Basically, 90% of their nutrition should derive from their balanced feed, and up to 10% can be any other non-feed food, for adult birds. I think I've read it suggested that baby chicks are more like 95/5, but I don't have a hard source for that. It's just that chicks grow so fast, and grow through so many sets of feathers, that good nutrition is really important.
Some think greens, like what they could find free-ranging, should be considered free-choice. One example - scratch grains - should be limited to 1-2 tablespoons per adult bird, per day, to get that 10% figure. It doesn't leave much wiggle room! But then, many think most of our birds are too fat anyway, due to the tendency to over-treat.
