That's probably a good thing to try first... Then Maybe oral anti-fungals. I just found this:There was recently a thread in this section using monistat for the problems you're dealing with. Try using the search box and see if you can find it. It might save you time and money. Good luck with your birds.
http://www.exoticpetvet.net/avian/budgietiel.html
Candidiasis
Yeast infections with Candida can occur, especially in birds that suffer from hypovitaminosis A. Many bacterial and yeast infections in budgies and cockatiels occur as a result of owners allowing birds to eat out of their mouths, or if they allow a bird to have contact with human saliva. Our mouths contain many microbes that can cause disease in birds, and contact with the human mouth should be strictly prohibited. I never use Nystatin alone anymore to treat candidiasis. Instead, I use a combination of fluconizole, 100 mg. crushed and added to 20 ml. of Nystatin, 100,000 U/ml. Dose at 0.5 ml. per 1000 gm. bw PO q 12 hr., and may be doubled in severe cases. This works very well for baby cockatiels with candidiasis, and 5 days of treatment is usually all that is needed. For prevention of candidiasis, when baby birds are on antibiotic therapy, this mixture works very well. It should be shaken prior to administration and should be stable at room temperature for six months.
It was written for small birds, but the same could be done for poultry.
-Kathy