I watched my mare teach her colt what to eat and what not to eat. Animals--even domesticated ones-- rely upon their instincts every day, rather than suppress them, as do humans.
Any older bird can/will show them what to eat/avoid even in a passive way.
That's what I think.
It's common sense: stay away from late instar insect larvae because they have lots of chitin which is extremely difficult to digest, stay away from too many commercially-produced grapes that provide not much more than water and maybe a few chemicals of a dubious nature, as the most worthwhile component, the seed, has been removed through the practice of plant breeding .....
Keep them hydrated and free of temperature extremes.
I recall from Animal Science 101 (seriously) that chicks will grow in Direct proportion to the quantity and quality of protein that is fed to them. Unfortunately, the endpoint of the study was, .....enough said.