Timothy Laum and Joe Corbo, Washington University
Birds have one of the most sophisticated color vision systems of any vertebrates. They have five types of cone photoreceptors (depicted here in white, red, green, blue and violet) each with a carotenoid-containing oil droplet used to fine-tune their color vision. These oil droplets act as intracellular microlenses that filter the incoming light before it reaches the photosensitive part of the retina, thereby improving color discrimination. Interestingly, carotenoids are an important class of pigment used by birds for the brilliant yellow, orange, and red colors of their feathers. There appears to be a fascinating relationship between the pigments in the bird's feathers and the means by which these colors are sensed by the bird’s visual system.