Fair enough, So just to be clear I should be quarantining the new bird for about a month or so before I have them out together and interacting directly, but it's ok to have the cages in the same room so they can see each other? Or should I be avoiding contact entirely until the quarantine is over?
And what should I be on the lookout for during quarantine?
If going completely by the book (which 99.9% of people don't do) aren't even supposed to TECHNICALLY be in a house with the same air-source/shared h-vac, but most people just put them in rooms as far apart as they are able. PDD, ABV and PBFD can all shed in the air via microscopic dander particles (which may also be transported on the clothing, hair and hands of care-takers). I know it's annoying...You may or may not see anything in an asymptomatic carrier-- the goal is just to separate them when stress=high and viral shedding is most likely. Either of your birds could possibly be carriers of those 3 diseases without showing symptoms (and sometimes even producing false negatives when tested in times of low stress).
The only way to rule out those diseases is through testing, but again testing is not perfect when it comes to ABV, PDD and PBFD (which is why they generally say to test if you see symptoms or if another bird is positive--- but even then, it can take anywhere from 2 weeks to 10+ years for a bird who is going to get sick to test positive post-exposure in some cases, so an exposed bird may not actually have enough of the virus to trip the test, even if they are infected ---that's why it can take a variable amount of time from exposure to know).
You will most def want to test for psittacosis
(Chlamydia psittaci) if your bird has not already been tested. That test is more common among breeders-- so do make sure you have a list of the tests that were already done when you get to the vet. They often won't take blood from a young bird, but they need check-ups at least yearly and blood-work (just a general CBC at least every 3 years after their initial CBC).
signs that you MAY see in birds sick from bacteria, viruses, genetic issues, parasites, yeast etc can include the following (any of the following should be taken seriously)-- even 1 of these symptoms should be watched closely. If you see tail bobbing or cage-bottom sleeping, those are urgent, red-flags--among others in this list:
weird, chunky poop or alarming urates...Lack of urates consistently, diarrhea that lasts
Poop that is red or black (or even neon) that lasts and cannot be attributed to colored foods
repeatedly straining to poop
tail bobbing when breathing but standing still
sleeping on the cage floor when old enough to perch
major issues with balance
changes to vocalization
audible sounds when breathing
nasal or eye discharge
appearing fluffed and sleeping most of the day
excessively quiet
plucking a very specific area (while this can be anxiety or hormones or both, it can also indicate pain in that area)
vomiting
star-gazing (odd head postures with a stare that lasts)
lack of appetite
loss of interest in once-preferred foods
a dirty butt ("vent")
excessive scratching of an area
a strange smell to breath
bald spots
feathers growing in abnormally
Constant shivering
lethargy
changes to the coloring in or around the eyes
open-mouthed breathing
weight loss or sudden weight gain
perching and sleeping with both feet on the perch, despite relaxing (I don't just mean standing there--- birds typically tuck a foot when resting and not doing so can be an indicator that they are too weak to support on 1 or that there could be other issues with balance.
Any visible sores, bumps or abnormal bald patches, along with any feather discoloration.
Crusty nares or inflamed nares
Mind you, an infected carrier will often show 0 symptoms in the case of certain viruses (much like some humans who get covid and don't know it, only the contagious period is longer-lasting, albeit less contagious when they are not under stress)
45 days is the minimum that I have heard of for quarantine, but many vets advise up to 3 months to be even safer