In the 200 Eggs A Year - Chicken Care Guide the author states that there are four ways the chicken keeper may discover the beginnings of sickness :
1. General appearance. A fowl in health is alert and active, plumage smooth, comb red, eye bright and clear, appetite good. A sick fowl is generally dumpish, plumage rough, comb pale or purple, appetite poor.
2. Respiration. Certain kinds of disease affect the respiratory organs and manifest themselves in the breathing. The best time to detect diseases of this sort is after the birds have gone to roost. If the birds on the roost sneeze from time to time, if the breathing is labored, if there is a rattling, bubbling or snoring sound, something is wrong and needs attention.
3. Excrement. The excrement is a pretty good index of the health of a hen. Where the bird is in perfect health the excrement is dark green in color, tipped with white, somewhat moist. Where the excrement is soft and watery the digestion is disturbed. Where it is light green in color there is danger of serious trouble. If upon examination the excrement shows that considerable food is passing through undigested, the bird needs to be supplied with grit.
4. Odor. Some kinds of disease - especially Coryza - are accompanied by an offensive odor, and can at once be detected by the smell.
Chicken Health Related Blog Posts