Found this on letstalkbirds.com and thought it might be useful here.
In the wild if budgies let their guard down and show they are unwell they will become the victims of predators. Therefore it is natural for budgies to conceal their illness as long as possible.
By observing your budgie daily you will learn its normal behavior and anything out of the ordinary should draw your attention. If your budgie looks ill it normally means it is really sick and can deteriorate extremely quickly.
When you detect any of the following signs of illness you should take your budgie to an avian vet as soon as possible - try to visit a vet experienced with birds so a correct diagnosis can be given.
Signs of illness to watch out for:
abnormal droppings (note the quantity, color, consistency)
abnormal feathers, feather growth, or molt
abnormal sleep pattern:
- both feet on the perch when normally one foot is tucked up
- head tucked under the wing
- head turned towards the wing with eyes only partly closed
any change in normal activities:
- talking or chirping
- playing with toys
- interaction with other birds
- interaction with humans
- energy levels
- different perching area
black spiky head feathers (except during molting)
discharge from the beak, eyes or nostrils
drinking a lot more water than usual
drooping head, tail or wings
dull or swollen eyes
excessive feather picking or plucking
face and head feathers coated with mucus and semi-digested seed
falling off the perch
fluffed up appearance
hunched over posture
loss of appetite
lumps or swellings on the body
sitting on the bottom of the cage