 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 . 
