Is it for certain the poop that yellow and foamy, or could it be the urine? I have a friend who works at an exotics clinic where they see a great deal of birds. Quoted directly from her:
Contact: Vet Tech :
First things first, determine what you're looking at, and if there's loose stool, no stool, or what.
Urine is the liquid; urates will be chalky. Loose stool will be akin to what we see with diarrhea.
She further recommends you check the ears, sclera and mucus membranes for jaundice. Also the exposed skin. If it's the urine that's yellow and foamy, and the ears, sclera and mucosa are showing signs of yellowing as well, then it's likely a liver problem.
If Not, then it could just be anomalous poop. My chickens can poop some super funky poops when they're stressed, overheated or drink too much water.
Do note, I am not a professional and she is not a veterinarian, so this is just and opinion on my part and an educated one on hers!