Can you try putting the chicks with the broody, and see how they do? I would offer some electrolytes in the water. Some water added to a little dish of chicken crumbles and along with the usual dry crumbles would be good to get fluids into her.
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Given that there are 7 chicks, and they are 3 weeks old, and the weather is hot enough to be causing problems for the hen, I think there is a good chance the chicks will be fine without her to keep them warm. Of course they need shade, and some shelter in case of rain or heavy wind, but you probably already provided those things.do you think with this warm weather the chicks will be fine in their usual coop at night?
That definitely would help!How warm is it? Can you give temps in both C and F?
And I will add the usual warning, to provide plain water in addition to the water with electrolytes. (Electrolytes can be helpful. Too much can be harmful. For situations like this, chickens usually make good choices when they have both options.)I would offer some electrolytes in the water.