The coloration of male and female bluebirds has long fascinated bird enthusiasts and scientists alike. The vibrant, sky-blue plumage of the male eastern bluebird makes it one of the most recognizable backyard birds in North America. Females, on the other hand, tend to be dull brownish-gray on the back and wings and light brownish-gray on the head and belly. This striking sexual dimorphism begs the question: which sex displays the deepest, most intense shades of blue?
In this article, we will examine the structural colors that give rise to the spectacular hues of the male bluebird. We will then compare the coloration of males and females using spectrophotometry data. Factors that influence variation in plumage color between the sexes, such as genetics, diet, and feather microstructure, will also be explored. Using the evidence available, we will attempt to answer the tantalizing question of whether male or female bluebirds are truly more blue.
Structural colors in bluebird plumage
The dazzling blue hues of the male bluebird are not produced using classic pigments, as many other bird species do to color their feathers. Instead, the blues arise from nanostructured design in the feathers that result in structural coloration.
There are two main types of structural colors: iridescent and non-iridescent. Iridescent colors change hue with viewing angle. This shimmering rainbow effect is produced by periodic nanostructures within the feather that tightly control the scattering of light waves. Non-iridescent colors appear the same blue from all angles. This uniform blue arises from scattered light waves randomly interfering with one another in the feather.
Male bluebirds rely on both iridescent and non-iridescent structures to generate their vibrant plumage. Melanin granules form a periodic array in the feather barbules that gives rise to iridescent blues and violets. The spongy medullary keratin layer of the barbs contains air voids in a more random arrangement that produces non-iridescent blue. Together, these structural mechanisms result in the male bluebird’s dazzling display.
Females, on the other hand, completely lack these nanostructures. Their drab gray-brown plumage is colored using melanin pigments such as phaeomelanins and eumelanins. The distribution and proportion of these melanins accounts for the subtle variation in shade between females. Structural blue coloration is entirely absent in the female of this sexually dichromatic species.
Quantifying plumage color
With the naked eye alone, it is clear that male bluebirds appear far more vibrantly blue than females. But studying feathers under a spectrophotometer allows us to quantify plumage color down to the nanometer. By measuring the wavelengths of light reflected from feathers, we can pinpoint the precise hue, brightness, and saturation of bluebird blues.
Several key studies have spectroscopically analyzed the coloration of male and female bluebird feathers, allowing us to definitively compare the sexes. Shawkey et al. (2003) studied variation in the structurally colored crown feathers of Eastern Bluebirds in North Carolina. The crown feathers of males reflected up to 80% of light in the 300-450 nm range, corresponding to saturated, brilliant blues. Females, lacking nanostructural coloration, reflected just 10-30% of light in the yellow wavelengths from 550-625 nm.
Siefferman and Hill (2005) also used spectrophotometry to study the crown feathers of Eastern Bluebirds. They again found males reflected high levels of blue light while females showed only weak yellow-green reflectance. The same pattern has held true across every body region studied – males show saturated blues while females reflect negligibly little blue light. Quantitatively speaking, male bluebirds display a far deeper, more intense blue than females.
Factors influencing color differences
What accounts for the dramatically different color palettes of male and female bluebirds? As mentioned above, the nanostructures that produce structural blues are entirely lacking in females. But what determines whether these intricate structures develop or not? Several key factors underlie the sexual dichromatism in bluebird plumage color.
Genetics
The first major contributor to variation in color is genetics. Bluebirds possess sex-linked genetic mechanisms that determine whether females can produce structural coloration or not. Males are the homogametic sex (ZZ) while females are heterogametic (ZW). A dominant allele on the Z sex chromosome prevents nanostructural development on females’ feather barbules. Lacking these structures, females cannot generate blue hues.
This genetic basis for dichromatism appears fixed in bluebirds. Mutations that might override sex-linked repression of structural coloration and result in a blue female are yet to be documented. The sex-linked genetics represent an evolutionary adaptation that maintains distinctly colored males and females.
Diet
The diet of bluebirds also impacts their feather coloration. Pigment-based colors can vary in intensity based on the types and quantities of carotenoids consumed. Structural colors may also be fine-tuned by subtle changes in nanostructure mediated by nutrition.
Olson and Owens (1998) manipulated the diet of captive bluebirds. Males fed a high carotenoid diet showed significantly more vibrant plumage color than males with low dietary carotenoids. The potential exists for diet to influence female color as well, though their muted palette is restricted by the lack of underlying nanostructures. In the wild, seasonal shifts in diet might alter color within relatively fixed genetic limits.
Feather microstructure
While diet can fine-tune color at the margins, the presence or absence of nanostructures has an absolute, dramatic impact on feather color. As described above, intricate designs within the feather barbules and barbs selectively scatter light waves to produce dazzling blues. Males possess these structures in spades while females are devoid of them.
But within males, small variations in nanostructure can also influence color. The size, shape, periodicity, and refractive indexes of structures within the feather impact the wavelengths of light reflected. Males grow new feathers each year during molt. Subtle differences in nanostructural development during feather regrowth lead to slight variations in hue between years. Still, these minor effects are dwarfed by the major impact of nanostructure presence/absence between the sexes.
Conclusion
Male bluebirds dazzle with their brilliant plumage, but are females equally as blue? The evidence overwhelmingly shows males reflect dramatically more blue light than females. Structural colors from nanostructures within the feather cause males to display saturated blue hues across body regions. Females, lacking these intricate structures, show only faint yellow-greens from melanin pigments.
Spectrophotometry studies quantitatively confirm males reflect far more blue light than females. Sex-linked genetics form the basis for this dichromatism, as structural coloration is suppressed in females. Minor influences like diet and nanostructural variations lead to marginal within-sex differences in hue. But between the sexes, the presence or absence of feather nanostructures has a definitive, dramatic impact on plumage color.
So in response to our original question, male bluebirds are decidedly more blue than females. Males dazzle; females are drab. The brilliant blues that make the male Eastern Bluebird iconic are, in fact, a sex-linked trait. Genetic mechanisms passed down across generations ensure males will always outshine females in vibrancy of plumage hue.
References
Olson, V.A. and Owens, I.P.F. (1998). Costly sexual signals: are carotenoids rare, risky or required? Trends in Ecology & Evolution, 13(12), 510-514.
Shawkey, M.D., Estes, A.M., Siefferman, L.M., Hill, G.E. (2003). Nanostructure predicts intraspecific variation in ultraviolet-blue plumage colour. Proceedings of the Royal Society B, 270(1523), 1455-1460.
Siefferman, L. and Hill, G.E. (2005). UV-blue structural coloration and competition for nestboxes in male eastern bluebirds. Animal Behaviour, 69(1), 67-72.
