Cedar shavings are bad for many small pets like rats and hamsters. Cedar gives off the scent from the cedar oil that can cause respiratory issues. It's not a stretch to think that it might not be good for chickens as well. Think about it: Cedar is used to make products like cedar chests and closet items meant to kill/repel moths and such.
Hold a handful of cedar shavings to your nose and do the same for pine.....BIG difference.
There is also a difference between the eastern and western cedar species.
Eastern Red Cedar is the highly aromatic one.
Additionally, it matters how old the cedar shavings are. I'm not worrying about a couple cedar saplings in my tree-trimmer wood chip pile that's been aging in my yard for 2 years. I wouldn't use the sawdust fresh from my neighbor's sawmill.
I'm asthmatic so it's it's strong enough to bother me I assume that it's going to be a problem for a bird since a bird's respiratory system is even more delicate.