Insisting on providing incorrect information which may get someone else's chicks killed is not helpful. I am removing myself from this conversation now.
Chicks that young cannot regulate their own body temperatures well enough to 'use each other as a heat source'. Down doesn't hold enough heat in for them to maintain a proper temperature which is why, without a Hen you need a source of heat in your brooder. Cold chicks will die much more easily...
Hi Josip! You will need some form of heat for them until they are around 4 to 6 weeks old. It doesn't have to be a light. Some studies have shown that a clear bulb light can cause chicks to be more aggressive than a red bulb light but I can't remember why right now. If you search this website...