Cornea

Transverse section of the cornea of a pigeon (HE)

The avian cornea plays an important role in eye accommodation. The curve of the cornea increases when the cranial part of the ciliary muscle (Crampton’s muscle) contracts.. See ciliary body

Many birds have UV vision. This implicates that UV light must reach the retina and cannot be absorbed at the level of the cornea and lens. To enable this, the density of keratocytes in the avian corneal stroma is less and the collagen bundles are better arranged.  

In the avian cornea, same layers are present as in the mammalian cornea. See next picture