This Simpsons episode was rated TV-G (meaning a rating for all ages of children, teens and adults) and is one of four episodes ever to be rated TV-G upon its original release. The rating was eventually upgraded to TV-PG.
Bart accidentally kills a mother bird with Nelson's BB gun, and tries his best to raise the mother's eggs on his own. But when the hatch-lings 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 with tickets (by cheating on the skee ball game). 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 the bird, but instead of punishing him, she refuses to have anything to do with his destructive ways and drives off. 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, 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.