Hi, Heather and Smokey, welcome to the forum! First of all, there is no such thing as 'overthinking' when it comes to parrots and kudos to you for doing it!

Second, I LOVE your question! It gave me the opportunity to do research on a subject that I had never done before and I am always grateful when somebody points out something that I had not 'caught' (I am a bit of a know-it-all who loves learning so this kind of questions are right up my alley

).
Now, according to my research, there are two schools of thought about this. Some ornithologists say that birds cannot hear ultrasonic sound waves and some say they either do actually hear them or perceive them somehow (vibrations?) because behavior shows they were affected. Of course, there is nothing (as usual) in terms of scientific studies done on parrots on this subject but there is some data available - and there are a number of commercial wild bird repellents that claim to scare birds away through ultrasonic sounds. I did find one species of bird that produces ultrasonic sound waves (lower than 20 kilohertz) but it's not a parrot. I would assume that the most relevant information would be the level that the humidifier produces and which level each species hears but, again, we don't have that info for parrots.
Personally, when in doubt, I don't do it (this is one of my bird 'mottos'). The thing with parrots is that we simply do not know enough about them. We have only recently begun to scratch the tip of the iceberg in terms of knowledge about birds (with the exception of chickens, we know a lot about them) and, in the past, we have made assumptions about them based on our mammal knowledge that we are now realizing did not apply at all so I always play it safe - just in case. When we first moved into this house, there were mice and getting rid of them in a humane manner (I do not kill any animal except cockroaches, fleas, mites, lice and mosquitos) was a problem so some people suggested the ultrasonic devices that scare them away but not knowing what effect they would have on my own animals, I decided not to use them (I took me years to get rid of all of them using only the humane traps but we finally did it). So, if it was me, I would use an old-fashioned humidifier - which is what I have.
Birds that produce ultrasonic sounds and echolocation study on them:
https://en.wikipedia.org/wiki/Oilbirdhttps://en.wikipedia.org/wiki/Swiftlethttps://www.ncbi.nlm.nih.gov/pmc/articles/PMC3664765/School that says they do hear or perceive it:
https://bird-x.com/wp-content/uploads/U ... ions-1.pdfhttps://www.prweb.com/releases/2012/3/prweb9265604.htmSchool that says they don't (the second link above refutes the below one):
https://www.prweb.com/releases/2012/3/prweb9244262.htmThis is a link to the Avian Hearing chapter on Sturkie's Avian Physiology book but I can't post a link to the actual chapter because it needs to be bought (I have it -you can buy it online- but can't share it

):
https://www.sciencedirect.com/science/a ... 1605000063