I just did this a few months ago. The younger the chicks the better. I got mine the day the arrived, so they were 1-2 days old. I kept the chicks warm with a heating pad (the kind you use when shipping) then after dark I went out and exchanged them for the eggs as quietly as I could. It went perfectly. All you can do is hope for the best and be prepared to deal with them if she doesn't accept them.
I grafted on 3 chicks to my broody who had been sitting for 4 weeks (1 week trying to break her and 3 weeks on what ended up being dud fertile eggs) Younger the chick the better. The hen will treat them as if newly hatched and not expect them to be up and around for at least a day and even 3 day old chicks are going to be mobile and independent lots which might give he mothering instincts some grief. It was funny to watch my first time broody accept those chicks (done after dark) and tuck them in while I removed the eggs. She held on to them for about 6 weeks then returned to the flock they still hang with her but she doesnt watch over them or call them to food anymore. My boy (Pita Pinta Cockerel about 30 weeks old) though does watch out for them which is really nice. I cant say enough good things about Pita Pintas. (Boys and Girls both)
Edited by caychris - 9/25/15 at 8:42am