Anybody know which traits of bird plumage are accurate indicators of a birds health? For example feather coloration, feather brightness, amount of feather wear, number of feathers, moult timing, moult duration etc.
This resource provides a short list of which general components of plumage are good indicators of a healthy bird.
Also, this website page discusses the signs of malnutrition or disease in plumage in more detail.
Stress bars, dark lines which transect a feather, are directly caused by an inefficient absorption or lack of required nutrients. Stress bars can be found on all feathers during the period of malnutrition, and remain even after this period.
Assymetrical moulting is also a sign of bad health. There should not be any bald patches present during the moult.
The resource states that if a moult is delayed due to malnutrition, an incorrect photoperiod or a metabolic problem, plumage will become depigmented. However, the appearance of "dirty" feathers can also be due to overpreening or damage done to feathers by physical contact.
Finally, a change in the usual plumage color of the bird in question is indicative of a lack or oversupply of a particular nutrient.