Hummingbirds are known for their iridescent plumage, which flashes brilliant colors as the birds hover and flit from flower to flower. The iridescence is caused by the physical structure of the feathers, which refract light at different angles to produce changing hues. Understanding what makes hummingbird feathers iridescent helps explain one of the most striking aspects of these birds’ appearance.
What is iridescence?
Iridescence is an optical phenomenon in which color appears to change based on the angle it is viewed from. It is caused by microstructures on a surface that split white light into its component colors through interference. As the viewing angle changes, the path length of the light traveling through the structures also changes, resulting in a shift in the dominant wavelength reflected back to the eye. This creates an effect of various colors sweeping across the surface.
Iridescent structures in hummingbird feathers
Hummingbird feathers generate iridescence through flattened, crystal-like structures called iridosomes or platelet structures. Iridosomes are made up of a protein called beta-keratin and are found in the barbules of the feathers. The flat surfaces of the platelets act like mirrors, reflecting light of specific wavelengths depending on the viewing angle.
There are two main types of iridosomes that create iridescence in hummingbirds:
Multilayer iridosomes
These are stacked, pancake-like platelets that form layers. Each layer has a different thickness and refractive index that reflects a different color. As the viewing angle changes, different layers and color combinations become visible, creating shimmers and shifts in hue.
Three-dimensional photonic crystals
These are crystal-like structures with a lattice-type arrangement. The shape and spacing of the lattice causes interference that selects specific colors to be reflected. Different lattice spacing results in reflection of different wavelengths.
How do iridosomes produce color?
The iridosomes in hummingbird feathers create iridescent colors through a phenomenon called thin-film interference. Here is how it works:
Light waves are reflected by the top and bottom surfaces of the iridosomes
When light strikes the iridosomes, some is reflected off the top surface while the rest enters the structure. Once inside, some of the light is reflected off the bottom surface.
Reflected light waves interact to produce interference colors
The light waves reflected from the top and bottom surfaces can interact and undergo constructive and destructive interference based on the distance traveled. Constructive interference occurs when the light waves are in phase and undergo amplification. This selectively boosts certain wavelengths, producing vivid colors.
Viewing angle determines which wavelengths constructively interfere
As the viewing angle changes, so does the path length difference between light reflected from the top and bottom surfaces. This favors constructive interference of different wavelengths at different angles. Thus, different colors appear from different viewing directions.
Thicker iridosomes reflect longer redder wavelengths
The thickness of the iridosomes helps determine which wavelengths are reflected. Thicker platelets favor reflection of longer wavelengths in the red end of the spectrum. Thinner platelets reflect shorter bluer wavelengths.
Combining multilayer and photonic iridosomes
Hummingbirds may have both multilayer and photonic crystal type iridosomes in their feathers. The combination of the two can enable control over both the hue and brightness of the reflected colors.
Photonic crystals produce directed iridescence
Photonic crystals reflect light directionally, similar to a laser. This makes the iridescent colors appear brighter in specific directions.
Multilayers scatter light in all directions
In contrast, multilayer structures produce iridescence visible from all angles, though it appears dimmer compared to photonic-produced iridescence.
Together, photonic crystals and multilayers create optimal iridescence
Having both types of nanostructures allows hummingbirds to combine the directional intensity of photonic iridescence with the multi-angle visibility of multilayer iridescence. This results in vivid, shining colors observable from any orientation.
Role of melanin in iridescent hummingbird feathers
While the iridosomes produce the iridescent colors, melanin pigments in the feathers also help strengthen and control the color.
Melanin absorbs non-iridescent wavelengths
Melanin absorbs light that is not constructively reflected by the iridosomes. This removes non-iridescent colors from the background, increasing color saturation and purity.
Melanin outlines the iridescent sections
Melanin forms borders around the iridescent parts of the feathers. This demarcates the iridescent patches and enhances the color contrast.
Functions of iridescence in hummingbirds
The dazzling iridescent colors of hummingbird feathers serve several important functions:
Attracting mates
The brilliant plumage helps attract mates. The males, in particular, often have showier iridescent feathers to appeal to females.
Camouflage
Iridescence allows hummingbirds to blend into flowers and foliage. The changing colors match the plants as the birds move around them.
Communication
Flashing certain colors at specific angles may help convey information to other birds. The iridescence essentially allows hummingbirds to signal with their feathers.
Thermoregulation
Some research indicates the iridescent feathers may help keep hummingbirds cool in hot environments by reflecting infrared radiation.
Colors produced by iridescent hummingbird feathers
By controlling the structure of the iridosomes, hummingbirds can produce a spectacular array of iridescent colors from their feathers. Here are some of the most common iridescent hues:
Ruby red
This comes from relatively thick multilayer iridosomes reflecting reddish light. Examples include the ruby-throated hummingbird and rufous hummingbird.
Emerald green
Thinner platelets reflecting bluish-green are responsible for emerald colors. Many hummingbirds have bright green feathers on their heads and throats.
Sapphire blue
Broad spacing in photonic crystals selectively reflects rich blues. This can be seen in species like the blue-throated hummingbird.
Violet
Combinations of platelets reflecting reddish and bluish light blend to create vibrant violets. Anna’s hummingbird produces a striking magenta-violet iridescence.
| Hummingbird Species | Location | Iridescent Colors |
|---|---|---|
| Ruby-throated Hummingbird | Eastern North America | Ruby red throat |
| Rufous Hummingbird | Western North America | Orange red throat |
| Anna’s Hummingbird | Western North America | Magenta purple throat |
| Emerald-chinned Hummingbird | Central America | Blue green throat |
Conclusion
The dazzling rainbow colors of hummingbirds are made possible by iridescent feathers containing nanostructured platelets called iridosomes. Multilayer and photonic crystal iridosomes use thin-film interference to reflect specific wavelengths of light based on viewing angle. Combinations of these structures, along with melanin pigments, allow hummingbirds to produce vivid reds, greens, blues, and violets that help attract mates, provide camouflage, and allow communication. The next time you see an iridescent hummingbird, you’ll know the secret behind its shifting, rainbow colors.
