I find the best age to integrate is 2-3 wks....they aren't so small they can't escape the big birds and they aren't so big the big birds see them as food competition. Especially if you have a free range setup, as the young chicks hang near the coop and the older birds are outdoors most of the day. I've never had a chick injured by integrating at this age and never even saw any overt aggression towards them at that age and they all eat out of the same trough together.
spoken from someone who has experience that I often listen too. Thanks for the suggestions on our young cockerels. So far things have gone as I had planned and hoped.
