Having just diagnosed and healed a budgie infected with AGY, I thought I'd share the symptoms and treatment of this common malady.
Disclaimer: AGY is a common cause of the symptoms listed below. There are other diseases with similar symptoms; this post is meant to inform you of a likely cause of illness.
I consulted expert avian veterinarians Dr. Jeff Olsen and Dr. Robert Speers in diagnosing my bird. These gentlemen are extremely helpful and friendly resources for those seeking expert advise!
Sleepy / lethargic
Puffed up / looks cold
Throwing up of not digested seeds
Not singing / socializing
Exposed keel bone / weight loss
AGY is a common yeast infection (especially for store-bought birds). The yeasts infect and inflame the digestive tract, forming a bio layer over the digestive lining, making it hard for the bird to eat and stealing nutrients. Consequently the bird gets weak because it can't hold down as many seeds and of those it eats the yeasts take their share of the nutrients. The bird is "going light" and will likely die if not treated.
The remedy is a fluid delivered orally (with a syringe) known as Amphotericin B (aka Fungilin or Megabac S). This kills the yeast.
After only 24-48 hrs there was a huge improvement in our birds energy levels, social behavior, eating habits, posture, and communication. The treatment period is about 10 days.
If your bird has these symptoms, AGY and Amphotericin B are likely the trails you want to be going down in terms of healing your bird. Again other diseases may induce similar symptoms, but according to the experts listed above AGY is the most common cause of "going light" in birds (particularly store-bought birds which are apparently riddled with the yeasts) and Amphotericin B is the best remedy.
Hope this helps!