The hen will overheat if we add a heat lamp. When she had 65 chicks we added a heat lamp to the other side of the 8'x8' brooder, but with 21 she can fit them all under her. We never add chicks that are more than 5 days apart in age. If they hatch late, we will move them to another hen. At 3 weeks, or fully feathered wings, everybody goes to a mobile brooder. At 4 weeks, or mostly feathered bodies, they free range all day and sleep in their mobile brooder at night. We give them hay or wood chips to nest in and conserve body heat. We are in north Florida, 70 degrees at night. We have a windbreaker for the brooder boxes, as we have found that wind will kill chicks more than just temperature.I’m assuming their area is well heated right? We made the mistake of introducing a batch of one week old chicks we incubated to the hen on the day that the eggs she was brooding were hatching. We lost 5/8 of the newly born because the hen was distracted by the one week old. Somehow the newly born chicks got chilled even though we had the heat lamp.
. Just in time for the first batch of chicks to graduate from the brooder with their mama. Now this batch has the whole brooder to themselves.