comparing to other pets (like cats and dog), birds are domesticated much more recently. In the wild, animals will not show any symptoms or weakness for the reason of safety. Unlike mammals, birds are much smaller (usually) and they do not store as much fat (energy) in their body coz they need a lighter weight to fly, but birds have much higher metabolic rate. So, in mammals we can see their health condition declining for weeks or months to death; but in birds, their body condition can be perfect and died the next morning.
