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?
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?
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.