They switched to singing at night because modern noise pollution is so bad during the day that they can't hear themselves chirrup. If you want to Do Something about that, you'll probably have to join the Black Ops Arm of the Noise Abatement Society.
I was aware that rats were growing immune to many poisons, but this is the first I've heard about them needing a blood thinner to survive. Which begs the question: If warfarin is now an essential dietary additive for healthy rats, why don't we simply stop giving it to them?
(S M) I think the story is true though there's a possibility it may be a scientific urban myth. Without Warfarin the rats presumably will rapidly evolve back to what they were before the introduction of this substance although a number will perish in the process. Something more subtle is needed.