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My mama hens with new chicks always keep their distance from the other hens -- and run off any hen who comes too close. It seems to be part of protecting her chicks.
There could be a number of reasons why your chicks are sleeping outside. If it is hot where you are, it could be that it's cooler outdoors. There could be something in the coop they are avoiding, like lice/mites, a rat, a snake, etc. If it has little natural light, like no window, and is dark in there, they won't go in. I'm sure there are more, that's just what comes to mind at the moment.
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