Humans are exceptional animals by any stretch of the imagination. Our special traits – from big brains to opposable thumbs – have given us great abilities and have allowed us to change our world considerably: we have created space rockets, developed the concept of time, and built computers to solve problems far beyond our capacity. And although there is a common thought that the human mind is infinite, there are simple things our minds are physically incapable of doing. For example, have you ever tried to imagine a colour that you’ve never seen before? It’s impossible.
Although we might not be able to see certain colours, they do exist. Birds see more colours than humans as they perceive parts of the ultraviolet (UV) spectrum that are invisible to our eyes. Along with this, they have better visual acuity and can filter wavelengths to establish subtle differences between similar shades of colour, shades that humans cannot distinguish.
Colour interpretation: Cone and rod cells
All animals have two different types of cells in their eyes that interpret both colour and light respectively and therefore allow them to see. The cells in the eye responsible for colour detection are cones and are present in the retina. Birds have four types of cones rather than the three that humans have. The exact number of cones varies in each bird species but is typically higher in birds than in most other mammals.
Diurnal birds (active during the day) have the most cones and the best colour sense. It is less crucial for nocturnal birds (active during the night) to perceive different colours, and many birds that are most active at night have a higher number of rod cells in their eyes instead. Rod cells are responsible for capturing the amount of available light, enabling nocturnal animals to capture more light and see better in low light conditions, though they may not see colours as clearly.
Why UV light matters
Being able to see ultraviolet light is an essential aspect of how birds see colour. For many years ornithologists believed that birds saw colours the same as humans, meaning that there were many elements of bird behaviour that was not easily understood. This was until birds’ sensitivity to UV light was recognised in the 1970s. Although it’s impossible for humans to see these differences under normal circumstances, birds’ ability to see UV light changes their perception of many features in their environment:
Many berries and other fruits have waxy coatings that reflect UV light, making them stand out vividly among green vegetation. Even if a red berry seems highly visible against a green leaf to human eyes, for birds this contrast is greatly enhanced, making foraging much easier. Some insects and flowers also reflect UV light, giving birds a clear advantage for finding those food sources.
Raptors also use UV light to hunt prey. Mice, squirrels and other rodents use urine splashes and trails to mark their territory. These splashes are highly visible in ultraviolet light. Raptors can therefore detect the exact position of prey, giving those birds the upper hand when hunting, even when the prey itself may not be visible.
Many bird species are dimorphic to humans, meaning that the sexes of the same species have different physical characteristics. Birds species with males and females who look similar to humans, however, may actually look incredibly different in UV light. Male Barn Swallows, for example, appear very similar to the females in the eyes of humans. However, to the birds themselves, the males are much more brightly coloured than the females, when viewed under UV light. Birds use these different plumage colours to help select mates, tell individuals apart, and defend territories.
Some brood parasite eggs have similar markings and colours to host bird eggs, and the only way to tell them apart is by viewing them under UV light. This allows birds to tell when an egg is not their own and lets them reject the intruder. For example, a Common Cuckoo’s eggs are very similar to those of a Great Reed Warbler, and the host mother needs the UV light to tell the eggs apart. While not all species that host brood parasites will reject unwanted eggs, the UV pattern may be a factor for those that do.
Not only can birds see ultraviolet light, but they also see visible colours more distinctly than humans can. Each cone in a bird’s eye has a droplet of oil in it that carefully filters out specific colours, giving birds higher sensitivity to various shades of colour.
Because of this, birds can see contrasts in their surroundings more easily. This helps them as they’re able to see through the camouflage of prey or other birds. This type of filtering or polarisation is also beneficial for waterbirds, as they can see deeper into rivers or watering holes, allowing them to find suitable prey and food sources.
A colourful world for birds
So to link this back to humans: we cannot detract from all that we are able to do, and our eyes, with over two million working parts, allow us to see a vast range of colours. But it is not the only way to see the world. Maybe a day in the life of a Barn Swallow would open our eyes to other ways of seeing, and just how colourful the world can be.