There does not seem to be much scientific study going on about ACV, possibly being that is has been around such a long time. Medical folks tend to concentrate on the newest of drugs to cure things.
But I did find this site, not much here, but in the middle of the page there are some studies that have been done, however not necessarily on gamebirds. However what works on humans may also work for the avian species. I think ACV is highly underrated and needs more study to be done. Right now I am doing my own personal study by using it on a sick chicken. If only I knew exactly what she has that is really ailing her, as if the ACV does cure her, it would be a perfect study and proof that ACV does in fact cure illness.
Those two sites may help some for folks wondering. I always am a ACV fan due to the fact that from personal experience my birds don't show problems and I use it.