Adding 10 week old pullet to pullet flock

[Mikski]

I have a flock of 12 pullets (although there are a couple possible cockerels) that are all around 8-9 weeks old. I want to pick up another 10 week old pullet tomorrow. Will I be ok to just put the new pullet directly in with the existing young flock? She’s at least a week older than the rest of them.

 

[aart]

Maybe, maybe not.
They are old enough to tell gender(most breeds) and also old enough to have territorial instincts.
Adding one bird often makes them a target forever, better to add multiples.

Might also depend on how much space you have in coop and run.

Pests and disease might also be a concern.
Consider biological/medical quarantine:
BYC Medical Quarantine Article


Here's 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/

 

[LaFleche]

I have a flock of 12 pullets (although there are a couple possible cockerels) that are all around 8-9 weeks old. I want to pick up another 10 week old pullet tomorrow. Will I be ok to just put the new pullet directly in with the existing young flock? She’s at least a week older than the rest of them.

In addition to @aart 's advice:

After the quarantine period you can just add one of the existing flock to live with the new bird and see how they get along.

After everything is settled, add another and wait a few days for them to get aquainted. Then add another.

After a week to 10 days they will have formed their own little hierarchy and you can integrate the new little flock much easier to the existing one.