The bones should be fine as long as they're raw. If you're going to cook the bones, cook them down enough (or can them) so the bones get soft - not just until the meat falls off the bones. If the bones are raw and the birds aren't swallowing huge bones, they will grind them up just fine.
Crush the heads and stuff with a hammer to make it more accessable - maybe even in front of them, it might peak their interest. The only real concern I would have is that some fish around here carry tapeworms - can chickens get tapeworm infestations? It's only a few kinds of fish, so it's not a huge deal.