I have several chicks that I incubated and hatched that are four weeks old tomorrow. There is one chick that is much smaller than the others (same breed) and seems to be breathing heavier than the other chicks. Not open-mouthed or in distress, but it just takes her a lot more effort. Her whole body moves as she breathes. I haven’t separated her and none of the other chicks have shown any symptoms. She has been about the same for at least a week or two - not getting any worse or better. Pretty sure she’s eating and drinking ok. She may stay under the heating plate more than the others.