Birds have amazing visual abilities that allow them to see a broader range of light wavelengths than humans. Their advanced color vision helps them find food, choose mates, avoid predators, and navigate their environments. But can birds see ultraviolet (UV) light?
What is UV light?
Ultraviolet light refers to electromagnetic radiation at wavelengths shorter than light visible to humans, but longer than X-rays. The wavelengths of UV light range from 10 to 400 nanometers (nm). By comparison, human visible light ranges from about 380 to 700 nm.
There are three types of UV light:
- UV-A: 315 to 400 nm
- UV-B: 280 to 315 nm
- UV-C: 100 to 280 nm
UV-A and UV-B reach the Earth’s surface. UV-C is absorbed by the ozone layer and does not reach us. Exposure to UV-B can cause sunburn in humans, while excessive exposure to both UV-A and UV-B can increase skin cancer risk.
Bird vision compared to human vision
Human vision relies on three types of color receptors (cone cells) that detect light in the red, green, and blue wavelengths. By combining input from these three receptors, we can perceive the visible light spectrum from violet to red.
Birds have four types of cone cells, allowing them to see a broader range of colors. The fourth cone detects ultraviolet light. Having UV sensitivity gives birds several advantages:
- Enhanced ability to find food – Ultraviolet wavelengths make some fruits, flowers, and insects stand out against foliage.
- Improved mate choice – UV color patches on potential mates may be visible to birds but not humans.
- Detection of UV reflective surfaces – Bird feathers, urine, and feces reflect UV. Birds may use UV signals for species or kin recognition.
- Enhanced predator avoidance – Some bird eggs and plumage have UV reflective or absorbent patterns to camouflage nests.
- Better navigation – UV cues in the sky may help migratory birds orient themselves.
Evidence that birds see UV light
Scientists have gathered extensive evidence that birds detect and make use of UV wavelengths:
- Microspectrophotometry and sequencing of the avian genome has revealed the structure and genetic basis of UV sensitive cone cells in birds.
- Behavioral experiments show birds can learn to discriminate between objects solely on the basis of UV reflectance.
- Birds prefer to feed at UV reflective feeders, even when all other variables are controlled.
- Songbirds are more attracted to UV reflective decoys than non-reflective decoys.
- Surgically removing the UV sensitive cone from pigeons impairs their ability to forage for certain seeds and grains.
Overall, multiple lines of research confirm birds see a visual world very different from our own with added UV information.
UV vision across bird species
The avian lineage can be divided into two groups based on the evolutionary development of UV vision:
- UVS birds – Have UV sensitive cone cells containing a light-sensitive protein called SWS1. Includes songbirds, parrots, pigeons, gulls, and others.
- Violet sensitive birds – Lack SWS1 and are maximally sensitive to violet wavelengths around 400 nm. Includes owls, eagles, and chickens.
There are also differences in UV sensitivity across species due to oil droplets in cone cells that act as filters. Birds that are most UV sensitive include the European starling, zebra finch, and budgerigar.
Most UV sensitive bird species
| Species | Maximum UV Sensitivity |
|---|---|
| European starling | 360 nm |
| Zebra finch | 373 nm |
| Budgerigar | 359 nm |
There are also some gender differences in UV perception. For example, female European starlings have higher UV sensitivity than males. This may help females assess male plumage as a sign of mate quality.
How birds use UV vision
Let’s look closer at some of the key ways birds utilize UV vision:
Finding food
The UV reflective and absorbent patterns on plants and fruits act as cues to help birds locate food sources. Many berries absorb UV, causing them to stand out against UV reflective foliage. Insects reflect UV as well, making them visible to birds from a distance.
Choosing mates
Brightly colored feathers and bare parts may take on very different shades and patterns in the UV spectrum. Birds use these UV signals to select and attract mates. UV reflectance may correlate to factors that indicate mate quality such as nutrition, health, social status, and genetic fitness.
Avoiding predators
Some bird eggs reflect UV light, while others blend in with their nest surroundings. Birds may choose nesting sites based on how detectable their eggs are. Incubating birds can also shield their eggs from UV to prevent reflection that might attract predators.
Navigation
The UV patterns and gradients across the sky may provide birds with additional celestial cues for navigation during migration. In one experiment with Savannah sparrows, individuals with UV blocking films over their cone cells were unable to orient themselves properly under overcast conditions.
Do all birds see UV light?
While most birds likely see some UV wavelengths, a few groups have limited or no UV sensitivity:
- Owls – Owls are violet sensitive rather than UV sensitive, perhaps because they are nocturnal and lack bright UV reflective plumage.
- Penguins – Filtering by corneal oil droplets is believed to block UV light before reaching their retina.
- Storks – Lack UV sensitive cone cells.
- Chickens and turkeys – Have violet vision typical of their ancestors, but may have some limited UV sensitivity.
Conclusion
In summary, most birds have the ability to detect ultraviolet wavelengths of light between 300-400nm thanks to a special UV sensitive cone cell in their retina. UV vision provides birds with many advantages for finding food, selecting mates, navigating, and evading predators. While all birds do not share the same UV visual capacities, the world they see is clearly richer in visual information compared to our own.
