Birds can see light beyond the visible spectrum that humans can see. Specifically, many birds likely have the ability to see some ultraviolet (UV) wavelengths. UV light refers to light with wavelengths shorter than visible violet light but longer than X-rays. Understanding bird vision, including their ability to see UV light, can provide insight into how birds experience and interact with the world.
Do birds see ultraviolet light?
Many birds likely have the ability to see some UV wavelengths, based on the structure and composition of their eyes. Birds have four types of color cones in their retinas, compared to only three in human retinas. This extra type of cone allows birds to see a broader spectrum of light. The UV cones in bird retinas contain a UV-sensitive pigment, allowing them to absorb UV wavelengths.
Multiple experiments have also demonstrated birds’ UV vision abilities. In lab experiments, birds learned to discriminate between UV wavelengths and visible wavelengths. Field studies have shown that some bird species use UV cues when choosing mates, finding food, and navigating. Overall, substantial evidence indicates many bird species can detect some UV wavelengths that humans cannot. However, sensitivity to UV varies significantly among different bird groups.
Sensitivity to UV light across bird species
While many birds can see UV light, their visual ranges vary based on ecology and evolutionary adaptations. Here are some patterns of UV sensitivity across different bird groups:
Songbirds
Many passerine songbirds, such as finches and sparrows, have high UV sensitivity. They see wavelengths ranging from 300-400 nm, on average. Certain species, including the blue tit and zebra finch, may detect wavelengths up to ~365 nm. Songbirds use UV cues in foraging, mate selection, and social interactions.
Birds of prey
Birds of prey, including hawks, eagles, and falcons, have relatively high UV sensitivity. Their visible range is estimated to be around 320-400 nm. UV vision helps raptors hunt small prey like rodents, which reflect UV light in their urine trails. Raptors such as American kestrels also choose mates based on UV-reflecting plumage.
Waterfowl
Waterfowl like ducks and geese have moderate UV sensitivity, though reduced compared to songbirds. Mallards see between ~370-400 nm, while Canada geese likely see from 360-400 nm. As aquatic feeders, UV sensitivity may help waterfowl find food underwater. It can also play a role in mate choice based on UV-reflecting plumage.
Seabirds
Seabirds appear to have the most limited UV sensitivity among birds studied. Albatrosses detect wavelengths only down to 360 nm, while petrels detect only above 380 nm. The reduced UV sensitivity may be related to a marine environment, where UV light rapidly attenuates with water depth. UV cues may be less useful to seabirds that forage over large ocean ranges.
Birds of paradise
Birds of paradise have excellent UV sensitivity, likely extending down to 300-310 nm. These birds use UV signals extensively in their elaborate courtship displays. Females likely assess UV plumage reflections when selecting mates. Species such as Lawes’ parotia even produce a “pseudo-UV” signal that is only visible to other birds of paradise.
Parrots
Parrots have relatively high UV sensitivity, though less than other land birds. Their visual range spans ~365 to 400 nm. Parrots’ UV vision may help them forage for fruit, grains, and other foods. Some parrots use UV reflecting plumage for signaling as well. Overall, parrots have moderately good UV vision compared to other birds.
Owls
Owls have quite limited ability to see UV light. Most owls can only detect wavelengths down to about 390 nm. However, they may have specialized color vision adaptations for low-light and nighttime conditions. The barn owl can still distinguish colors at extremely low light levels. But overall, owls are not as adept at seeing UV as diurnal raptors.
Pigeons
Pigeons can see UV wavelengths down to about 355-380 nm. This gives them moderately good, though not excellent, UV vision compared to other birds. Rock doves are able to use UV cues in foraging for grains and seeds. UV signals are also used in courtship rituals between pigeons. But pigeons do not rely on UV signals to the same extent as some songbirds and parrots.
Anatomical basis of UV vision in birds
Birds’ ability to see UV light stems from the structure of their eye anatomy. Here are some key features that enable avian UV vision:
More color cones
Birds have four types of color cones, compared to only three in humans. The four include long-wavelength-sensitive (LWS), medium-wavelength-sensitive (MWS), short-wavelength-sensitive (SWS), and either violet-sensitive (VS) or UV-sensitive cones, depending on the species. The additional cone type extends birds’ color vision into the near-UV range.
UV-transparent ocular media
The cornea and lens in birds’ eyes transmit UV wavelengths, unlike the UV-blocking lenses in human eyes. With transparent ocular media, UV light can reach the retina and be detected by UV-sensitive cone cells.
Oil droplets
Birds have colored oil droplets in their cones that filter incoming light. Specific oil droplets in the VS/UV cones filter out longer wavelengths, making the cones most sensitive to UV. This tuning by oil droplets enhances UV signals reaching the VS/UV pigment.
UV-sensitive cone pigments
The visual pigments within bird VS/UV cones are tuned to absorb maximally within the near-UV range around 360-420 nm. Shifts in the pigment molecule cause shifts in absorption spectra that tune cone cells to UV wavelengths.
Denser cone distribution
Birds have a very high density of cone photoreceptors in their retinas compared to humans. More cones increases their spatial acuity and light sensitivity. The concentration of cones also enhances spectral sensitivity, including in the UV range.
Benefits of UV vision for birds
Seeing UV wavelengths provides a range of advantages for birds in various behaviors important to survival and reproduction:
Foraging
Many foods reflect UV wavelengths, especially those rich in nutrients like fruits. UV sensitivity aids birds in discriminating food sources like ripe berries. It also enhances visibility of foods like seeds and insects.
Predator avoidance
The urine and feces of mammalian predators reflects UV light. UV vision enables prey birds to readily detect and avoid areas marked by the waste of predators.
Mate choice
In many birds, UV-reflecting plumage functions as an indicator of fitness for sexual selection. Females likely use males’ UV ornamentation to assess reproductive potential.
Social signaling
Some social interactions between birds rely on UV signals that humans cannot perceive. UV vision allows complex communication using these channels.
Migration and orientation
Some birds utilize UV patterns in celestial markings and gradients to orient themselves during migration. UV sensitivity provides more navigational cues.
Brood parasitism
Brood parasites exploit UV signals to infiltrate host nests. UV vision also helps hosts identify foreign eggs. The coevolutionary arms race drives enhanced UV sensitivities.
Measuring avian UV sensitivities
Researchers use several approaches to study the extent of birds’ UV vision:
Microspectrophotometry
Measuring spectral absorbance of photoreceptor cells reveals peak sensitivities of visual pigments. This demonstrates UV vs visible light tuning.
Electroretinograms
Recording electrical responses in photoreceptors shows wavelengths that elicit strongest stimulation. The spectral sensitivity curve indicates UV sensitivity.
Behavioral experiments
Training birds to discriminate different wavelengths establishes visual detection thresholds. Comparing performance at UV vs visible wavelengths is used.
Molecular sequencing
Identifying amino acid substitutions in opsins infers spectral tuning of pigments. Site-directed mutagenesis can test effects of substitutions.
Anatomical studies
Analyzing ocular media density and oil droplets’ filtering establishes optical conditions for UV transmission to the retina.
Bird group | Estimated UV sensitivity range |
---|---|
Passerines (songbirds) | 300-400 nm (high) |
Raptors (hawks, eagles) | 320-400 nm (high) |
Waterfowl (ducks, geese) | 360-400 nm (moderate) |
Seabirds (albatrosses, petrels) | 360-380 nm (low) |
Birds of paradise | 300-310 nm (excellent) |
Parrots | 365-400 nm (moderate-high) |
Owls | 390 nm and above (low) |
Pigeons | 355-380 nm (moderate) |
Comparisons with other animal groups
Birds’ UV sensitivities compare to other animals as follows:
Reptiles and amphibians
Many reptiles and most amphibians lack UV sensitivity. However, some lizards and turtles have UV vision comparable to birds. Snakes in particular have very limited UV perception.
Fish
Most fish cannot see UV wavelengths. A small number of shallow water reef fish have some UV sensitivity, but far less than most birds.
Mammals
No mammals have the visual pigments to see UV light. Some can see violet wavelengths barely below 400 nm. But mammals lack the UV sensitivities found in birds.
Insects
Many insects including hymenopterans and Diptera see into the ultraviolet range extensively, down to 300 nm or below. Insect UV vision exceeds most birds, except some passerines.
Crustaceans
While poorly studied, some crabs and prawns have UV sensitivity comparable to or potentially broader than birds. Stomatopod crustaceans have twelve color cones and likely excellent UV vision.
Limitations and future research
While we are learning more about avian UV vision, there are still many open questions:
Individual variation
Most studies look at species averages, but UV sensitivities likely vary between individuals of the same species based on ecology. More focus on individual variation is needed.
Mechanisms
The complex interactions of oil droplets, opsin spectral tuning, and neural processing that shape UV vision are not fully understood. More physiological work is needed.
Function
We lack complete understanding of the functions and benefits of UV sensitivity for most species. Testing UV’s impacts on survival and reproduction in the wild is important.
Technology constraints
Humans cannot perceive UV, making it difficult to study. Tools to convert UV signals into human-visible ranges would aid research significantly.
More species
We only have UV sensitivity data for a small fraction of extant bird species. Expanding measurements across the avian phylogeny is an ongoing task.
Conclusion
In summary, most birds likely see ultraviolet wavelengths to some extent, due to the structure of their eyes. Exact sensitivities vary across species based on ecology. UV vision provides benefits for tasks like foraging, mate selection, and migration orientation. However, many open questions remain regarding the mechanisms, functions, and diversity of avian UV vision. Overall, sensitivity to UV light is an important aspect of birds’ sensory world that differentiates them from humans. Continuing research on avian visual systems will provide deeper understanding of their sensory experiences.