This question comes up often ... "how much apple cider vinegar to add to my bird's drinking water?" ... A while back while I was at the vet's office, I found out that two of my birds have some budding yeast in their droppings. Everything else was perfect (blood, bacteria, physical exam, etc) so the vet did not see any need to use medications to treat this problem. It was not surprising to me, because they both eat lorikeet powder which has a higher amount of sugar in it, as compared to other parrot foods. So these birds often have issues with yeast.
His recommendation was to go back to adding apple cider vinegar to their water and recommended a ratio of 1/2 to 1 tablespoon per cup of water. In metric units, that would be about 15 ml apple cider vinegar in 240 ml water or 30 ml apple cider vinegar to 1 liter of water. I have decided to mix up a gallon to keep in the refrigerator. Using 1/2 tablespoon as basis means adding 8 tablespoons of apple cider vinegar to a gallon of tap water. If I had a bigger problem to deal with, I would have increased the amount to 16 tablespoons per gallon of water.
I use Bragg brand of apple cider vinegar & get it at Whole Foods (also at other grocery stores). It's not very expensive. I think this 1 liter bottle was about $4.00. There are other brands and you can find it at all sorts of markets. This particular brand is unfiltered. Meaning it will be cloudy and contain a fluffy material. This is the bacteria that is used to create the vinegar.
Basically, when you make apple cider vinegar, you allow bacteria to convert the sugar in the apple cider to alcohol. Another type of bacteria converts the alcohol to vinegar.
The vinegar helps to acidify the digestive system in the birds and controls the overgrowth of yeast and other undesirable organisms.