It is kind of funny to see all the comments about whether it is OK to feed chickens this or that. The better question is probably what wont they eat?
When we moved to a home with a 5 acre lot and the chance to have chickens we had also planned to have a compost pile --- we have nothing to compost because the chickens eat just about everything. We will chop up some of the veggies to make them easier to eat, but everything goes to the birds.
Last holiday season I was helping clean up after the office pot luck. There was a ton of leftovers that no one wanted to take home. I got a box and lined it with trash bags. We put all the leftovers in the boxes. Everything from salad to pizza to potatos, and desserts, etc. went in. One lady was afraid to throw in a meat gravy that had ground meat in it -- I told her to add it to the pile. The chickens went crazy and with it being winter they worked the pile until every single scrap was gone.
My chickens love watermelon and pumpkins and will eat them until they are just a shell. They eat onions if they are diced. About the only thing that they leave alone are avocado seeds - probably because they are too big. They also seem to love shrimp tails. When I got a deer I had the trimmed out the rib cage and with the chickens running free, they discovered that they also liked venison and were hitting the few scraps of meat.
So, what don't chickens eat?