I don't necessarily worry about respiratory - anything is possible, especially with wild migratory birds. Damp conditions I think of worms and Coccidia. I take up all feed daily, but spilled feed can mold fast, so I always look for that - mold will take them down.Like havoc said about it if all else is kept up. I'm one that unless I had to I wouldn't touch antibiotics especially for respiratory Havoc's knows I'm against such I'd simply cull but if it came down to my brood stock I'd probably have to try everything I could. I'm a worrier crap like this is always in the back of my mind lol.
Now mine are for the most part under cover and dry, but there is still mucky areas that have no roof (metal) and you know how chickens are, they got to get in there and mess about.