I agree it is a personal decision. Quarantine is a great tool when used correctly but let’s look at what quarantine accomplishes.
Flocks, herds, or packs (any group of social animals) can develop flock immunities to certain diseases and parasites. With chickens Coccidiosis is a good example but there are others, some of them a lot more serious than Coccidiosis. The animals can be carriers and infect others but won’t show symptoms. You can quarantine these animals all you want and not be able to tell if they carry the disease. As with everything there is an exception to that general statement, on occasion with certain diseases the stress of relocating can weaken them enough so they will show symptoms, but that is pretty rare. What quarantine is really looking at is diseases that the animal has recently been exposed to. It’s extremely valuable for chickens coming from chicken swaps where they have been exposed to a lot of new chickens or say cattle from an auction barn. If they are coming from a closed flock or herd that has not been exposed to new animals quarantine is unlikely to show anything. Does that breeder take chickens to chicken shows where they are exposed to lots of other new chickens or bring in other adults? Or do they have a closed flock where they are not exposed to other chickens?
One way to check for these “hidden” risks is to select a potentially sacrificial chicken from your flock and house it with the new chicken for a month. If one of them gets sick there is a pretty good chance flock immunities are involved. It could be your flock infecting the new chicken.
The more isolated the new chicken is the more effective the quarantine. Diseases can be spread by sharing food, eating each other’s poop, sharing drinking water, or by air. Marek’s can carry a long way on the air. An effective quarantine means housing them for enough apart that they never breathe the same air. You need to change clothes (at least shoes) when you go from one to another, and use different feed so you don’t track or carry diseases from one to another. The better your separation the better your quarantine. Just doing part of this has some benefit.
If your chicken is coming from a closed flock and that breeder would recognize a disease if they saw it, the chicken has essentially been through quarantine. The most likely things the new chicken would bring in anything if they do at all is something like mites or lice, maybe worms, from some wild critter but the same is true for your flock. If the breeder is any good they should know if their flock has those and if they have integrity they should be willing to tell you. These are treatable and are generally nuisances, not life threatening if you treat them, but they are a nuisance. Yes, it is possible they could have some devastating disease that could wipe out your flock, you are dealing with living animals, anything can happen, but the odds of that are in my opinion really low. I would not worry about that at all, especially since you already have some chickens from that breeder.
If that breeder does not keep a closed flock then all this changes and the risks go way up.
As I said, a personal decision. I certainly cannot make that for you. Good luck!
