Hummingbirds are known for their iridescent plumage that shimmers and changes colors in the light. This is caused by specialized feather structures that reflect light. When light hits these structures, some wavelengths are reflected while others are absorbed, producing brilliant metallic colors that shift between green, blue, purple and even red.
Hummingbirds have evolved iridescent feathers for several important reasons:
- Attracting mates – The flashy plumage helps male hummingbirds attract female partners.
- Camouflage – Iridescence allows hummingbirds to blend into their surroundings by reflecting the colors of flowers, leaves and branches.
- Communication – The changing colors may convey information to other hummingbirds.
- Thermoregulation – Some theories suggest the iridescent feathers help regulate body temperature.
So how do hummingbird feathers create these stunning rainbow effects? Let’s look at the specialized structures involved:
Feather Anatomy
Hummingbird feathers are complex microscopic structures. A feather has a central shaft called a rachis. Thin branches called barbs extend off the rachis, and from these branch even thinner barbules. The barbules have projections called barbicels that allow them to lock together.
Normal feathers have smooth barbules, but hummingbird barbules have specialized platelet-like structures. These thin plates are made of melanin pigments and are precisely arranged within the barbules. When light hits them, some wavelengths get reflected while others pass through. This selective filtering of colors produces iridescence.
Melanosomes
The melanin platelet structures in hummingbird barbules are called melanosomes. These melanosomes come in two types – eumelanosomes which reflect blues and greens, and pheomelanosomes which reflect reds and oranges.
By varying the shape, size, density and distribution of eumelanosomes and pheomelanosomes, hummingbirds can fine-tune the wavelength filtering to produce exactly the iridescent colors they want.
Multilayer Reflection
In addition to melanosomes, some hummingbird barbules also contain layers of keratin and air that enhance the iridescent effect. As light passes through these layers, it bounces off the keratin-air interfaces. Constructive interference between the multiple reflections produces stronger colors.
By adjusting the thickness and number of keratin layers, hummingbirds can amplify and shift the hue of the iridescent display.
Iridescent Structures
Different types of iridescent structures create unique effects:
Multilayer Melanin Platelets
- Stacked melanosomes form multiple thin plates
- Reflects greens, blues and ultraviolets
- Found in throat and crown feathers
Multilayer Air Bubbles
- Air bubble layers interspersed with melanin
- Reflects golds, oranges, reds
- Found in gorget feathers
Fractured Melanin Rods
- Elongated eumelanosomes split into fragments
- Reflects blues, greens and bronzes
- Found in tail feathers
By mixing the different iridescent structures, hummingbirds can create stunning color combinations and patterns across their plumage.
Diffraction Gratings
Some hummingbird feathers have fine regular grooves, similar to the lines on a compact disc. These grooves form a ‘diffraction grating’ that breaks up light into spectral colors.
As the angle of light changes, different wavelengths get reflected off the grating, producing a shifting rainbow effect. Hummingbird tail feathers often display this pure spectral iridescence.
Iridescence Through Motion
Hummingbirds have the unique ability to dynamically control the iridescent display as they move. Here are some ways motion enhances iridescence:
Change Viewing Angle
- Shifts the hue, intensity and pattern of colors
- Rapid wing flapping creates color flickering
- Used in courtship displays
Making Feathers Flutter
- Air rushing over feathers makes them tremble
- Fluttering enhances the iridescent effect
- Achived during aerial courtship dancing
Transition Between Light Environments
- Swiftly flying through sun flecks and shade
- Alternating light environments
- Creates flashing and pulsing iridescence
By moving through complex environments, hummingbirds can dynamically control their color displays to attract mates and startle predators.
Molecular Basis of Iridescence
The magical iridescence of hummingbirds originates in the molecular structure of feather proteins:
Pseudocone Nanocrystals
- Formed from beta-keratin protein molecules
- Assemble into hollow nanocone shapes
- Act as photonic crystals splitting light
Ordered Layers
- Keratin and air layers have regular spacing
- Spacing determines reflected color
- Spacing differs in diverse iridescent structures
Precisely Arranged Melanosomes
- Size, shape and packing of melanosomes scatters select wavelengths
- Creates pure spectral colors
- Pattern determines iridescent effect
At the molecular level, iridescence arises from the self-assembly of feather proteins into photonic nanostructures that filter and reflect specific colors.
Evolution of Hummingbird Iridescence
Hummingbird feathers have evolved highly specialized iridescent structures through natural selection:
Ancestral Monochromatic Feathers
Early hummingbird feathers likely had simple melanin pigmentation and no iridescence. Solid matte blacks, browns and rufous colors provided camouflage.
Development of Iridescence
Initial iridescence arose through:
- Changes in melanosome shape and distribution
- Origin of air layers in feather keratin
- Periodic branching of barbules
This conferred brighter plumage and better mating success.
Diversification Over Time
More complex iridescent structures evolved through incremental modifications:
- Increasing layers and melanosome types
- Precise arrangement into lattices
- Tuning of layer spacing and thickness
This allowed fine color control and dazzling iridescent effects.
Differences Between Species
Closely related hummingbird species evolved distinct iridescent designs:
- Adaptations to specific visual backgrounds
- Visual cues for species recognition
- Specialized courtship displays
This promotes reproductive isolation between populations.
Through gradual improvements, hummingbird iridescence evolved into an elaborate ornament for mating success.
Conclusion
In summary, hummingbird iridescence arises from specialized nanoscale structures in their feathers that filter and reflect light. Melanosome platelets, air layers and diffraction gratings produce shifting metallic colors used in courtship displays. Motion further enhances iridescence through color flashing and feather fluttering. At the molecular level, feather proteins assemble into photonic crystals and multilayers that select specific wavelengths. Iridescence evolved through incremental improvements conferring mating advantages. So science reveals the magic behind hummingbirds’ dazzling rainbow feathers!
