This is what I have gathered,
"Broadly speaking anemia can occur one of three ways:
- Loss of red blood cells: The most obvious of which is blood loss (bleeding) following trauma, however blood can also be lost in less obvious ways through the bowel and urogenital system.
- Destruction of red blood cell: Animals may eat certain toxins such as lead (common in older household paint), zinc (found in some bird toys and household items), or onions which can cause red blood cells to rupture within the bloodstream. It takes time for these ruptured cells to be replaced, and in the meantime anemia can develop. Another type of destructive anemia can occur when an animal’s immune system targets red blood cells and destroys them. Fortunately this type of anemia is very rare in avian and exotic pet medicine. Diagnosis of this last form is difficult and may require advanced diagnostic tests.
- Decreased production of red blood cells: One relatively common cause of decreased production of red blood cells is iron deficiency. This can occur when pets are on substandard diets. Diagnosis and treatment of this condition must be made by a veterinarian since excess iron supplementation may be toxic. Chronic inflammatory or infections disease may also lead to this type of anemia."
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http://www.poultrydvm.com/condition/anemia