My brother lives in the Pacific NorthWet (just south of Seattle) and he uses straw all the time. Never has much of a problem. The birds seem to scratch it around enough to keep the airflow going. Mold grows better when undisturbed, so I would say if you have some in a place where the birds aren't paying it much attention then maybe getting in there with a stick and stirring it up daily might prevent any mold?I use alfalfa hay in my coop, nest boxes, and then I push out into run. It cost only about $4 more than straw per bale in my area. $10 vs $6 I get it at $4 per bale from a farmer directly if I'm at his farm. Replenish with fresh and clean on a as needed basis. I get more rain than my AZ friends put together. Never had a mold concern. I'm sure there is some that forms . Not all mold is dangerous. Mold is everywhere and unstoppable. If you have known allergies to mold like the kind that occurs in hay, grass, compost piles, then need to avoid for sure.
BTW talking about alfalfa for chickens to eat. I buy alfalfa pellets intended for rabbits and offer free choice to my girls. I do this during winter when nothing is growing in my lawn. It does not get consumed as compared to other treats, but does go down.
I have given the rabbit pellets to my birds too, and since they already get layer feed in pellet form they don't mind eating it up. I just put a pan out with some in it and it's usually gone by the end of the day. But i found it's expensive, and I can get the alfalfa pellets for horses for alot cheaper, but those pellets are too big so they get soaked or go in the fermented feed instead.