It has made me so sad over the past few days to hear of peoples frustrations with sick birds, to the point of wanting to give up before they have barely begun.
I fully understand the addictiveness of hen-keeping, especially in the beginning. You plan for 3, in a week or two you decide you want a couple/few more... please, please before you go out and buy more consider if you have the facilities and space for quarantine.
New hens should be quarantined from your existing flock for at least two weeks. They will need a separate pen and house, either within your exisiting run, or in a separate run all together.
One of the main risks of bringing in new birds is bringing in new disease. Coccidiosis is a latent disease. Once your birds have had it (and it is possible that they may catch it and not necessarily show symptoms) they have it for life and become carriers. Now although people talk about coccidiosis often, they do not often say that there are many, many "strains" of coccidiosis, and though your birds may have immunity to one strain this does not mean they have immunity to all strains, so the likelihood of your birds coming down with cocci upon rapid introduction of new birds may be quite high.
Mycoplasma is another disease which is very readily transferrable between birds. It is responsible for a lot of the runny noses and bubbly eyes you see with a "cold."
Hens are very prone to being "stressed", moving from one place to another, being shut in a box, maybe a car journey etc are huge stressors and therefore for a few days or even weeks your new hens may appear, quiet, withdrawn, depressed, show little interest in food or water.
During this time I would keep a very basic diet, so layers pellets or mash (for POL or older hens) with a vitamin tonic in the water, but no "treats." If your chooks are realy not eating mashed up hardboiled eggs mixed with a little tinned beef or fish catfood will often go down well.
It is during this period of stress that any latent diseases may flare up and cause problems, which is why it is so important that the new birds be separated from the exisiting flock.
If you do not have the space for a quarantine, have you thought about what you would do if one of your birds gets sick or injured and needs to be separated from the main flock anyway?
While you have two separate flocks, you should ensure hygeine control. i.e. disinfect hands, change clothes and shoes before going from one flock to another... yes it is hard work and time consuming maintaining a proper quarantine, but we are trying to maintain the health of your birds!
This is not meant to "get at" anyone, but there seem to be a lot of new hen-friends one the board at the moment, so I just thought I'd mention this to try and avoid the frustrations and sadness we have seen over the past couple of weeks.
Katrina
