Bird Senses. Graham R. Martin

the whole of the retina. However, they are usually absent from the fovea, which is where cones are found at their highest concentration. In species which have the highest acuity, such as raptors, double cones are also absent from their foveas.

      In birds, while there are rod receptors containing a single type of photopigment (which is very similar to that found in humans), the photopigments of the cone receptors are typically of four types, giving them tetrachromatic vision. Three of these photopigment classes show a high degree of similarity across a wide range of species, but the fourth type can occur in two forms. One has maximum sensitivity in the UV part of the spectrum while the other has maximum sensitivity at violet wavelengths but does have some sensitivity into the near UV. It is this photopigment that gives certain birds vision in the UV.

      The basic uniformity in visual pigments and receptor types across bird species provides evidence that there has not been adaptive evolution of visual pigments among birds. It suggests that photopigments and colour vision arose early in bird ancestry. Indeed, they were probably inherited from dinosaur ancestors, and their properties have changed very little over at least 150 million years.

      A clear example of the uniformity of photopigments across modern birds comes from a study which showed that the visual pigments found in the eyes of a species of pelagic seabird (Wedge-tailed Shearwater Ardenna pacifica from the Procellariidae, order Procellariiformes) are very similar to those found in a phylogenetically distant species that lives in open forest habitats (Indian Peafowl Pavo cristatus from the Phasianidae, order Galliformes). This suggests that colour vision in birds has general, all-purpose, properties, that are not tuned to specific tasks performed by different species in different environments.

      Colour through birds’ eyes

      Just how the world might look when viewed through the birds’ tetrachromatic colour vision system is of considerable interest. For example, does tetrachromatic vision mean that particular parts of the spectrum have different salience to birds compared with our own view of the world? Certainly, knowledge of the tetrachromatic system suggests that birds’-eye views are likely to be different to how humans detect colour information in the world. Of course, it is not possible to see these colours through birds’ eyes, but computational methods have sought to compare colour patterns viewed through trichromatic and tetrachromatic vision systems, and have given some clues as to which kinds of differences between natural colour patterns are changed by a tetrachromatic system.

      All of these differences in spectral sensitivity and colour vision, however, apply only at high (daytime) light levels when the cones are active. At lower (twilight and night-time) light levels, only the rods function and they have very similar characteristics across all birds and across mammals. Thus, at low light levels a similar sensitivity across the spectrum, and an absence of colour vision, is found in all bird species and indeed in humans and other mammals.