Different diseases and parasites spread different ways. Some float in air, some are spread by drinking from the same bowl, eating each other’s poop, or just being in the vicinity. The more you can isolate them the more effective your quarantine is going to be. If you let them roam the same ground, even at separate times, they are still going to share poop. If they can get next to the pen when roaming the yard, you really don’t have much in the way of quarantine. If you don’t change shoes or use different feed and water buckets for each group, you could be the one transmitting pathogens. A lot depends on what diseases or parasites they have, if they have any at all. Just do the best you can, you can’t do any better than that.
Quarantine is an effective tool but it has its limits. A flock can develop flock immunities. The entire flock is infected and are carriers but since they are immune they won’t show any symptoms no matter how long they are isolated. The stress of relocation may weaken the immune system enough to let some symptoms show but not necessarily. Coccidiosis is a good example but there are others. Either flock could have a flock immunity. It’s possible your flock could infect the newcomers.
Quarantine is mainly checking the birds for any disease they have just recently come into contact with. If your new chickens are coming from a flock that has not come into contact with any other chickens for the past month or more, quarantine is not likely to do you a lot of good, especially if the person keeping that flock would recognize a disease if they saw it and would admit it. Quarantine’s strength is checking animals that have come from a chicken swap, auction, or a flock where new birds are often being added. If the birds have been isolated for a month or more, they have basically already been quarantined.
One trick to check for flock immunities is to take a potentially sacrificial bird from your flock and put it with the newcomers. If that bird gets sick, then get rid of the newcomers. If the newcomers get sick but yours doesn’t your flock is the one that has the immunities.
Lots of people are like Free Feather. They bring in new chickens without quarantine and do OK, maybe the worst thing is occasionally having to treat for mites or lice. Maybe not even that. It is certainly possible you could bring in something that totally wipes out your flock. That doesn’t happen that often but it does happen. It’s a risk and how you approach that risk is a personal decision.
Good luck!
