Giardia

In the wild, birds are masters at hiding illness or weakness, because to demonstrate ill health makes the bird an easy target for predators. As a result, by the time many parrots show obvious signs of illness, they are really quite ill and they may not be able to be nursed back to health.

Giardia is caused by a protozoan called Giardia psittaci. Signs of a Giardia infection include loose droppings, weight loss, feather picking (especially under the wings), loss of appetite, and depression. Because the Giardia organism is difficult to detect in a bird's feces, arriving at a proper diagnosis may be very time-consuming. The disease can be spread through contaminated food or water, and birds do not develop an immunity to it. Your veterinarian can recommend an appropriate medication to treat Giardia.

Reference: "Why Does My Bird Do That?" by Julie Rach Mancini