 Terns have red oil droplets in the cone cells of the retinas of their eyes. This improves contrast and sharpens distance vision, especially in hazy conditions. Birds that have to see through an air / water interface, such as terns and gulls, have more strongly coloured carotenoid pigments in the cone oil drops than other avian species. The improved eyesight helps terns to locate shoals of fish, although it is uncertain whether they are sighting the phytoplankton on which the fish feed, or observing other terns diving for food. Tern's eyes are not particularly ultraviolet sensitive, an adaptation more suited to terrestrial feeders like the gulls.
