Transverse section of the iris of a pigeon (HE): detail of the stroma (2)
Birds often have the most brightly colored eyes and in some species, the color varies according to age and sex. Both pigmentary and structural colors contribute to the coloration of the avian iris.
The iris on the micrograph above is from a piebald pigeon that has unpigmented areas of white feathers on its body and dark brown eyes (bull eyes). Piebald or partial leucism is a genetic disorder that causes absence of pigment cells in parts of the skin and in the anterior iris stroma whereas the posterior (retinal) pigmented epithelium is unaffected, which results in brown colored eyes.
Pigment cells in the anterior part of the iris stroma: see next picture