My opinion at this time, specific to my flock, is that a heavy rain quickly cools the chicks beyond their ability to reheat themselves. Not a problem if momma is there, but a big issue if they have no momma or heat source to run to. But for older chicks and even adults, it activates pathogens (Marek’s in my case) they’re already carrying and the pathogens quickly wreck their internals. I believe this because Marek’s related stumbling begins the morning after a heavy rain or a sudden cool snap that was preceded by a hot day. If you have ever messed with aquarium fish or tournament-caught bass that can quickly be shocked to death by being changed from hot to cold to hot again, that’s sort of what the sick chickens remind me of after a storm or a cool night.
So far I see no losses or sick acting birds. One of my newest mommas is missing some chicks, but they may have been missing prior to the storm. We got 1.1 inches here yesterday evening. Its going to put a new lease on life for much of the plant life.