Quarantine is important because regardless of initial health, the birds have microbes that their bodies are used to, but the other group of birds may not have been exposed to, which could cause sickness in either group. When we quarantined last, we were in a small one bedroom apt, we had the new bird (My tiel Ruby) in the bedroom, and the others in the living room. We got her vetted, so we knew she had no diseases (testing is SUPER important, people have lost birds to PDD and other diseases gone undetected when mixing flocks without vet check ups and disease testing) and then for three weeks kept her in the bedroom, changing her dishes in the bathroom and giving her flight time in the bedroom. It gives the birds time fir their systems to get used to the microbes in the air in the new environment without added stressors like getting along with flock mates. It's a big deal, seriously.