Hummingbirds are unique among birds in their ability to see ultraviolet (UV) light. This allows them to see colors in flowers that are invisible to humans and most other animals. Understanding how and why hummingbirds evolved the capacity for UV vision can provide insight into their ecology and behavior.
How do we know hummingbirds can see UV?
Scientists have conducted multiple experiments demonstrating that hummingbirds can see UV light. In one type of study, researchers put out two feeders containing the same concentration of sugar water – one with a UV-reflecting pattern and one without. Hummingbirds consistently chose the feeder with the UV pattern, indicating they could distinguish between them based on UV cues.
Other studies have tested wild hummingbirds’ ability to locate UV-reflecting flowers. When given a choice between artificial flowers with and without UV reflectance, the birds preferentially visited the UV-reflecting flowers. These experiments confirm that hummingbirds use UV vision to locate nectar sources in their natural habitat.
How does hummingbird vision compare to human vision?
Humans and most other mammals have three types of photoreceptor cells in their eyes that allow color vision – sensitive to red, green and blue wavelengths of light. Hummingbirds have a fourth type of photoreceptor that is sensitive to ultraviolet light between 300-400 nanometers (nm) along with the three photoreceptors found in humans.
Having an extra photoreceptor allows hummingbirds to perceive a wider range of colors than humans can see. Flowers that appear uniformly red or pink to us often have UV-reflecting spots or patterns that are visible to birds but not humans. This helps guide the birds to nectar concealed within the petals.
Why did hummingbirds evolve to see UV light?
There are two main evolutionary drivers that likely led to hummingbirds gaining UV vision:
- Foraging – UV vision allows hummingbirds to locate nectar sources from flowers more efficiently. Many flowers have UV patterning to help guide pollinators like hummingbirds and bees to their nectar. Seeing UV gives hummingbirds an advantage in seeking out nutrition.
- Mate selection – Hummingbird feathers reflect UV light. Males that appear the most vibrant in UV to females are more likely to attract a mate. UV vision helps hummingbirds assess potential partners.
The advantage UV vision confers in terms of foraging and mating success is strong enough that this trait underwent positive selection as hummingbirds evolved from other ancestral bird species.
How do hummingbird flowers use UV markings?
Plants pollinated by hummingbirds and insects often have UV markings on their flowers that help attract the pollinators. These markings take various forms:
- Central spots – Flowers may have a UV-reflecting spot that helps guide the pollinator to the middle of the flower where nectar is located.
- Stripes or lines – Petals may have UV-reflecting lines or patterns leading pollinators to the nectar.
- UV halos – Some flowers have UV halos around the center that only pollinators can see.
These cues allow pollinators to locate a flower’s nectar more efficiently. This benefits both the pollinator, which gets rewarded with food, and the flower, which gets its pollen spread to other plants by the pollinator.
How do hummingbirds see UV?
Hummingbirds see UV light because of special pigments called oculars in their photoreceptor cells that are sensitive to those wavelengths. Here is how it works:
- Light enters the eye and passes through the cornea.
- It travels through a clear lens which focuses the light.
- The light hits the retina, which contains photoreceptor cells called rods and cones.
- The photoreceptors contain visual pigments called opsins. Hummingbirds have four types of opsins, one of which picks up UV light.
- When the opsins detect the light, they trigger neural signals which travel to the brain via the optic nerve.
- The brain processes these signals into colors and images.
The specific opsin in hummingbirds that enables UV vision is called UV opsin or VS opsin. This allows the photoreceptor cell to send signals to the brain when it detects UV light.
How do oil droplets enable UV vision?
In addition to special opsins, hummingbirds have oil droplets in their cone cells that allow UV vision. These oil droplets act as filters that narrow the spectrum of light entering each cone.
Most birds have three types of cone oil droplets that filter light into red, green and blue ranges. But hummingbirds have a fourth cone type containing a clear oil droplet that allows UV light to pass through to the UV opsin below.
Without this clear “UV-transmitting” droplet, the UV wavelengths would be filtered out before reaching the visual pigment. The oil droplets enable hummingbirds to have much more refined color vision, including in the UV spectrum.
How do hummingbird feathers reflect UV?
Many hummingbird feathers reflect UV light. When illuminated by sunlight, areas of feathers glow brightly in the UV range to bird eyes but appear dull to humans. The main ways feathers reflect UV include:
- Pigments – Some pigments in feathers absorb most light but reflect UV. These create UV-bright patches without affecting color in human vision.
- Nanostructures – Microscopic physical structures of the feather can selectively reflect UV through interference effects. These nanostructures enhance UV colors.
- Combination – Some feathers use pigments and nanostructures together to fine-tune both color and iridescence.
Male hummingbirds often have the most UV-reflecting feathers. When they spread their tail and fan their feathers during courtship, the UV reflectance becomes a sexual signal to attract females.
How does UV vision help hummingbirdcourtship?
UV vision helps hummingbirds assess potential mates. Males have brightly colored iridescent feathers that reflect UV light. During courtship displays, males will fan their tails, spread their wings or make dive displays to show off these flashy feathers.
Females observe these displays and the UV reflectance helps indicate a male’s fitness. Females preferentially mate with males that have higher UV reflectance, as it often correlates with factors like:
- Health and nutrition status
- Physical condition
- Reproductive maturity
- Experience and survival skills
By selecting mates with brighter UV plumage, females likely gain both good genes and paternal care benefits for their offspring. The UV vision helps female hummingbirds make mate choices that increased reproductive success.
How does UV vision vary among hummingbirds?
Research has found some variation in UV sensitivity among hummingbird species. For example:
- Ruby-throated hummingbirds have some of the strongest UV sensitivity. They see wavelengths down to 325 nm.
- Rufous hummingbirds see UV light down to about 350 nm wavelength.
- Some cloudforest hummingbird species may not see UV light below 370 nm.
In general, lower wavelength sensitivity allows a hummingbird to perceive a greater variety of UV cues. Ruby-throats have an advantage in discriminating among flowers or mates based on fine gradations in UV patterning.
There are also some gender differences. In Broad-tailed hummingbirds, males see about 20 nm further into the UV spectrum than females. This extra perception likely evolved to help males spot small variations in UV plume signals from competing males.
Conclusion
In summary, hummingbirds evolved UV vision primarily to help them find nectar and select mates. Specialized photoreceptors and oil droplets in their eyes allow hummingbirds to make use of UV signals from flowers and feather plumage. This unique visual adaptation provides hummingbirds with a more vibrant view of the world that facilitates essential behaviors in their daily lives.
