Birds have 4 types of cone photoreceptor cells in their eyes, allowing them to see a broader range of colors than humans. This grants birds several advantages that aid in their survival.
The 4 Cone Types in Bird Eyes
The 4 cone types in bird eyes are:
- Long wave sensitive (LWS) cones – detect red light
- Medium wave sensitive (MWS) cones – detect green light
- Short wave sensitive (SWS) cones – detect blue light
- Ultraviolet sensitive (UVS) cones – detect ultraviolet light
Humans and many other mammals only have 3 cone types that detect red, green and blue light. Birds, reptiles, amphibians, fish and some insects have the additional UV cone that allows them to see into the ultraviolet spectrum.
Advantages of 4 Cone Vision
Having 4 cone types gives birds a number of advantages:
Broader Range of Color Vision
With an extra cone type for ultraviolet light, birds can see a broader range of colors than humans can. They have tetrachromatic vision that allows them to perceive up to 100 million more colors. This helps birds distinguish between objects, find food and detect camouflaged predators.
Enhanced Detail
The UV cone allows birds to see more visual detail than humans. Things that look identical to us may be easily distinguishable for birds under UV light. This helps with identifying food, differentiating between species and choosing mates.
Improved Foraging
Many fruits, seeds, nectar and insects reflect UV light. Birds make use of their UV vision to more easily find food sources. Certain patterns, markers and guides on plants are only visible under UV and can direct birds towards nutritious nectar or ripening fruit.
Social Cues
Birds use their ultraviolet vision to recognize each other. Certain feathers, beaks, skin patches and urine markings reflect UV and allow birds to identify and select potential mates, see their young, maintain social hierarchies and recognize territorial boundaries.
Predator Detection
UV vision helps birds spot the urine trails and scent markings of certain predators. This allows them to avoid potential danger long before the predator is in visible sight. Birds also use UV when checking that their eggs and young are camouflaged and hidden from threats.
Cone Type | Light Detected | Advantages |
---|---|---|
Long wave (LWS) | Red | Food identification, mate choice, camouflage breaking |
Medium wave (MWS) | Green | Food detection, predator avoidance, detail perception |
Short wave (SWS) | Blue | Detail detection, foraging, social signals |
Ultraviolet (UVS) | Ultraviolet | Enhanced foraging, social cues, predator detection |
This table summarizes the 4 cone types in bird eyes, the light they detect, and the advantages they provide.
Evolution of 4 Cones in Birds
It is believed birds evolved a 4th cone type around 120 million years ago as they adapted to daytime activity. This gave them advantages in seeking food, avoiding predators and breeding:
Transition to Daytime Living
Early birds were ground-dwelling predators that transitioned to being active during the day rather than night. Tetrachromatic vision helped them better operate in daytime light environments.
Fruit and Nectar Diets
Many modern bird species consume fruit, nectar and seeds. UV sensitivity helps target nutrition sources advertised by UV markings on plants.
Wide-Ranging Hunting
Broad color vision aids birds of prey and insectivores in spotting camouflaged or distant prey while hunting over large territories.
Partner and Offspring Recognition
As birds evolved complex social and mating behaviors, UV vision helped with recognizing kin, choosing mates and differentiating species.
In the early stages, having 4 cone types conferred many advantages that made birds better adapted to daytime vision and foraging. This drove the expansion and dominance of diurnal bird species and has been strongly selected for in modern birds.
Distribution of 4 Cones in Bird Retinas
The 4 cone types are not evenly distributed across bird retinas. Some areas have greater densities of certain cones:
- LWS cones peak in density in the dorsal retina.
- MWS cones are distributed evenly throughout.
- SWS cones are concentrated in the ventral retina.
- UVS cones peak in the ventral retina.
These uneven densities are believed to match how birds visually scan their environment. Upward vision used for navigation and predator watch tends to have more LWS and MWS cones. Downward vision used for foraging and food handling tends to have more SWS and UVS cones.
Foveas
Some bird species have areas called foveas with very high densities of cones. Foveas provide enhanced visual acuity in the center of gaze, much like human vision. However, foveas when present tend to contain increased UVS and SWS cones rather than LWS and MWS cones.
Comparison to Human Vision
Human vision differs from bird vision in a few key ways:
- Humans have 3 cone types (lack UV sensitivity).
- Humans see less color variety (only about 1 million colors).
- Humans see less visual detail and cannot detect UV markings.
- Humans have one central fovea optimized for red and green cones.
- Humans cannot visually see magnetic fields.
However, human color vision and detail detection is enhanced compared to many other mammals. Tradeoffs between UV vision, color range, visual acuity and night vision depend on the ecological niches of different species.
Cones in Other Animal Groups
Other animals show a variety of cone numbers and types:
- Mammals – most have only LWS and MWS cones.
- Reptiles – some have SWS, LWS and UVS cones.
- Amphibians – some have SWS, MWS, LWS and UVS cones.
- Fish – some have SWS, MWS, LWS and UVS cones.
- Insects – some have UVS, SWS and LWS cones.
However, spectral tuning of cones and genetic differences mean we cannot always draw direct comparisons across distant species. For example, what we label as “UVS” may detect different precise wavelengths in different species.
Primates
Other than birds, primates are the only vertebrate group with trichromatic color vision from three different cone types. However, primates vary in what cones they possess:
- Humans – LWS, MWS, SWS cones.
- New World monkeys – SWS, MWS, LWS cones.
- Old World monkeys and apes – LWS, MWS, SWS cones.
This demonstrates how different evolutionary paths can lead to varied visual systems, even among closely related species.
Genetics of Cone Types
The different cone classes are produced from distinct types of opsin photopigment proteins that react to specific light wavelengths:
- LWS – LWS opsin gene, reacts to yellow/red light.
- MWS – RH2 opsin gene, reacts to green light.
- SWS – SWS2 opsin gene, reacts to blue light.
- UVS – SWS1 opsin gene, reacts to UV light.
Differences in spectral tuning arise from amino acid substitutions in the opsin proteins. Shifts between cone types occur when mutations cause switches in opsin gene expression over evolution.
In birds, most spectral tuning differentiation occurs on the LWS pigment, while SWS1 remains highly conserved. The UVS cone is thought to have re-evolved in early birds after first appearing in ancestral reptiles.
Oil Droplets
Birds also contain colored oil droplets inside their cone cells. These filter the incoming light, fine-tuning the spectral sensitivities of the different cones.
- Red droplets in LWS cones.
- Yellow droplets in MWS cones.
- Colorless droplets in SWS and UVS cones.
The oil droplets effectively narrow the bandwidth of light reaching the cones, reducing overlap between cone classes. This enhances color separation and allows fitting four cone types into a compact space.
Conclusion
In summary, birds possess four cone types optimized for different wavelengths that equip them with superior daytime color vision and visual detail compared to humans and many mammals. Their ability to see a wider range of colors and into the UV spectrum provides numerous behavior and ecology advantages for finding food, recognizing mates and avoiding danger. This tetrachromatic visual system evolved alongside birds’ adaptation to daytime activity and more complex social lives. The distribution of cone densities and oil droplets in the avian retina facilitates spectral separation and high acuity vision that aids their survival. While not universal in the animal kingdom, multiple cone classes have independently evolved in different vertebrate and invertebrate lines – suggesting tetrachromatic vision confers strong fitness benefits to birds that adopted a diurnal niche.