When Bart is hanging out with Nelson, he accidentally kills a bird sitting in her nest. Seeing that the bird has left behind two perfect eggs, Bart is overcome with guilt and takes it upon himself to hatch the baby birds. He keeps the eggs warm anyway he can, sometimes with a lightbulb, sometimes by sitting on them. Eventually the eggs hatch, but there are no birds inside. Instead two nasty-looking lizards emerge. Confused by what happened, Bart and Marge take the lizards to the Springfield Birdwatching Society and ask them why the lizards were inside the bird's nest. Principal Skinner, a member of the Society, explains that the lizards are Bolivian tree lizards, a species that steals bird's eggs and leave their own eggs to be watched after by the mother bird. Skinner wants to kill the lizards since they're bad for the ecosystem, but Bart flees with his lizards and sets them free. The lizards go on to eat all of Springfield's pigeons.