Hi, I'm new on this site. I joined because on of my cockatiels has been diagnosed by an avian vet as having megabacteria. The poop test showed negative, but based on symptoms, she diagnosed him, anyway. He had an antibiotic shot, and has just finished a week of twice daily doses of amphotericin...

I think you mean a symptom, not a cause. And yes, but these symptoms fit many GI problems, not just AGY/megabacteria.