Mine get everything...as I have mentioned before in other posts, cows can get mad cow disease. Chickens do NOT get mad chicken disease. Put a dead cow in a pasture, and al its herd mates will move away from it. Put a dead chicken in the run and the others will come out with forks, bibs, and wet naps to wipe their beaks.
They are omnivores...given the chance if you were dead in the pen they would try to eat you too.
I give mine beef, turkey, chicken, pork etc as well as tonnes of greens and grains and layer pellets. I have actually seen my granddad's chickens running across the field as fast as their little feets would go to get to a deer carcass that was on the edge of the pasture. In fact a few weeks back my husband took me to dinner...it used to be a good place but they are not as good as they used to be...so I took my prime rib home. I ended up giving it to the girls, and they went mad over it. Forget the potato, the apples, the meal worms... oh no there was meat!! Their little birdie eyes lit up like the dinosaurs that they are. And its true... they are closely related to dinos...so why wouldn't they eat meat?
Feeding them chicken isn't going to hurt them... after all many chickens eat their own eggs. I am guilty of serving an over abundance of eggs back to my girls for a high protein treat.
And as my granddad would say "Chickens love to eat chicken...even they know they taste good!! Why you think they go for blood every so often?"