Blue-winged warblers are small songbirds found in North America. As their name suggests, the wings of these birds have a distinctive blue-gray color. But what causes their wings to be this hue?
Blue Structural Coloration in Feathers
The blue color of blue-winged warblers is produced by structural coloration in their wing feathers. Structural coloration is caused by microscopic structures in feathers that reflect specific wavelengths of light. In the case of blue-winged warblers, their wing feathers contain a nanostructure that reflects blue light.
There are no blue pigments in the feathers of blue-winged warblers. If you look at a blue-winged warbler feather under a microscope, you would see that the feather barbs contain stacks of hollow melanosomes. Melanosomes are organelles that contain the dark pigment melanin. The melanosomes in a blue-winged warbler’s wing feathers are arranged just right so that when white light hits them, the blue wavelength is reflected back while other colors are absorbed.
This selective reflection of blue light makes the wings appear blue to our eyes. It is similar to the light phenomenon that makes the sky look blue during the day. The structured arrangement of the feather barbs and melanosomes produce the brilliant blue color that gives the blue-winged warbler its name.
Color Differences Between Males and Females
There are some subtle differences in wing color between male and female blue-winged warblers. The wings of the male are a brighter, more vivid blue compared to the female. This is because males have a higher concentration of the nanostructured melanosomes in their wing feathers.
The muted blue-gray wing color of the female is an example of sexual dichromatism, where males and females of the same species display different color patterns. The brighter blue hue helps males stand out and attract mates.
Blue Wings in Juveniles
Juvenile blue-winged warblers when first fledging from the nest do not yet have the vibrant blue wing coloration. Their new flight feathers are a drab gray-brown. As they undergo their first prebasic molt and replace their feathers, the nanostructure that produces the blue color will develop.
By their first spring, the wings of young blue-winged warblers will take on the brilliant blue tint that gives adults their name. The development of the structural color takes time as the intricate arrangement of melanosomes must form as the feathers grow in.
Blue Wings Year-Round
The wings of blue-winged warblers maintain their bright blue coloration year-round. Unlike some other birds that only display vibrant plumage during the breeding season, blue-winged warblers do not have a distinct pre-alternate molt.
Their blue wings can be observed whether it is spring in their breeding grounds, summer on their northern range, fall during migration, or winter in their southern range in Mexico and Central America. The blue hue does not fade or change shade through feather abrasion during the year.
Variations Across Range
The shade and intensity of the blue wings can vary slightly across the wide breeding range of the blue-winged warbler. In general, populations in the northeastern parts of their range tend to be brighter blue. Southern populations may be a more grayish or muted blue on the wings.
There are also two hybrid zones where blue-winged warblers interbreed with the closely related golden-winged warbler. In these hybrid zones, there can be great variation in wing color, with some individuals showing an intermediate greenish-blue hue on the wings.
Diet Does Not Affect Color
The diet of blue-winged warblers does not have any influence on their wing color. Unlike some reddish or yellow plumage that comes from pigments in food, the blue color is structural and determined by feather nanostructure. A blue-winged warbler with a diet poor in carotenoids or other pigments will still maintain its bright blue wings.
Color Persists After Molting
During the molting process, blue-winged warblers shed their feathers and grow new ones. Even with this replacement of plumage, the new wings feathers that emerge keep the same brilliant blue coloration. This is because the structural color arises during feather growth, encoded by genes that direct the patterning of melanosomes as the feather forms.
No Sexual Selection for Brighter Blue
Researchers have investigated whether female blue-winged warblers exert sexual selection for brighter blue wings in males. However, studies have found that females do not favor males with wings that reflect more intense blue wavelengths. This indicates that the difference in wing color between sexes is likely explained by genetic mechanisms only.
Wing Color Differs by Subspecies
There are four recognized subspecies of blue-winged warblers that show subtle differences in the blue coloration of their wings:
- Vermivora cyanoptera cyanoptera – breeding range in southeastern Canada and northeastern US, wings are a violet-blue color
- Vermivora cyanoptera bermudezi – resident breeder in Bermuda, wings are a pale blue gray
- Vermivora cyanoptera chrysoptera – breeds in eastern Manitoba, wings are a greenish-blue cast
- Vermivora cyanoptera pinus – southern Great Lakes region, wings are medium bright blue
The varied hues are likely genetic adaptations to differences in habitat across the breeding range of the species.
How the Blue Color Functions
The bright blue wings of the male blue-winged warbler function to attract females and ward off competing males from their breeding territory. The contrast of the blue against the yellow body plumage helps the wings stand out.
The more subdued female wing color likely provides better camouflage while incubating eggs in the nest. Drabber wings help the female blend in with vegetation to avoid predation.
The blue wings may also serve other functions like species and kin recognition. The unique color can help blue-winged warblers quickly identify other members of their species.
Threats to Blue Wings
The brilliant blue wings that give the blue-winged warbler its name face threats from climate change and habitat loss. As forests are cleared, nesting habitat declines. And studies show the blue wings are difficult for females to produce when temperatures rise beyond optimal levels.
Conservation of mature forest tracts and reduction in carbon emissions are needed to ensure blue-winged warblers can continue displaying their spectacular blue wings far into the future.
Conclusion
In summary, the blue color of blue-winged warblers is a structural color created by nanostructured feather barbs rather than pigments. Males have brighter blue wings to attract mates, while females are duller blue-gray. Juveniles acquire blue wings after their first molt. The blue wings are maintained year-round across the species’ range. While diet does not affect the color, there are subtle differences between subspecies. This unique coloration helps blue-winged warblers communicate and reproduce, but is threatened by climate change and habitat loss.