Birds can see colors that humans cannot. This is due to differences in the photoreceptor cones in bird and human retinas. Birds have four types of cone photoreceptors that allow them to see into the ultraviolet spectrum, whereas humans only have three types of cone photoreceptors that limit our vision to the visible light spectrum. This means that birds can perceive many more colors than humans can. In this article we will explore how many more colors birds are able to see compared to humans.
The Avian Visual System
Birds have excellent color vision that is superior to human color vision in many ways. One key difference is that birds have four types of cone photoreceptors in their retinas, compared to only three cone types found in human retinas. The four avian cones contain visual pigments that are sensitive to:
- Long wavelength light (L-cones, peak sensitivity ~570 nm) – sensitive to red light
- Medium wavelength light (M-cones, peak sensitivity ~505 nm) – sensitive to green light
- Short wavelength light (S-cones, peak sensitivity ~435 nm) – sensitive to blue light
- Very short wavelength light (V-cones, peak sensitivity ~415 nm) – sensitive near the UV spectrum
Having four types of cones allows birds to perceive a wider range of colors across the light spectrum compared to humans with only three cone types (L, M, S cones). In particular, the additional V-cones give birds sensitivity to ultraviolet (UV) wavelengths of light. This expands the visible color spectrum for birds into the UV range, allowing them to see UV colors that humans cannot.
Estimating the Number of Colors Birds Can Perceive
So how many more colors are birds able to see compared to humans? There are a few ways we can estimate or model the relative number of colors birds can perceive:
Tetrachromatic Color Space Model
One method is using mathematical models of tetrachromatic (4 cone types) versus trichromatic (3 cone types) color space. According to one estimate that mapped out the tetrachromatic color space of birds and compared it to human trichromatic color space, birds may be able to distinguish somewhere between 2 to 5 times as many colors as humans.
Specifically, based on calculating the number of discriminable colors for humans (~1 million) and birds (~2 to 5 million), birds can potentially see about 2 to 5 times more colors than humans.
Connecting Color Range to Cone Sensitivity
We can also relate the expanded color perception of birds to the wider spectral range of light their 4 cones can detect compared to human 3 cones.
| Species | Cone Types | Spectral Range (nm) |
|---|---|---|
| Humans | S, M, L cones | 390 – 700 nm |
| Birds | S, M, L, UV cones | 300 – 700 nm |
As the table shows, birds have a spectral range that extends further into the UV wavelengths down to around 300 nm, whereas human visible range only extends down to about 390 nm. This wider spectral coverage means birds have access to many more shades of colors, especially in the UV range.
Some estimates based on the extended spectral range birds can see calculate that birds may perceive up to about 12 times as many colors compared to humans! However, estimates vary on exactly how big the color difference is.
Accounting for Neural Processing
However, we also have to keep in mind that perceived color is not just about the range of wavelengths detectable by photoreceptors. Neural processing in the brain also influences how receptors signals are interpreted as discrete perceived colors.
So while birds may collect more visual information from a wider spectrum, additional neural processing likely compresses this input into a smaller range of discrete color perceptions. This means the full range of detectable wavelength differences does not directly translate into the total number of color perceptions.
Accounting for neural processing, the actual enhancement in birds’ perceived color discrimination space is likely closer to the range of 2 to 5 times more colors vs. humans, rather than the upper limit of 10-12 times as many colors.
Impacts of Broader Color Vision
What benefits does an expanded range of color perception provide for birds? Here are some of the advantages broad spectrum color vision offers birds:
Enhanced Contrast Perception
The addition of UV sensitive cones allows birds to perceive increased visual contrasts between colors, patterns, and textures that appear similar to human vision. UV reflections and absorbance can create visual patterns and contrasts that birds can detect, but humans miss. This enhanced contrast helps birds discern important visual cues in mate selection, foraging, predator detection, and navigation.
Foraging and Feed Selection
Birds rely heavily on color cues when foraging for food. Certain foods and ripened fruits have UV reflective markings that guide bird feeding behavior. UV vision helps birds discriminate between edible and poisonous foods. Birds can also use an expanded color space to find flowers and plants with nectar guides that lead them to nutritious nectar rewards.
Communication via Colorful Displays
Many bird species use brightly colored plumage and displays to communicate with each other. Additional UV reflective patches or UV absorbent feathers provide birds with a private visual communication channel that other animals cannot perceive. Brighter colors enabled by tetrachromatic vision assist birds with mate attraction, territorial defense, and social signaling.
Camouflage and Mimicry
Some predatory and prey species of birds use UV vision to help camouflage themselves through UV reflective or absorbing patterns on their plumage. This helps birds blend into their surroundings and avoid detection. Mimicry of other species’ UV reflection patterns also occurs, letting some birds copy the appearance of unrelated, dangerous species to deter predation.
Conclusion
In summary, birds can see many more colors than humans due to possessing four types of cone photoreceptors including a UV sensitive cone. This tetrachromatic vision gives birds an expanded perceptual color space compared to human trichromatic vision.
Estimates based on photoreceptor spectral sensitivity, modeled color space size, and neural color discrimination suggest that birds can detect around 2 to 5 times as many colors as humans. However, the upper estimates extend as high as 10 to 12 times as many colors that birds can potentially distinguish relative to humans with our limited visual system. Access to broader spectrum color vision provides birds with advantages in contrast perception, foraging, communication and camouflage within their environments.