The sooner you do it the better. I would not put a lamp on the chicks and broody. She will keep them warm provided you have a big enough broody and few enough chicks. I had 1 small EE broody who hatched 3 LF chicks and she did a great job keeping those babies warm in 30F weather in an unheated not insulated coop. But I've never slipped chicks under a broody so not sure of the timing. Take away the eggs though when you slip her the chicks. And keep ready to intervene if necessary. My guess is if they are young enough and she's desperate enough to be a momma, she'll take them in like her own hatched.I need opinions please. My 7 eggs hatched within the last 24 hours. The last about an hour ago. I was going to go out tonight and place them under a broody. My dh thinks that because it will be so cold tonight that we should put a heat lamp in there,which to defeats the purpose of doing it at night. SO....should I wait until tomorrow night when they are just a bit older, tomorrow in the AM so I can watch( would she react immediately? ). She is a 10 month old buckeye and this is a first for both of us. My dh is afraid that she will not cover them up and they will freeze to death.
Edited to say, the chicks were hatched in the incubator and not under her.......sorry for the confusion