Many 'dull' birds are spectacularly colored in ultraviolet. Females see elaborate patterns invisible to predators; secret love letters written in hidden ink.
A World Unseen
To the human eye, a male and female blue tit (Cyanistes caeruleus) look nearly identical. To each other, they are brilliantly distinct. This difference is due to vision in the ultraviolet (UV) spectrum, a range of light from roughly 300 to 400 nanometers that is invisible to people. Birds are tetrachromats, meaning their retinas have four types of cone cells for color vision—red, green, blue, and one sensitive to violet or ultraviolet light. Humans, as trichromats, lack this fourth cone. Each cone in a bird's eye also contains a tiny droplet of colored oil that filters light, allowing them to perceive far more subtle variations between colors.
This capability changes the appearance of many birds. The blue tit’s unassuming blue cap is, to another tit, a radiant patch of UV-reflectant feathers. Research linked to the University of Oxford, particularly through its Edward Grey Institute of Field Ornithology (EGI), has examined this phenomenon in detail. The EGI runs the Wytham Tit Project, a continuous study of great tits and blue tits in the nearby Wytham Woods that began in 1947. This long-term project, with over 1000 nest boxes spread across 385 hectares, provides a large dataset for understanding how traits like UV plumage affect survival and reproduction.
The UV coloration in a tit's crown is not created by pigments like melanin (which produces blacks and browns) or carotenoids (reds and yellows). Instead, it is a structural color. The feather barbs contain a spongy layer of keratin and air pockets with a nanostructure that selectively scatters short-wavelength light, producing the blue and UV hues.
The Selective Advantage
The blue tit’s UV crown is a signal in sexual selection. Studies, including those analyzing Oxford's Wytham Woods population, show that females prefer males with more intensely UV-reflective crowns. This preference is not arbitrary; the brightness of the UV signal indicates male quality. This trait is linked to the male sex hormone testosterone. Males with brighter UV crowns tend to be in better physical condition and are more successful at securing within-pair paternity, meaning they are less likely to be cuckolded.
The situation is different for females, who also possess this UV ornamentation. Research shows that female blue tits with more UV-reflective crowns fledge more offspring and show lower levels of stress hormones during the demanding chick-rearing period. This suggests the trait is a mutual signal, allowing both sexes to assess the quality of a potential mate.
This communication channel is also relatively private. Many avian predators, such as sparrowhawks, have visual systems that are less sensitive to UV light compared to their smaller passerine prey. This allows blue tits to display meaningful signals to one another that are less conspicuous to the animals that hunt them. The UV signal can fade over the winter, and the ability of a male to maintain his coloration is another indicator of his fitness for prospective mates in the spring.
💡Fun Facts
Some flowers have UV patterns called "nectar guides" that are invisible to humans but lead pollinating insects and birds directly to the nectar.
Reindeer use UV vision to spot lichen, a major food source, and predator fur, as both absorb UV light and stand out sharply against UV-reflective snow.
The human eye lens naturally blocks UV light to protect the retina. Aphakic patients—those who have had their lens removed for cataract surgery—can sometimes perceive UV light, often described as a whitish-blue color.
The mantis shrimp has one of the most complex visual systems known, with up to 16 types of photoreceptor cells that allow it to see UV, visible, and polarized light.
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