HALL, M.I.; ROSS, C.F.; Stony Brook Univ.; Univ. of Chicago: Ecological Influences on Avian Eye Shape

Avians are extremely visually dependent vertebrates that have radiated into a wide variety of ecological niches. These niches place diverse demands on their visual system resulting in changes in gross eye morphology within the basic vertebrate eye design. Morphological adaptations are predicted for aquatic versus aerial niches, styles of flight, substrate use, diet, predation, and activity pattern. Here, the variance in eye shape explained by these ecological variables is analyzed for eight groups of extant avians, the Strigiformes, Falconiformes, Caprimulgiformes, Apodiformes, Psittaciformes, Passeriformes, Columbiformes, and Procelleriformes. In general, ecological variables are significant factors in explaining the variation of eye size and shape seen in these groups. For example, Falconiformes exhibit greater eye length than slower-flying or non-predatory birds. Activity pattern, the time of day when an animal is active, is an especially significant ecological factor affecting eye shape. Diurnal birds have eyes with larger axial lengths relative to their corneal diameters, an adaptation for increasing focal length, which enlarges the image and heightens visual acuity. Nocturnal birds exhibit enlarged corneal diameters relative to eye length, an adaptation for maximizing the number of photons captured and increasing sensitivity over diverse, particularly low, light levels. Anaysis of these differences in a phylogenetic context will determine whether the objective conditions surrounding vision under specific ecological circumstances are more significant than phylogenetic constraint in determining eye shape.