Summary: Smith et al. (2002)

Humans have three types of cone receptor, while birds have four and are sensitive to ultraviolet light (UV). It is thus likely that birds require four primary colours for colour vision and are thus tetrachromatic. Like humans, birds have short, medium and long wavelength sensitive single cones. Birds use UV signals for a number of tasks, such as mate choice and foraging, but few studies have investigated a role for UV in the detection of chromaticity differences. Achromatic mechanisms are involved in the perception of brightness while chromatic mechanisms are responsible for the perception of colour differences. Smith et al. (2002) developed a 'colour blindness' test, which they administered to a passerine, the European starling (Sturnus vulgaris), and a non-passerine, the Japanese quail (Coturnix c. japonica). This was an associative learning technique where the bird had to learn to discriminate between patterns of random intensity with or without UV reflectance. Both species were able to distinguish between orange and red stimuli, and between UV and non-UV stimuli, and were making the discrimination using chromaticity differences, not achromatic cues. This indicates that the output of the UV cone is involved in chromatic colour vision.