They provide necessary nutrients in the water bowl for bacterial growth to occur. If you have to use vitamins in the water, change and disinfect the water bowl every day. So you see, you fee your pet bird bacteria every day. So Cecil gets a gram stain probably every 3-4 months. Gram stains differentiate between gram positive and gram negative bacteria.
Gram positives are the good guys and gram negatives the bad guys when our birds are concerned.
This tells me what bacteria are present and the exact best antibiotics with which to treat this bacteria. Birds can carry bacterial infections for years and still appear healthy. Many people look at me with disbelief when I make this statement. Look at it historically. Dogs and cats have been domesticated since before Egyptian times.