The rats survived but cats and birds did not
Towards the end of the film, there's a scene where they are eating rats. The large number in the barrel suggests that the rat species has not become extinct. Shortly afterwards, there's the scene where they're watching TV and learning about the skeletons of cats and birds. I've taken this to mean that cats and birds have become extinct, and there are only bones of them left now.
If the cats and birds died out, why did the rats survive?
My knowledge of biology is negligible. Just genuinely curious as to whether this is plausible or just a mistake.