74 . Eye Colour in Birds 



branching processes and gradually lose their colour when submitted to 

 the action of 5 "/^ formalin solution. 



In the Brown Owl (Strix alues) and the Grey Eagle Owl, and some 

 other brown eyed species, the brown colour of the iris is due to the 

 presence of branched pigment cells containing brown granules. The 

 same occurs in the Eagles (Aquilinae), the Kites (Milvus) and some other 

 birds of prey. The Egyptian Kite (Milvus cegyptius) and Bateleur's 

 Eagle (Helotarsus ecaudatus) both have dark brown irides, and in both 

 the pigment is contained in branch cells. In no case at present has 

 any deposition of yellow or brown pigment granules been observed in 

 the striated muscle-fibre cells in the iris of any wild bird. . * 



The Brown and Black Eyes. 



Amongst Birds, as amongst Mammals, quite a large number of species 

 possess anterior iris pigment which passes through all grades of brown 

 up to black. The black iris is associated (with some exceptions, notably 

 the Silky Fowl) with black feather colour, and in its deeper grades with 

 black leg colour. Histologically the brown (in its darker shades) and 

 the black iris are produced by a well-defined layer of characteristic 

 branching cells, which contain dark brown or black pigment, on the 

 anterior surface of the iris. These cells intercommunicate by their 

 branches and form a plexus of pigmented cells thickest over the capil- 

 laries in the peripheral or middle zones of the iris. 



The cells are much alike in different species. The body of the cell 

 and its processes are usually crowded with pigment granules of a rounded 

 outline and fairly uniform size. Under favourable conditions a central 

 nucleus can be made out more or less free from pigment. These pig- 

 ment cells permeate the thickness of the iris wall for some depth and 

 are often found encircling the striated muscle cells. They are structurally 

 continuous with the branching pigment cells in the sclerotic and the 

 outer surface of the choroid behind the cornea, and are quite distinct 

 from the hexagonal cells on the posterior surface on the iris. As has 

 been already stated, they are especially numerous and heavily pigmented 

 in birds of dark or black plumage. They are found in nestlings belonging 

 to the Corvidae and allied species. They are also present in the newly 

 hatched chick in the black breeds of fowls. 



Genetically the black eye with its deeply pigmented branching cells 

 is dominant over the yellow iris and other grades of iris pigmentation 

 thus : 



The daw eyed Malay hen crossed with the Black Orpington cock 



