In the chicken the eyes have it, a very special type of retina, and so more convergence, a seventh type of eye:-"Evolutionarily speaking, the researchers' results show that nature found a unique workaround to the problem of cramming all those cones into the compact avian eye, Corbo said. The ordered pattern of cells in most other animals' eyes are thought to be the "optimal" arrangement, and anything less would result in impaired vision. Yet, birds with the arrangement studied here—including chickens—have impeccable vision, Corbo said.
 
"These findings are significant because they suggest that the arrangement of photoreceptors in the bird, although not perfectly regular, are, in fact, as regular as they can be given the packing constraints in the epithelium," Corbo said.
 
"This result indicates that evolution has driven the system to the 'optimal' arrangement possible, given these constraints," he said. "We still know nothing about the cellular and molecular mechanisms that underlie this beautiful and highly organized arrangement in birds. So, future research directions will include efforts to decipher how these patterns develop in the embryo.'"-
 Read more at: http://phys.org/news/2014-02-eye-chicken-state-view.html#jCp