I get loads of rain year round here.
Most of my chickens go under a tree, bush, or under the coop during down pours. A few wander around. It's the ones that just stand out in the rain doing nothing that worry me. If I see that, I suspect something is wrong. I may bring them in, give them sugar water (or molasses), dry them off, and check them out. If I think they are sick, I have a pretty low threshold and I won't keep sick birds. At most, I'll give them a day to perk up with heat, TLC, warm fluids and food.
I had three birds this winter that froze in the snow. Frustrating. I think they just "fell asleep" under the coop. One of the doors to the coop was closed, and they didn't go in the other. I found them, obviously too late.
So the lesson is, I would just keep an eye on them. If they seem to be strutting around happy and content, I wouldn't worry. If they just stand in the rain, or seem lethargic, I'd be concerned. I would give them the opportunity to perk up with heat, fluids (I spike it with some sugar energy, whatever is handy), and food. If that doesn't do it. Suspect illness.
