If anyone had told me how big the girls would be in four weeks I'd have fallen on the floor laughing...but from fluffballs 12 hours old on arrival to this? They outgrew our brooder at two weeks, good thing the coop was ready. We have a lamp in there but have used it only one night. Astonishing. They're sweet birds, very friendly, probably production reds with RIR heritage. They were 'mixed bin', so I can't be sure yet.
Wait until you get a really big heavy breed. At 3 weeks old my orp boys are larger than a quart jar feeder.