Sneezing hen and chicks
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Chickens can sneeze due to dusty conditions, dust in feed, poor air circulation, mold, and with some respiratory diseases such as infectious bronchitis and MG. If they start to look lethargic, have more symptoms, or stop eating, you might want to use an antibiotic such as oxytetracycline or Tylan. I would let the hen decide when to leave her chicks. Here is a good article about common diseases including MG and IB: http://edis.ifas.ufl.edu/ps044
Just make sure that the feed is not too dusty or powdery, or that it doesn't have an "off" odor or color which can indicate mold. I have used some store brands of feed, and even name brands which occasionally were too powdery. You can add water to them (which the chicks usually love) and make it less dusty. I accidentally bought a bag of food recently with 3 fresh bags, that was dated 3 months earlier, and it was moldy. I didn't realize it until the chickens ate some. That can be dangerous.
The chicks are old enough to fend for themselves, but most chicks with stay with the other chicks in a group. I like to raise my chicks in with my other chickens. That way, when the mother hen walks away from them, they are already members of the flock and are accepted (although kept in their place.) If she is pecking them, you should let them out of the pen.