I'm unsure... we often lose the bag of bread (in the cubbard) and typically give it to them when they expire and they seem to love it, but I haven't fed them in excess. At least, they STOP when they're full?
I wasn't sure about feeding them deer meat, until I was told by a lady who showed chickens that it was alright. I've had hens wolf down some serious meat. I try to make sure they actually get some grain into them and sadly, they ignore most of the layer and any crumbled looking feed.
Giving them a deer bone is a treat... makes naughty chickens divert their mischevious minds elsewhere lol instead of the garden.
I'd be worried about feeding them something like peanut butter lol they could choke on it? Or milk... I don't know if chickens can process milk lol and I've heard old wive's tales, don't feeed pigeons rice or they'll explode... so I dont feed the chicken's rice too much, unless it's cooked... I don't want exploding chickens!!! O.O