You'll get to know your parakeet's "normal" behavior by spending a lot of time with him. That way, it becomes easier to notice changes in behavior so that you can get the bird to the veterinarian quickly. Parakeets normally are quite perky, with bright eyes and singing. When they are sick, these behaviors change dramatically.
A sick parakeet's eyes will stay half-closed or will blink a lot; his feathers will puff up and look ruffled. An ill bird will keep his head tucked under the wing as if asleep. Difficulty breathing or breathing, with the beak open as the bird tries to get more air, is common. A sick parakeet will sit on the bottom of the cage rather than on a perch. Constant shivering is indicative of fever. A sick parakeet looks listless and hunches up.
Check the Droppings and Appetite
Any sudden changes in color, texture or smell of a parakeet's droppings signal a problem. Mucus in the droppings, a dark green color, streaks of blood and watery droppings are indicators of avian illness. Also, if the bird is straining more than usual while relieving itself, he's showing a sign of constipation. The sick parakeet will likely have no appetite and may stop eating.
Vocalizations and Enthusiasm
A parakeet who is normally vocal but suddenly stops singing and spends a lot of time lying listlessly at the bottom of the cage is not feeling well and should be taken to the vet. Lack of enthusiasm over anything the bird normally takes delight in, such as playing with toys, singing or playing with people, is often the first warning that something is seriously wrong.
Changes While Molting
During molting, which takes place in the spring and fall, a bird is under more stress than usual. While this is normal, it can leave the bird open to illnesses that can be devastating. Old feathers may fall out faster than they are supposed to. New feathers coming in can indicate illness in their growth patterns, in color changes or by looking drab rather than fresh and shiny.