I pet sat for a lady whose birds were on 4 T ACV to one gallon of water. Refrigerated and used as her parrots' permanent water source. Prescribed by her avian vet because her 2 conures were very prone to giardia. Can't imagine why because they were on an excellent diet. I don't know how the birds were able to drink that much ACV, but they did. Her birds were in excellent feather and shape.
When I get a new bird, I always put it on ACV for at least 2 weeks. 1 T to 1 gal water. If the bird has a bacterial infection, fungal infection or giardia just starting that makes a condition in the gut where it can't reproduce. This worked for me with a bourke with diahhrea too. Cleared it up in 3 days.
But if the bird is really sick, it will need an antibiotic prescribed by your avian vet....
When I first got Luna, her gram stain came back positive for gram neg. bacteria. So the vet and I talked and our solution was to use ACV and water to do just that, make her body inhospitable for bacteria. Worked like a charm!