My daughter asked me this, and I of course had no clue. My "chicks" are 9 weeks old, and though they still peep like chicks much of the time, they look like chickens. Dual purpose breeds mostly, except for one dark cornish rooster. They have about 180 square feet of run to keep active in, they get free access to starter/grower feed, plus a few small meal worms apiece in the morning, and sometimes assorted fruit and vegetable scraps, and the bugs they find, but no scratch. How do I tell if I need to stop the free access and limit their rations? What will an obese young chicken look like?