Since I'm having some slow down in the aviary activity (pairs of gouldians, stars, owls, societies, keets have been set up but most don't seem that interested yet), I decided to take this opportunity to post some of my experiences for treating illnesses in birds. Majority of sick birds in my aviary are gouldians. They just seem to get sick more easily than other birds. Otherwise, higher mutations of any bird species do tend to have health problems more easily than their non-mutant normal counterparts.
The first problem with treating illness is to figure out if a bird is sick. As part of their survival instinct, birds like many animals will try to hide signs of illness to avoid predation. This means the vast majority of the time, when a bird is showing obvious signs of illness, the infection (most common problem) may be fairly advanced already. Constant monitoring of poop, as unpleasant as it may seem, can be very helpful to detect illness earlier on. Normal droppings are fairly dry and solid mostly consistent of half chalky white urate and half dark green/brown (depending on diet) fecal matter. I've noticed my grasskeets tend to have greener colored droppings and the shaft tails tend to have a reddish/orange tinge. If droppings are extra wet, off colored, bad smelling, or have seeds in them, sure signs of illness.
A fluffy bird may be a cold bird, but if it stays fluffy, it's a bad sign. A bird that is constantly sleeping is not healthy either. While we can not catch each bird all the time to check, but catching a bird that is not looking 100% is important because one needs to make sure the bird has not lost a lot of weight - this can be easily done by feeling the breast area - if the keel bone feels sharp the bird is too thin.
Treatment of birds illnesses depends on the cause of the infection obviously. If one has a good avian vet, then one is very lucky. Sometimes they can at least do fecal smears. Most of the times, I've found I have to be my bird's vet - majority of the cases will be bacterial, but under the right conditions, protozoal/fungal infections may be the cause as well. In general, however, I've found that giving anti-bacterial antibiotics require higher dosing than what is on the label. Perhaps due to the birds not drinking as much water when they are sick or perhaps due to higher metablism of the birds; I've not had much luck curing any infections in my birds until I started to make medicated water 2 times as concentrated as what's on the bottle.
If a bird is lethargic, not eating/drinking much, and/or is very thin, crop feeding is usually needed to keep it alive - I crop feed medicated water as well as baby bird formula at least twice a day. This, in addition to heat lamp in a hospital cage with lots of eggfood and millet can give a sick bird a better chance at recovery. This is especially important for smaller birds - the smaller the bird (finch) the less reserve it has, and the more likely it'll die if measures are not taken quickly.
Hope everyone's birds stay healthy and happy!