Well, chickens are little pigs with feathers, so when they're growing they eat a lot. Once they reach mature sizes, the amount of feed they consume stops growing exponentially. And if birds range freely, they do eat much less feed.
They get lots of niacin (bugs have niacin) and protein and other good stuff, which is healthy for them.
It's simply amazing how bug-free my back yard is, now. I have to put boards down and leave 'em there for a couple days, then turn 'em over to even find bugs... practically.
Oh, I can still find earwigs in rolled up newspaper or under boxes and other hidden places - but earwigs are like candy to chickens! (Except that they're better for 'em than candy is for us.)