Many species of birds have cones sensitive to ultraviolet light, which is invisible to humans. Foveae are extra dense concentrations of cones in special retinal pits. Over a million light-responsive cells can be packed into a single fovea. These foveae are thought to fine tune the image, giving it sharpness, improving definition and better detecting movement. The oil droplets are red, orange, yellow, green, or clear.
Oil droplets filter out certain wavelengths making it possible to detect subtle hues.
Bird feeding behavior often provides clues to the abundance of these oil droplets. Birds like kingfishers, needing to see through the surface of water, have an abundance of red and yellow oil droplets. Birds that spend much time underwater, like goldeneyes, also have an abundance of yellow droplets. Night feeding birds tend to have only clear oil droplets, as the colored ones reduce brightness. It's exciting to think about the different adaptations and possibilities as we watch an Osprey see past the sparkling veneer of a stream and monitor several trout, hoping one will swim closer to the surface.