How Do Birds See Colour?
Vision is the most acute sense in birds, vital for finding food and even breeding. They are able to see more colours than humans in a number of ways. Not only can they perceive colours that are invisible to the naked human eye, they also have better visual acuity.
The cells in the eye responsible for detecting colours are called cones. They are found in the retina and birds have four different types of cones compared to humans who have three. The exact number of cones varies in different species but it is typically higher than in mammals.
Each cone in the eye of a bird contains a coloured oil droplet, something also found in some amphibians, fishes and reptiles but not present in mammals. The droplets vary in colour and are transparent, clear pale or intensely red or yellow due to various concentrations of carotenoid pigments.
The pigments cause strong spectral filtering and a distinctly increased refractive index at longer wavelengths which means the oil droplets act as powerful spherical microlenses. Light passes through the droplet before reaching the visual pigment, enabling them to act as filters, removing some wavelengths and increasing the number of colours that a bird can discern.
This means, that although birds have less sensitivity to contract than mammals including humans, they are far more responsive to colours and can effectively see colours that we can’t including those in the ultraviolet spectrum. They are able to determine subtle differences between similar shades of colour which humans are unable to do.
The vast majority of birds including all diurnal birds have both violet sensitive (VS) and ultraviolet (UVS) vision thanks to a visual pigment in the cones of their retinas that absorbs UV light. It is less important for nocturnal birds to have acute colour sense and birds that are active at night tend to have a greater number of rod cells in their eyes which allows them to capture more light and see better in low light conditions.
This is useful for a number of reasons. Many species of birds have plumage patterns that are only discernible in UV light. Species in which male and female birds appear identical to us have UV reflective patches on their feathers that enable the birds themselves to tell the sexes apart.
Male blue tits, for example, have a UV reflective crown patch which is displayed in courtship by posturing and raising their nape feathers, advertising themselves to females. In blackbirds, females respond more strongly to males which good UV reflectiveness, while male blackbirds competing with each other over territory are more concerned with the degree of orange in the bill.
It had been thought that kestrels and other birds of prey u se their UV vision to detect the urine trails of potential prey such as mice or voles, although recent research suggests that raptors have relatively low UV sensitivity.
For other species, UV vision may enable them to find certain fruits and berries. Some fruits and berries have waxy coatings that reflect UV light so they stand out more against green foliage and birds can see them more easily. Some flowers and insects also reflect UV light giving birds an advantage for finding those food sources.