I've had this question posed to me before. And with an unfortunate listing on a webpage (where we were not asked for authorization of what got listed - don't worry, this has been resolved) our farm was listed as feeding ONLY grass to all our animals.
That just is not true. And that is NOT healthy for birds. Yes, chickens DO eat grass. We find it in their gizzards all the time. It's fine for them to eat grass. But it is not fine for them to only eat grass. They are not ruminants, this does not provide them with the nutrition they need.
So why do people have this impression or ask me this question? The biggest reason I see for being asked if my chickens ONLY eat grass is an emergence in food allergies. Apparently folks who are sensitive to grains, are also sensitive to chicken who also consume these grains. This has given rise to more and more questions about if I raise chickens on grass only.
Chickens, especially my young, growing meat birds, need a high protein content in their diet. Grass does not provide this. They do also eat bugs, but I would have to grow my own bugs to ensure there were enough for the birds to get the protein they needed. Just, not going to do that, folks. I'm getting the creepy crawlies considering it, and considering the quantity of bugs I would need to make available DAILY for the batches of 100-150 chickens I raise at a time. And I imagine I have to feed those bugs, too, whether they be worms or something else.
Since feed is readily available and formulated to meet the vitamin and protein needs of my birdies, they get free choice of this and all the bugs and grass they can want.
If you don't believe me, I have found some interesting articles online that reiterate my point. And folks out there, if you are putting your chicken out in your backyard and NOT giving it some sort of feed - this is not healthy for your bird. As this article states, grass and bugs usually only contribute to about 10% of their dietary needs! (And as someone who processes birds for other folks, I have seen birds that have been starving to death, it's not pretty. Please feed your birdies!) Do you really want to learn more about bird digestion, check out this detailed and graphic article discussion avian digestion systems.