Introducing Chickens
Chicken math has stuck! Now you have older chickens and new younger chicks. How do you introduce them together?
Chickens can be kind of mean to newcomers. So I find the playpen method to work the best. You do this by setting up a separated spot for your new chickens. This can be a fence or dog cage or little chicken run. Have the chickens meet each other through that barrier so they can get to know each other without anyone getting hurt. This takes time and every chicken introduction can go differently. It usually takes 3-14 days.
Once the chickens are ignoring each other. Let the new chickens out. watch and see how they do. A little pecking is normal but extreme aggression I would separate them again then wait a couple more days. Things to help ease the transition, set up multiple feeders and waterers, set up more roosting spaces, add more space if possible, add things enrichment like little feeders around or a flock block.
The two main contributor to aggression are space and diet. So as long as you have plenty of space and the chickens have access to feed, the chickens will eventually get used to each other. Be patient