Small Brown Shell Houses

"Knock knock Knock, Who is staying inside?"

Only one is still occupied, the other four shell houses had been broken. They cemented firmly onto the side of a plastic container in my farm. Guess who were the occupants?  Bumpy bees ? Wasps?

I had the urge to break the last one and checked what lived inside. But finally, I refrained from doing so. Nature, just be it, should be able to find out from other sources.