This is the chick now:
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I never found a cause but it stopped on its own about a week later. I suspect it was environmental, but will never know.
It definitely helped putting him a small brooder with less chicks- whether it was the limited excitement or less competition I don’t know. Now he is a normal rooster in a normal flock. He is a lower energy guy who lays around more than my other roosters, but just like the others, he chases after pretty hens, fights naughty cockerels, and runs away from the dominant rooster without any problem and no sign of being winded or breathing trouble.
I would advise separating as suggested, perhaps with a friend or 2 to limit stress. I personally would not administer an antibiotic at this stage. Monitor the chick and also closely monitor the others. Hopefully it clears up on its own like in Citronella’s case. Sorry I can’t tell you anything more concrete than this. If you haven’t already, you may want to try starting your own thread in the sick chicken forum to get more visibility. Welcome to BYC!