The first two classes of birds are very straightforward to deal with, but it is the sub-clinically ill bird that is most problematic for most avian practitioners.
Birds are masters at hiding signs of illness as a protective mechanism.
A sick bird will attract predators to the flock, so a bird will hide that it is ill for as long as possible. A sick bird will usually be pushed out of the flock when it can no longer keep up with it. So, the ill bird will act as normal as it possibly can for as long as it can. This means that the bird that is presented to you for clinical illness has been sick for quite some time. Inexperienced or unknowledgeable owners are likely to miss the subtle signs of illness, such as decreased vocalization, change in volume, character or consistency of droppings, change in appetite, weight loss or decreased activity level.