On her back, above her right wing. Just noticed it today, though she has been with her mother and siblings and so we haven't had a chance to observe her too much. Her mother is a witch and doesnt tolerate much messing about with her babies, she's a first time momma and a very good one! She has been taking excellent care of the chicks, but probably partly because of the winter weather, she has lost two. We have had them in a wooden wall enclosed elevated pen with two screen doors, and a heat light on them during the day. We don't think it's a heatlight burn because they haven't been able to get that close to it, and the pen is big enough for them to get away from the heat light if they choose to. We just moved them into a bigger pen today, which is why we noticed the thing on the chick's back. All the chicks seem to be healthy although this one is smaller and not quite as robust as the others. She is eating and drinking though. I have read about air sacs showing on the backs of newly hatched chicks, but obviously this one is older. What would be the cause, and should we be treating her with something?