Hollow
The tree’s bark is whole.
But inside it rots away.
Surprise! It falls down.
______________________

A lot of the time, once a tree starts to hollow out, there are visual clues that this is happening, but seeing requires one to be close and to pay attention. If the fall is a surprise, you weren’t doing those things.
Birds might not know, or care, if the structure wherein they are nesting is dying from the inside out. Maybe they should. Of course, birds can fly so the collapse really only matters while the eggs are being sat upon or in that small window after hatching before the young learn to fly.
I sometimes read about how smart birds are, but that’s rarely what I see out in nature. Anecdotally, “bird brain” makes a lot of sense to me as a pejorative. That said, I don’t have any intention to get on bad terms with crows or ravens. I don’t intend to get on good terms with them, either. I’m going to maintain a ‘no terms’ policy of distance kept unless or until they force my hand to offer them friendship.
The haiku above can be literal or it can be a metaphor. I leave that to the discretion of the reader. If you suspect a friend is hollowing out, it means you’re standing close to him or her and paying attention. I don’t pretend to know what to do for someone, after one makes that observation, but I think it’s good practice to be close and to pay attention to the people we care about. Life has a way of keeping us busy, and self-occupied, and treating each other like the birds treat a tree.