No Neil I don't know why either. Fancier use ACV to lower the ph but I never found out why? A pigeons gut ph is 4.80, chicken is 4.41. Nobody has ever said what the benefit is to the bird by artificially lowing it?
EVERYTHING fanciers put in the water WILL lower the ph level. Meds/Vitamins ,Supplements ect. Tap waters ph level will change from area to area, 6.5 and 9.5 so you will need to know what it is before you reduce it.
Wild birds drink from rivers which is around 7 - 8,
A lower ph level does reduce ecoli growth but giving ACV will not reduce it lower than the birds already natural gut level of 4.8?