Age doesn't matter, they can start being territorial at just a week or two.
How long was the injured separated?
Was she separated within sight of the flock or out of sight?
That can make a huge difference.
Out of sight will make a stranger of the injured,
while being just separated by wire much less so.
You might be able to just put her back in....or that might not work so well.
With a such a large flock, and hopefully
lots of space, it might work just fine.
You could try it and see, but be ready to intervene if she gets beaten too badly.
Places for her to hide would help a great deal.
You might need to put one of the flock in with the recovered bird,
if they get along, put in another one or two,
get a good half dozen to form a group then add them all back into the flock.
Do this over a couple few days....Chicken Juggling!
This might help:
https://www.backyardchickens.com/articles/introducing-a-single-hen-to-an-existing-flock.71997/
So might this.
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.
This used to be a better search, new format has reduced it's efficacy, but still:
Read up on integration..... BYC advanced search>titles only>
integration
This is good place to start reading, BUT some info is outdated IMO:
http://www.backyardchickens.com/a/adding-to-your-flock
**Edited by Staff**