When can i put my 15 week old chicken in with my full grown rooster?

[shannon84]

Got a 15 week old today at swap. When can I put her in with the big chickens?

 

[Blayto012]

I don’t think you can, or should. At least not for a few weeks to make sure she doesn’t have anything that could infect the rest of your flock. Also, they could really injure her if she’s not slowly introduced. I would put her in a pen around your flock at minimum for several weeks. I’m no expert, but introducing new chicks has been tricky for me and has taken up to six weeks of slowly introducing before I could safely put them all together. Good luck with your new chick

 

[aart]

Probably too late, but....
Consider biological/medical quarantine:
BYC Medical Quarantine Article

Integrating a single birds is the hardest integration.
How many other birds do you have and how much space do they have?

This might help:
https://www.backyardchickens.com/articles/introducing-a-single-hen-to-an-existing-flock.71997/
Then here are some tips about......
Integration Basics:
It's all about territory and resources(space/food/water).
Existing birds will almost always attack new ones to defend their resources.
Understanding chicken behaviors is essential to integrating new birds into your flock.

Confine new birds within sight but physically segregated from older/existing birds for several weeks, so they can see and get used to each other but not physically interact.

In adjacent runs, spread scratch grains along the dividing mesh, best if mesh is just big enough for birds to stick their head thru, so they get used to eating together.

The more space, the better. Birds will peck to establish dominance, the pecked bird needs space to get away. As long as there's no copious blood drawn and/or new bird is not trapped/pinned down and beaten unmercilessly, let them work it out. Every time you interfere or remove new birds, they'll have to start the pecking order thing all over again.

Multiple feed/water stations. Dominance issues are most often carried out over sustenance, more stations lessens the frequency of that issue.

Places for the new birds to hide 'out of line of sight'(but not a dead end trap) and/or up and away from any bully birds. Roosts, pallets or boards leaned up against walls or up on concrete blocks, old chairs tables, branches, logs, stumps out in the run can really help. Lots of diversion and places to 'hide' instead of bare wide open run.

Good ideas for hiding places:
https://www.backyardchickens.com/threads/a-cluttered-run.1323792/