I'm sure there are reasons for not feeding ground up animals as animal feed to your cattle or sheep etc, but birds will eat meat if found, though it's usually bugs, the odd mice etc.
As tuna isn't usually fed on chickens, I'm happy that my chickens are not eating animal by-products that can harm them. But, might have a google tonight and see if I can find the actual guidelines on animal feed.
I'll let you know what I find out Parson.
Best wishes
Christine