Bluebirds And Fall Foliage Have Something In Common

Photo © Raisa Kochmaruk

By Raisa Kochmaruk, Cornell Class of ‘21
and Robyn Bailey, NestWatch project leader

From mountain peaks to foothills all across North America, this fall has brought a particularly vibrant symphony of color—both in birds and the leaves of deciduous trees. While remnants of the autumn migration pass overhead, songbirds are busy in the fiery foliage that rolls down hillsides into meadows and backyards below.

Change isn’t only for the leaves, however. One songbird in particular draws the eye with its flashes of blue, starkly contrasting the golden hues of the fields in which it lives. Similar to the fall color phenomenon, the plumage of the Eastern Bluebird appears more vibrant during some autumns than others. In fact, this year brought out a wave of ‘mega-watt’ bluebirds which look quite out of place alighting on their usual posts. Despite the months of cold and shortening days to come, bluebirds put on fresh attire for the last, glorious days of fall, and they won’t molt again until next fall.

How are bluebirds like fall foliage?

Bluebirds In Autumn
Bluebirds In Autumn

Raisa Kochmaruk illustrated this article in colored pencil, graphite, and watercolor (though not necessarily in that order). Feel free to click the image and download a large version, so you can enjoy the ephemeral beauty of fall for a little while longer.

The answer lies in their responses to weather: bluebird plumage patterns are correlated with the vibrancy of leaf pigments in autumn, and they share the same biological causes. Under optimal conditions, the loss of chlorophyll from leaves gives way to reveal orange-tinted carotenoids and red-hued anthocyanin pigments which account for the drastic change in color. And in the same way that an abundance of sun, rainfall, and frostless nights bring on the most vibrant autumns, these favorable conditions also lead to a plethora of insects and natural forest foods. Birds feast during harvest season to build the stores of energy that will carry them through the winter—and also to enrich their bodies with nutrients for building strong and healthy feathers. So while leaves are not directly related to this late-season molt, they coincidentally are a great indicator of the amount of food available to birds from year to year. Because bluebirds rely on insects as a main source of nourishment, years that have favorable conditions mean lots of available, nutrient-rich food. In short, more bugs can equal more colorful birds (Siefferman and Hill, 2005), and better ornamentation is linked to better reproductive success in both males and females (Siefferman and Hill, 2003, 2005).

All bluebirds are sexually dimorphic, meaning that there is an obvious difference in color between the male and female of the species. While both sexes have melanin-based chestnut coloration and molt in the fall (Gowaty and Plissner, 2015), male Eastern Bluebirds develop a much greater proportion of ultraviolet blue plumage on their heads, backs, and wings. Ultraviolet plumage actually results from microstructures on the keratin shaft which reflect the shortest wavelengths on the visible light spectrum: blue, green, and iridescent purple (Siefferman and Hill, 2003). Even though this structural plumage coloration does not involve pigment, feather composition nevertheless depends on the quality of nutrients obtained during the fall (Siefferman and Hill, 2005). Since each harvest season brings different levels of precipitation and warmth, the abundance and nutritional quality of food fluctuates from year to year. Particularly fertile autumns, such as we are experiencing, can leave bluebirds looking gem-like well into the winter. Unlike us humans, bluebirds can see into the ultraviolet range, and good feather quality is a signal that the wearer is in good condition (Gowaty and Plissner, 2015).

Why molt in the fall?

Fresh Versus Worn Plumage
Fresh Versus Worn Plumage

These two historic (ca. 1930s) specimens from the Cornell University Museum of Vertebrates perfectly illustrate the contrast between fresh and worn plumage. The duller bird on the left has newer plumage (collected in October) than the brighter bird on the right (collected in March). The buff color will break off to reveal more blue feathers as the breeding season approaches (click to enlarge).

You might think that the birds would wait until spring to adorn themselves with shiny new feathers, but in this case, structural plumage coloration involves a bit of delayed grandeur. Freshly-molted bluebird plumage is tipped with buffy-orange (that’s the melanin) giving them a duller appearance, and this part of the feather actually wears away by the time breeding season comes around.

As the orange tips wear down, blue feathers underneath are revealed such that the bird has an overall brighter appearance just in time for the breeding season. In much the same way, autumn leaves don’t really add color but rather lose some pigments thereby revealing others that were always there. As we transition into winter’s subdued color palette, enjoy these final days of blazing leaves and know that when spring returns, the bluebirds will become as the leaves—even more brilliant versions of what they already were.


References:

  • Gowaty, P.A., and J.H. Plissner. 2015. Eastern bluebird (Sialia sialis), version 2.0. In The Birds of North America (P. G. Rodewald, editor). Cornell Lab of Ornithology, Ithaca, New York, USA.
  • Siefferman, L., and G.E. Hill. 2003. Structural and melanin coloration indicate parental effort and reproductive success in male eastern bluebirds. Behavioral Ecology 14(6):855–861.
  • Siefferman, L., and G.E. Hill. 2005. Evidence for sexual selection on structural plumage coloration in female eastern bluebirds (Sialia salis). Evolution 59(8):1819-1828.

Filed under:


13 Comments

  1. SDBuebirds says:

    Very nice scientifically based article, with lovely illustration!

  2. Jason says:

    This artist’s illustration provided something that I have not seen in nature. Bluebirds have been elusive to me in Pennsylvania, but her artistry combined with the vibrant detail of her article have given me a rare glimpse into the nature of bluebirds. Well done. I look forward to your next writings and illustrations.

  3. Paula says:

    Thank you for the download of this beautiful illustration! 🙂

  4. Spring Hollow says:

    Jason-I wish you could come to our part of PA. We have groups of bluebirds all around our property. They’re always fun to watch and beautiful to spot!

  5. Chrisula Stone says:

    Hello Raisa, thank you for this lovely article and for the beautiful artwork. It is now my desktop background!

  6. Emily says:

    Beautiful artwork and a very informative article! I’d guess that these facts about pigment and plumage change also apply to Western Bluebirds; I’ve noticed them getting brighter in spring.

  7. Susan Otto says:

    Excellent article. I love birds and bluebirds are my favorite bird, even though I rarely have bluebirds at my home. That is a little strange though, as I live about one mile from where the late Herman Bressler lived. Mr. Bressler was known as Mr. Bluebird as he built more than 31,300 bluebird nestboxes. Has there ever been a postage stamp for bluebirds? Bluebirds are the official New York State bird.

    • Robyn Bailey says:

      A couple of years ago, there was a “songbird” postage stamp set that included the Mountain Bluebird. It looks like there have been some historic ones, too.

  8. Jefferson Shank says:

    Looks like we have a good artist!

  9. Diana Koutinas BS CNIM R.EP T says:

    Very informative! I never even thought of the color changes in animals relating to the foliage.

    • Robyn says:

      Thanks for reading, Diana. The relationship is more coincidental than cause-and-effect, but we thought the parallels were interesting.

  10. Janie says:

    What an excellent and informative article. I am very fortunate to have bluebirds nest in my bluebird box in the spring. They occasionally visit my heated birdbath in the fall and winter, especially when it snows. I’ve never noticed the fall color changes before, but will pay attention in the future. I love watching all of the birds, but the bluebirds are my favorite.

  11. Merna says:

    Three bluebirds (Eastern) are visiting the suet at my feeders every day now. The males most definitely have a heightened appearing royal blue in these drab winter days. They are a joy.

Post A Comment

Your email address will not be published. Required fields are marked *