I'm one of those "better safe than sorry" people.
I'm a mom, and one thing I learned quickly was that when your kid gets exposed to other sick kids it doesn't simply build up their immune system and make a tough kid, it causes them to get whatever the other kid has 9 times out of 10 and makes them feel like crap. And it never seems to help them avoid getting sick in the future. Not fun for the kid, not fun for the parent dealing with the sick kid. I'm quite sure the same is true of parrots and other birds. I'm going with my gut on this one, sorry
There are so many bird diseases out there, and I highly doubt there isn't something that could be transmitted to parrots even if they aren't normally from the same area. And while socialization is important, that can be done within our own homes in our own way from the start. A just weaned baby is far from over the learning limit and be taught these things by us. As for the outside birds coming into outdoor aviaries, since most people cover them with some sort of roof doesn't that make it less likely contamination would occur? The difference to me is that when I see wild birds of all sizes in the wild, they fight and get into scuffles where nails and beaks get dug into both birds. I live in the country so I see this a lot. I see little birds attacking birds ten times their own size mid-flight. Exchange of disease is far more likely in that case then one where a little bird perches on the fencing used on a covered aviary outside. Call me over cautious if you will, but I'd rather start out with an unsocialized baby free of potential disease and do the socialization myself.
Please be advised there is a curse word in this video, but it shows how easily other birds can attack something even as large as a macaw while flying:
Krestrel hawk dives at Camelot macaw - YouTube
If you are one of those "better safe than sorry" people then you will not get a baby parrot that was forced weaned, not fully fledged, not extensively socialized (and yes, once the parrot is weaned, even forced weaned, critical socialization windows have been missed than cannot be regained) and not allowed to interact with older/adult parrots during its development period. There are literally tons of articles available addressing how all of the above is
critical to the psychological development and long term mental health of large parrots. No article has said that you can't get a mentally healthy parrot who has been raised with the above practices, but it is harder and the chances are exponentially decreased when large parrots are
deprived of these things.
Do I think you should get a bird from this guy? Not if your gut is telling you no. But I do think we, as buyers, should be supporting breeders who are actually setting their babies up for longterm success. Screamers, pluckers, self-mutilators, are not born, doesn't happen in the wild, they are created and while it can be from a crappy adult life I've seen enough in fantastic homes, providing
everything they should have, to know that what happens in the absolutely critical fledging stage, when neurons are proliferating to an unprecedented degree, can have a very real detriment on longterm stability.
And is more of a lifetime risk than the off chance of catching some vague disease by being outside.
There are studies done on human children, if deprived of basic, biologically hard wired, rearing practices when small infants you cannot make that up. Period. You can certainly get improvement, but they are damaged. They've been recreated with an assortment of animals, including some avians, and the results are the same. Deprivation during a time when they would naturally be establishing lifelong routines, thought & behavior patterns cannot be reversed.
I can point to literally 100s of experienced, longterm, established members of the avian community who free fly their parrots and do not have a single case of disease caught from wild birds. Established, respected breeders (like best of the best) who have birds they allow to fly freely and undirected around their homes....for decades. Once again no diseases. Cockatoo Downs has parrots living free, outside at all times and zero "wild caught" diseases.
Some people with aviaries certainly have covers, but most home aviaries don't, as they are not permanent housing and roofing is expensive and also requires a different support structure. I've never seen a breeder with outdoor setups where at least half the aviaries are not uncovered, including M&D. Many zoos now have walk thru aviaries with a simple game netting strung across poles. They don't have rampant diseases. My zoo has vast flocks of roaming swans, flamingos, peacocks, and other exotic birds that mingle daily with the various wild bird populations. Once again no bird diseases being transmitted. They treat for lice and then parasites occasionally, but those parasites are very likely from the ground and not from bird-to-bird transmission. My vet works at the zoo.
My baby macaw came from a well-known breeder who is doing as many of the "better" rearing techniques as anyone else. The difference between a baby macaw from such a breeder (and the later adult from such a breeder) and babies available from any number of the larger scale "farms" is so stark it is actually a little shocking. When you are used to the "farmed" babies (which is what most people have only experienced) you don't even realize how different those babies should be had they been raised with different, more biologically appropriate techniques. It's like the difference between a puppy-milled dog and one bred and raised by Guide Dogs for the Blind.
TL;DR If you (general you) believe that buying a parrot from a mill is safer, better or wiser, by any measure (psychologically, physically, medically, etc) than getting one from a carefully sourced, conscientious small scale inhome breeder you are sorely mistaken.