We are often so busy doing other things, that the most common indicators of a sick bird are overlooked. For this reason I start this article with the back to basics approach to diagnosis of a healthy/sick bird. When buying new birds these are the signs we should be looking for before we spend our hard-earned money. Remember also that you should quarantine all new birds for as long as possible to avoid infecting the rest of your stock. Quarantine should be an absolute minimum of 30days.

Be jaunty and brisk in movement and move freely around the cage or aviary.
Be clean, bright & colourful in appearance.
The eyes should be clear and bright.
May not be singing but should be chirping and making noise.

Be puffed up and sitting motionless.
Have watery or sore eyes; eyes may also be red and inflamed.
Should not sneeze or shiver.
Have watery discharge from the beak.
Should not have droppings that are white and watery.
Be 'light' with little or no body weight.
Have droppings/faeces around vent.

This link is to a copyright article belonging to DR. TONY GESTIERíS BVSc. (HONS) MACVSc. BMSc. (PHARMACEUTICS) - Vetafarm, which gives some helpful advice.

Below is a list/menu of the most common problems that you may come across. Remember if you keep your birds well fed with a varied diet, and quarantine new birds, you should avoid most of the diseases, which leaves just the major problems.
Below is a list/menu of the harder problems to solve that you may come across. Remember if you keep your birds well fed with a varied diet, and quarantine new birds, you should avoid most of the diseases. You may need to consult a veterinarian.