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Classifying the different types of cone photoreceptors in the retinas of all animals is done by reference to the position in the spectrum of the peak sensitivity of the photopigments that they contain (Box 3.1). In our own retinas there are rods that contain a single type of photopigment, but our cones are of three types defined by which one of three different photopigments they contain. These pigments, and hence the cone types in which they occur, are commonly labelled red, green, and blue, to indicate the spectral regions in which they are most sensitive. It is these three types of cone receptors that provide the basis for human colour vision. It is referred to as trichromatic vision, because it is based on three receptor types.

Box 3.1 The photoreceptors of bird retinas

At one level of analysis bird retinas are highly complex. Even the smallest eyes contain many millions of individual photoreceptors, the rods and cones. However, at another level there is relative simplicity. This is because these photoreceptors are of few types and they are very similar in all bird species. Their essential features are depicted in the accompanying diagram. What varies between species are the relative numbers and distributions of the different receptor types across their retinas. As discussed in the ‘The image analysis system‘ section above, it is these patterns of receptor distribution that are the primary foundations of differences in the visual abilities of species.

The cone receptors are the ones that function primarily at higher, daytime light levels. Rods function at low light levels. During twilight both rods and cones may function, depending on the exact light level. In birds, rods are of one type, but the cones are of five different types: four types of single cones, and double cones. The different types of single cones are classified by reference to the position in the spectrum of the peak sensitivity of the photopigments that they contain.

The top portion of the diagram depicts the types of retinal photoreceptors as they appear when viewed through a relatively high-powered light microscope. The outer segments are extremely narrow, generally between 1 and 2 μm (microns) in diameter, but they are relatively long. Each outer segment contains many millions of photosensitive pigment molecules, and each individual molecule of photopigment can absorb the energy from a single photon of light. When this happens, the receptor triggers a signal to the brain. Of course, at high light levels many millions of photons are simultaneously absorbed by the pigment molecules scattered throughout the outer segment.

The four types of single cone provide the fundamental mechanism upon which colour vision is based. The double cones provide a neural channel that is thought to signal luminance (brightness) and they are not part of the colour vision system. Within all cone types there is an oil droplet. As depicted in the diagram, most oil droplets are highly coloured. The colour is due to carotenoid pigments which are derived from the bird’s diet. Birds fed on carotenoid-free diets have colourless oil droplets.

The blue arrow on the left indicates the direction in which light travels from the optics of the eye to the focused image on the retina. In one sense the retina would seem to be back to front, because light does not reach the photopigment until it has travelled through the neural layers of the retina. However, this arrangement means that the light that makes up the retinal image must pass through the oil droplets before it enters the outer segments.

The fact that the oil droplets are coloured means that they have light-filtering properties: they let light of certain wavelengths pass through and absorb light of other wavelengths. In fact, the oil droplets act as cut-off filters, that is they allow light only above particular wavelengths to pass through to the pigment molecules, while light of shorter wavelength is absorbed. The combination of photopigment types and oil droplet types results in there being four main types of single-cone photoreceptors in birds’ retinas, with each cone type able to absorb light only within a particular part of the spectrum, although there is overlap between them.

The lower section of the diagram shows the resultant ‘photoreceptor sensitivities’ and the labels used to describe them. These are LW (long wave), which absorb light at the orange–red end of the visible spectrum; MW (middle wave), absorbing light in the green–yellow spectral region; SW (short wave), absorbing light in the blue–green spectral region; and VS (violet-sensitive), which absorbs light in the violet-ultraviolet spectral region.

The photoreceptor pigments in the LW, MW, and SW types of cone receptor differ one from another, but each cone type is highly similar across all birds species. However, the VS cones can contain two different types of photopigments. One type has a peak sensitivity in the violet at about 410 nm, which is within the human visible spectrum. The other type has its sensitivity centred around 360 nm, which is in the part of the spectrum not visible to humans. It is referred to as the UVS pigment. Cone photoreceptors that contain UVS pigment are found only in songbirds (oscine passerines) and in some non-passerine species: gulls, ostriches, and parrots. It is those species which have the UVS photopigments that have true ultraviolet vision.

Double cones are widespread in vertebrates but are absent from mammals. In birds, the double cones always contain one type of pigment. It has broad sensitivity across the spectrum centred on about 565 nm in the yellow region of the visible spectrum. The rod photoreceptors are found across the retinas of all vertebrate eyes and have a similar broad sensitivity centred about 500 nm, in the green part of the spectrum. Rods are considerably larger than cones and are usually found across 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.