Sorry for the delayed reply! So what you could do if you don’t have a great heat source is have them out in the coop during the day and bring them in at night.
You could also block the coop in half, set it up like you would for the babies and put a thermometer in there and check temps before you actually put them out.
I hear ya on the heat lamps, I won’t use one. I have a heating pad meant for brooding
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I put that heating pad into a little bunny hutch
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And then the hutch inside the half of the coop blocked off for them, that way they have a nice enclosure that is warm but can come out of it into the coop if it’s too warm I wish I had a pic but I don’t and I don’t even have the same coop any more to show you dang it.
Hopefully that all made sense!