They are omnivores...if it is edible and they are hungry, they WILL try it! I have seen chickens head first in a deer carcass going to town on the meat and flies <this was a somewhat fresh kill btw>. I have also seen them go nuts over hotdogs and McD's cheeseburgers lol They LOVE meat... I think it is an instinct in them to eat protein...othewise why would they go nuts over catching bugs, frogs, lizards, and mice?
I would never give a vegetarian animal <ie cows> meat, cos they were not meant to eat it. Chickens tho? Yup...I give them meat scraps and they love it and have had no issues. As my grandad used to say... "Put a dead cow in a field and the other cows stay away. Put a dead chicken in the coop and the chickens will get their knives and forks. It's how they are put together!"
If they were w/out humans to feed them... you can bet they would be eating anything they could catch....or come across. I have yet to see a chicken who chose to be vegan!