sardines are not an animal by-product. Where in the relevant legislation are they banned? and why?
They contain lots of good nutrition, including significant quantities of protein, calcium, and all the amino acids chickens need.
Meanwhile commercial operations can dump the cheapest by-products they can find at any given time into their feeds, and given the inability of anyone to identify those ingredients from the homogenized mass extruded, go unchallenged. We now recognized that highly processed foods are bad for us, so why should we believe that highly processed feed is good for our chickens, who are not (in most cases of people reading this site) being kept in food factory conditions, for the most productive and shortest time?