It looks like you already have a separate run area, is that your Rooster? That looks like an ideal place if you covered it. Even blue jays or crows will take chick.
Anyway, an idea now is to give them a box or something to go in at night in their brooder and give up the artificial heat source. Put some bedding inside. Train them to go in at night by putting them in their box. They won't like it at first. But this prepares them to be in outdoor temps and where to go to get warm. And you can move the box outside with them to until they're integrated into the flock. Summer and into fall is your perfect time to get them 70's is not going to harm them.
If you set the box open on it's side, pull the side flaps out and rest the top flap on them, add some tape to hold it and it makes a nice little porch. The bottom flap can be pulled up/in to close the entrance when you put them in. You can kind of see it here. I kept puttimg them in even once they were outside until they went in on their own.
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