Introducing new chickens straight into flock?

Hi Andy, just a couple of questions you should ask yourself first...

1) Are your older/original girls vaccinated
2) Will the new girls be vaccinated
3) Have you every had any sniffles/bubbly eyes with your original girls?

Just let me say, that I'm a great believer in quarantine no matter what the vaccination status is of your birds old and new, just to be on the safe side.

If you mix vaccinated with unvaccinated (either way round, doesn't matter) you risk the vaccinated birds actually being carriers of the diseases they were vaccinated against, if the vaccine was live or attenuated. (Actually this is more likely if you are bringing in newly vaccinated birds to an unvaccinated flock).

Mycoplasma infections have become almost endemic in backyard flocks, because the only real way of removing it is to cull the flock, let the ground rest and bring in new birds, and most back yard keepers don't really want to do that (understandibly).  Mycoplasma is very easily passed from bird to bird, and once a bird has been infected they can't get rid of it (a bit like humans and herpes/cold sores).  It lays dormant until the bird is stressed and then you see a resurgence of symptoms (which can range from very mild to severe).  Even during the dormant phase, the birds can shed disease particles which then infect any new comers, and so the cycle continues.

So the main reason for that waffle is... if your older girls have had any signs of respiratory disease, they may have mycoplasma, and it will almost certainly be passed to any new birds which are introduced.  While it doesn't seem to effect egg production in any way (unless there is active infection), it can be debilitating at the time of active infection.  So you may want to consider keeping a "closed" flock and not intorducing any new birds until the old ones have died/been culled, or take the risk of passing on the infection to new birds.

Of course if your older girls have never had any sign of respiratory disease... my waffle is meaningless

Personally I would never put new birds directly in with old ones, no matter what else I decided.  I would always have them sectioned off with a cage/fence etc for a couple of weeks so the old ones get to know the new ones first.  Hens can be pretty vicious and can even kill another if the squabbling draws blood.

Thanks for the thorough responses!

Hmmm I know my old birds are vaccinated as will the new ones but I take on board the unpredictable nature of establishing a new hierarchy. I guess I'll somehow have to divide my run and let the new girls find their bearings in piece to start with, might have to buy some netting from the garden centre or something and divide the inside of the run for a while, god knows where they will roost tho... will have to develop some kind of covered roost as I only have one coop.

Thanks folks