Once quarantine period is over...
People tend to introduce carefully, like in separate pens/cages where chickens can see each other but not touch. Do that at day. Move closer & closer to see if problems arise. If not, put them in at night once everyone has roosted. Waking up together seems...
Read this: https://www.backyardchickens.com/forum/viewtopic.php?id=17696
Peruse the "Emergencies" threads about quarantine - both from people who did & those who didn't. Just yesterday someone posted that she was glad to have isolated 2 new birds because they did get sick.
I have to admit to...
Quarantine
Quarantine
Quarantine!!
Don't let the birds in with your existing flock until you're CERTAIN that they carry no diseases. There are too many stories on the forums of people infecting their flocks and losing a lot of the birds as a result.