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Birds
Birds have an elaborate variety of cones cells, with more morphological complexity and diversity than those of mammals.
In birds, the cones are complex. The inner segment characteristically contains a colored oil droplet adjacent to the base of the outer segment, forming a filter through which light must pass before reaching the visual pigment. The oil droplets are of several colors, due to the presence of different carotenoids. In addition to single cones, there are also double cones, consisting of two closely contiguous cells known as the principal and accessory members of the pair.
The structure of the bird retina is also more complex than humans'. Multiple foveae and other specialized regions that are suggested by non-uniform distributions of oil droplets (such as the red quadrant of the pigeon's retina) indicate a degree of complexity that is not present in the human eye and for which our own sensory experience provides little intuitive understanding. Birds may have a generalized system of color vision, but individual species may also have features of their eyes adapted to specific visual tasks or conditions, and attention to this ecological dimension in formulating hypotheses about visual function is likely to be critical.
As in primates, pigments absorbing at relatively long wavelengths are the major contributors to photopic sensitivity, but at least some birds probably discriminate short wavelengths in the violet region of the spectrum rather better than primates. The presence of a receptor process in the near UV, long thought to be the exclusive province of insects and foreign to our own visual experience, now seems to be commonly present in many nonmammalian vertebrates and has recently also been found in rodents. Birds can detect near UV, and it may have a chromatic quality of its own; the possible role of UV in avian color vision needs to be explored.
The information that is available on the receptor substrate and on color abilities such as discrimination and color matches shows that at least four chromatic channels are likely to be very commonly present in the retinas of birds. This makes birds true tetrachromats, and perhaps the only pentachromats in the animal kingdom.
The evolutionary history and radiation of vertebrate color vision present us with enormous diversity, and the mammalian or even primate perspective is a narrow pedestal from which to view and understand this evolutionary scene. The tetra- or pentachromatic color space of birds appears to be the most complex in nature and is likely involved in virtually all areas of the animals' lives, from the discrimination and recognition of objects to more complex behavioral tasks such as navigation, the classification of objects, and social and sexual behavior. The next years should see considerable progress in understanding these various evolutionary, physiological, behavioral, and ecological factors and in solving the mysteries of this appealing facet of natural history.
