I have nine 8 week old chicks. Five roos and 4 hens. I want to keep one roo, but get rid of one hen in exchange for another. I know that it's hard to introduce a single chicken to an already established flock, but I figured that if I got rid of the roos I didn't want and introduced the new pullet in the same day, that the flock would be so disrupted that the new addition would go a little un-noticed. When the flock was 4 weeks old, I introduced a new chick (same age) by already having her in the brooder box while the others were exploring the big coop. When I put the flock back in the brooder box, the new chick was completely ignored and started eating with everyone right away.