Starlings are small to medium-sized passerine birds that belong to the family Sturnidae. They are native to Europe, Asia and Africa, but have been introduced widely across other parts of the world like Australia and the Americas. Starlings are best known for their striking plumage that can appear black or have a metallic sheen of green and purple. But can starlings truly be black? Let’s take a closer look.
What determines a bird’s feather color?
A bird’s feather color is produced by pigments as well as the reflection and refraction of light through the structure of the feather. Melanin is the key pigment responsible for darker colorations like black, brown and gray. Carotenoids produce red, orange and yellow hues. But the microstructure of the feather also affects the color we perceive by scattering and reflecting light. So in the case of starlings, the interplay of melanin pigments and structural color produces their dark iridescent plumage.
Melanin pigments
Melanin is the primary pigment responsible for dark coloration across many bird species. There are two main types of melanin: eumelanin which produces black to brown tones, and pheomelanin which produces reddish-brown to yellow hues. The relative concentration and distribution of these melanins creates variation in the dark coloration of feathers. Species with predominantly eumelanin-based feathers, like crows, appear solid black due to the dense saturation of this dark pigment. But in starlings, the melanin concentration is lower, allowing for other effects to influence the hue.
Structural color
Structural color refers to colors produced not by pigments, but by the physical structure of the feather. Tiny nanostructures within the feather barbules selectively scatter and reflect light waves, generating iridescent and metallic colors like those seen on starlings. The shape, arrangement and density of these structures determine the type of light waves that are reflected, and the resulting color produced. In starlings, the combination of melanins and structural color results in their unique dark, but glossy and iridescent plumage.
Starling plumage variations
Common starling
The common starling (Sturnus vulgaris) is the most widespread and familiar starling species. Adults have a black plumage with an iridescent sheen ranging from purple to green. The feathers have a high concentration of melanins, but not enough to produce a purely black coloration. When light hits the feather, some wavelengths are absorbed by the melanins, while other wavelengths are reflected and refracted to produce the iridescent effect. The tips of the feathers are generally more iridescent, while the base is darker black.
Spotless starling
The spotless starling (Sturnus unicolor) is aptly named for its lack of pale spotting that is seen in other starling species. Its plumage appears darker and more uniformly black. However, it is still not completely devoid of the iridescent effect. The high eumelanin content and dense layering of melanosomes produces a deeper black color. But the nanostructures in the feathers still impart a subtle purple-green iridescence. The sheen is just not as brightly colored or conspicuous as in other starlings.
Wattled starling
The wattled starling (Creatophora cinerea) has black plumage with a blue-green iridescent sheen. The feathers have a similar melanin content to other starlings, but the structural color produces a more blue and green colored gloss. This is caused by slightly different nanostructures that preferentially scatter blue and green wavelengths of light. The influence of structural color combined with the melanin pigmentation gives rise to their deep blue-black plumage.
Violet-backed starling
The violet-backed starling (Cinnyricinclus leucogaster) is named for its distinctive violet iridescence. While the feathers contain melanins that absorb most visible light wavelengths, the specialized feather structures refract and reflect violet light, producing this unique color effect. The violet sheen exhibits different hues depending on viewing angle. In some lights, the feathers can also produce a green or blue iridescent glow.
Can starlings produce true black feathers?
Birds cannot produce purely black feathers devoid of all structural color or sheen. There are several reasons why starlings and other iridescent black birds cannot achieve true black plumage:
Melanin concentration
While starlings have a relatively high melanin content, it is not as dense as in birds like crows and ravens. A higher concentration of melanins leads to increased light absorption and less reflectance. But starlings express enough melanins to produce a very dark color while allowing iridescence.
Nanostructures
The specialized nanostructures in starling feathers will always impart some level of structural color by scattering particular wavelengths of light. Feathers without these structures would appear matte and not produce an iridescent effect.
Feather overlap
Overlapping feather layers may help reinforce a darker appearance by absorbing more light. But gaps between feather bases would still allow light to reflect and produce a sheen.
| Bird | Melanin concentration | Nanostructures | Feather overlap |
|---|---|---|---|
| Crow | Very high | Minimal | High |
| Starling | High | Extensive | Moderate |
Blue structural color
In rare cases like the violet-backed starling, the nanostructures may produce a blue or violet iridescence. While this helps deepen and saturate the black color, it means the feathers are still not completely black with a total absorption of visible light.
Adaptiveness of black plumage
While starlings may not be capable of producing feathers that are pure black, their dark iridescent plumage still serves important functions:
Thermoregulation
The black melanin pigmentation provides camouflage by absorbing light, helping the birds avoid detection.
Signaling
The iridescent sheen provides conspicuous flashing patterns used in communication and display.
Sexual selection
The striking plumage is used to attract mates, as those with the most vibrant iridescence tend to be preferred.
Structural strength
Melanin strengthens and toughens feathers, reducing wear and tear. This helps the feathers last longer before needing to be replaced.
So while not truly black, the coloration confers key advantages related to thermal regulation, visual signaling, sexual selection and structural integrity.
Mimicry of black coloration
While starlings cannot produce feathers that are pure black, some other bird species have evolved alternative mechanisms to mimic the appearance of black plumage:
Birds of paradise
In birds of paradise, the barbules of dark feathers are packed very close together, eliminating air gaps that would otherwise reflect light. This creates a velvety, matte black that mimics true black coloration.
Bowerbirds
Male bowerbirds apply a mixture of saliva and charcoal to their display sites. By wetting their black feathers, females spread this dark pigment to their plumage, appearing more deeply black during courtship rituals.
Blackbirds
The coloration of blackbirds is produced through a combination of eumelanin pigmentation and an internal spongy structure within the feather barb that helps trap light. This creates their deep black coloration.
So while structural limitations prevent starlings from producing true black plumage, other evolutionary adaptations can help birds approximate and mimic the visual effect of black coloration.
Conclusion
In summary, starlings are incapable of producing purely black feathers due to the combined effects of melanin pigmentation and structural color. While their plumage appears black to the naked eye, microscopic examination and iridescence reveal a more complex interplay of light absorption, reflectance and refraction. However, the dark coloration serves essential functions related to heat regulation, signaling, sexual selection and feather strength. So starlings provide an excellent example of how limitations at the molecular and nanostructural level ultimately shape the evolution of visible traits and adaptations. While not truly black, their glossy plumage confers advantages that help explain the broad success and distribution of these remarkably colored birds.
