I wondered this myself, until I found out that chickens are "opportunistic feeders" as my husband likes to say. As a new chicken mommy (I've only had my girls for about 6 months), I had no idea how "opportunistic" they really are. One day I saw all six hens fighting over something slimy. I thought it was a worm, but upon closer inspection, I realized it was the entrails from a gopher carcass left in the yard by my cats! Yuck. They truly will eat anything that doesn't pose a threat to them.