Published September 1991 by Howell Book Hoose. An excellent resource for anyone who keeps companion birds.
Parrots are very good at masking signs of illness.
This is a survival mechanism, because in the wild a sick bid would be easily preyed upon. You need to know how to recognize the subtle signs of illness, from the color of droppings to changes in appetite. Published September 1993 by Barron's Education Series. By far, the best text on avian nutrition. Also lists toxic substances, herbicides, pesticides, toxic and nontoxic plants, salt toxicity, and specific diet-related diseases in birds.