It was about a crazy lady who lived above his flat in Pimlico. She would let pigeons into her flat so she could feed them. Apparently she didn't want her presence in the flat to interfere with the natural behaviour of the pigeons, so she would let them nest and shit in there and she wouldn't clean it up, because it wasn't natural to do so. The pigeons would die, but apart from the smell and the sludge and the gas, the corpses weren't really a problem. It was the rats that came to eat them. The rats would eat the rotting pigeon corpses mixed in with the rotting pigeon shit and they would get ill and die too. New rats that came through wouldn't mind though, and they'd start to eat the mass, only to get sick and die in it later on. The population grew steadily as more pigeons and rats came from in the cold, to live naturally. They fed the mass further.
i haven't read 100 book s so i'm probably not getting the depth of all of what you're saying