The colored oil droplets in the retina of 30 avian species with different ecological habits and different taxonomic positions were examined by microspectrophotometry and histology, and their role or significance was conjectured. In diurnal birds such as the Night Heron, Jungle Crow, and Japanese Lesser Sparrow Hawk, four kinds of colored oil droplets, red, yellow, orange, and pale green, were discriminated ; whereas in nocturnal birds, only pale green oil droplets were found. Cluster analysis of the distribution of droplets showed that the color of the oil droplets in the 13 species of wild birds correlates with their ecological behavior. The absorption spectra of the red, yellow, and orange oil droplets displayed only minor interspecific differences. The spectra of the pale green droplets differed significantly between the Eastern House Swallow and the Japanese Jungle Nightjar. In the King Penguin, the retina contained only pale green droplets, whose spectrum was completely different from that of other diurnal birds. The upper half of the retina of the Japanese Jungle Nightjar was lined characteristically with the tapetum lucidum of nocturnal birds ; and the under half, with the diurnal birds' pigmented epithelium. Our findings suggest that each colored oil droplet has specific advantage for visual perception and that each bird has the retina that best matches its natural environment and feeding behavior in each habitat.