizards on the sidewalk.