Cedar is the one it's often recommended not to use. It's not a matter of poison but of "fumes" which can be irritating to their respiratory tract. However -- actually any "smelly" wood can be irritating, including pine, and many, many people on BYC use pine shavings in their coops routinely. The trick is that the coop (or brooder or whatever) needs to be well ventilated. Actually, some people use at least some cedar, knowing the problem. I use hay and pine shavings, and when I had a couple of bags of cedar shavings given to me, I used them as well -- in small amounts and with ventilation to the point of a breeze actively blowing through the coop.