them, whom Bart had already named Chirpy Boy and Bart Jr. Bart and Marge take the lizards to the Springfield Birdwatching Society, where Principal Skinner explains they are Bolivian tree lizards, a breed that steals a bird's eggs and leaves their own to be watched after by the mother bird, which is then eaten by the offspring once they hatch. As Skinner wonders how their mother ended up in Springfield, a nervous Apu secretly remembers that a pair of the lizards had come with his shipment of Bolivian donuts and escaped his store while he was putting them on the shelf. Skinner says the lizards must be killed by law because they have killed many bird species. Bart escapes and runs away with the lizards, but Skinner catches up to him and they struggle on top of the society building. The lizards fall off the side but to everyone's disbelief, glide to the ground. The lizards grow in population and start to eradicate pigeons in Springfield. Since the town considered the pigeons a nuisance, they are delighted and Bart is thanked and honored by Mayor Quimby with a loganberry scented candle. Lisa worries the town will become infested by lizards but Skinner assures her they will send in Chinese needle snakes to eat them, followed by snake-eating gorillas, which will "simply freeze to death" when wintertime rolls around. Production "Bart the Mother" was the last full-length episode David X. Cohen wrote for The Simpsons. Shortly after the episode aired, Cohen teamed up with The Simpsons creator Matt Groening to develop Futurama, where he served as executive producer and head writer for the series' entire run. Cohen's idea for the episode started out as a B story for Homer. In the original concept, pigeons nest outside Homer's window at the Springfield Nuclear Power Plant. Mr. Burns exterminates the pigeons and Homer climbs out onto the ledge of the window, in the middle of a blizzard, to nest the eggs. Cohen said the idea "[did not] fit in anywhere", so he decided to pitch it as an A story instead, with Bart nesting the eggs instead of Homer. The Simpsons writer George Meyer thought the idea of Bart shooting the bird and simply nesting the eggs was too straightforward, and he did not think it had "enough of a twist" to be a good episode. Cohen and Meyer eventually decided the eggs