You can, but I would avoid it unless you have no trees. Any water I collect is jammed up with tree debris and I'm sure it is full of fungus and all sorts of other things.
That being said, chickens are always drinking out of ditch water.
Was wondering the same. I have 4 rain barrels and I water my garden with it and it has a screen to collect most debris, that being said it still has much, but it's WAY down at the last couple inches of water. I but Mosquito dunks in them and the water is pretty clear, not green, not brown (unless it's almost empty). Would the mosquito dunks hurt the chickens microbiom in their guts?