Why are some bluebirds bluer than others?

Some bluebirds are bluer than others. There are even some that look dark. 

Several factors contribute to such variation in color perception, including the fact that bluebirds acquire new plumage every year, the quality and availability of food during the molting period, and being blue in color! 

The color blue in birds is not an actual color or a color derived from pigments. The color blue is a structural color whose appearance is affected by the quality and angle of light.

In this article, I go over the main reasons bluebirds can often look bluer or duller than others. 

The color blue in eastern bluebirds can vary as a result of plumage replacement, food availavility during the molting period, and the and angle and quality of light. Photos: Frank, George Thomas, Fishhawk, and Niki Jordan, Flickr/CC by-SA 2.0

Some bluebirds may be bluer than others for the following reasons:

  1. Bluebirds molt into a new plumage every fall.

  2. The availability and quality of food during the molting period affect the brightness of bluebird plumage.

  3. The color blue appears differently depending on the angle and quality of the light.
  1. Bluebirds molt into a new plumage every fall.

    Adult eastern bluebirds in North America replace their entire plumage once a year between July and October, corresponding to the summer and part of the fall season. It usually starts at the end of the breeding season, continues through the fall, and in some cases, occurs during migration in bluebirds that migrate.

    Compared with worn-out and somewhat faded feathers worn the previous year, fresh plumage is brighter. After the fall, adult bluebirds become bluer.

    Juvenile of the year also replace their gray plumage for the adult plumage during the same period as the adults. The first adult plumage, which is kept until the next fall, is usually duller and more grayish than the plumage of older birds.

  2. The availability and quality of food during the molting period affect the brightness of bluebird plumage.

    The weather and food quality can significantly influence the color of the bluebird’s plumage during the molting period from July through August. Warnock (2017) studied the molt of eastern bluebirds for 12 years in Alabama. 

    Her study revealed that bluebirds gained brighter blue and more vibrant plumages after molting in rainy years, while their plumage was not as bright in less rainy years. Thus, some years eastern bluebirds are bluer and more colorful than others. 

    Because the weather patterns vary across the eastern bluebird’s range, some regions in some years produce brighter bluebirds and others duller ones.  

  3. The color blue appears differently depending on the angle and quality of the light.

    Bluebirds’ vivid blue color is not actually blue, but a trick of nature and an optical illusion.

    The plumage patterns and colors of bird feathers are produced either through selective absorption of specific wavelengths of light by pigments or through the scattering of light by micro-structures arranged at a tiny scale called structural colors.

    The colors of bird feathers are either pigment-based or structure-based. Melanin and carotenoid pigments are among pigment-based colors. 
Abundant food of good quality during the molting period result in bluebirds with brighter plumages. Photo: Festive Coquette, Flickr/CC by-SA 2.0

Melanin pigments are the most commonly observed pigments in bird plumages, resulting in gray, black, brown, rusty, and rufous colors. Actual pigments produce these colors. The color of a feather is reflected in the light that strikes it due to its pigments. Together, the human eye and brain translate light into color.

Carotenoids are different types of color-producing pigments. Carotenoids are molecules that birds acquire through their diet. Birds cannot digest carotenoids and are deposited on their feathers. Red and yellow, as well as their many combinations, are the most common carotenoid-based colors. 

Structural colors are produced through the scattering of light by micro-structures arranged at a tiny scale. These micro-structures are tiny pockets or bubbles of air, each trapped in a film made of keratin, the material that makes the feathers in birds. The film layer of the tiny bubbles scatters the light by selectively absorbing other wavelengths but refract or reflect the blue wavelength, which our eyes and brain together translate into the color blue.

There isn’t really an actual blue color or blue pigment in nature, and both plants and animals have to perform tricks of the light to appear blue.

Is a Bluebird really blue? 

Orioles, wrens, sparrows, and warblers get their colors from pigment colors we see in their feathers. Indigo bunting, blue grosbeaks, blue jays, and of course, bluebirds don’t have any blue pigment. Their feathers perform the trick of selective light scattering that we see as blue.

Hence, the color blue, or structural colors for that matter, are subject to vary in tone and hue. 

Depending on the angle and intensity of light hitting these tiny bubbles in bluebird feathers, the resulting blue can vary from a dark color through the vivid deep blue we see in ideal light conditions.

When it comes to breeding, a bright plumage is essential.

The blue color in bluebirds is structural, while the orange-brown of their breasts is a melanin-based color.

The blue coloration is likely linked to the state of health as it is sensitive to nutritional stress. The intensity and brightness of the color blue are important traits during pair formation. 

By having bluer and brighter plumage, males advertise their health and suitability for breeding. As a result, these males can breed with brighter-plumaged females, which in turn influences reproductive success.

It is unlikely that the melanin pigment in the breasts of eastern bluebirds is an indicator of mate quality due to being less sensitive to environmental conditions during molt.

Conclusions:

Some bluebirds may be bluer than others and such differences in color may be explained by either physiological reasons and our perception of the color blue. Physiological reasons include the fact that bluebirds molt into a fresh plumage every fall, which looks brighter than the old plumage. Also, food quality and availability during the molting period affect the brightness of the bluebird plumage.

Being a structural color, the color blue is prone to be perceived differently depending on the quality of light and the observer’s angle.

References:

  • Liliana D’Alba, Caroline Van Hemert, Karen A. Spencer, Britt J. Heidinger, Lisa Gill, Neil P. Evans, Pat Monaghan, Colleen M. Handel, Matthew D. Shawkey. 2014s. Melanin-Based Color of Plumage: Role of Condition and Feathers’ Microstructure. Integrative and Comparative Biology, Volume 54, Issue 4, Pages 633–644.

2 thoughts on “Why are some bluebirds bluer than others?”

  1. Great article but what word(s) are missing here: “ Conclusions:
    Some bluebirds may be bluer than others … the fact that bluebirds molt into a new plumage every fall.”

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