A male Swallow Tanager, “one of the most gorgeously colored species of the entire Neotropical region,” would seem to be an easy target for avian predators.
By Rex Graham
However, it morphs from conspicuous to camouflaged at the speed of light. One instant it’s a blue beauty, but when soaring hawks, Harpy Eagles and Bat Falcons look down they see an emerald oval amid a forest background.
Scientists described the male tanager’s lenticular feathers in The Auk: Ornithological Advances.
Swallow Tanager lenticular trick
Birdwatchers occasionally mentioned that the brilliant blue bird sometimes looks emerald green. Many thought the time of day led to a strange color effect. Argentine and Canadian scientists didn’t discover the lenticular blue-green effect with their own eyes. Instead, they used a spectrometer instrument that measured the intensity and wavelengths of light reflecting from tanagers’ feathers at various angles.
The scientists created a virtual bird’s-eye view that included wavelengths from the ultraviolet (UV light is invisible to humans) to the visible spectrum of blue, green and red. They fed their measurements into a model of how the avian retina works to try describe what birds actually see.
Human love of blue
In addition to the tanager’s blue torso, its velvet-black forehead, face and throat contrast with its pure-white abdomen. Humans love blue flowers, blue fish and particularly blue, jays, buntings, rollers, bluebirds, and the Swallow Tanager.
The striking colors of birds, particularly male birds, were impossible for Charles Darwin to reconcile in his groundbreaking theory of evolution in On The Origin of Species by Means of Natural Selection. Still, he was confounded by almost insanely colored birds. Their brilliant plumage and ornaments would seem to make them easier targets for predators and offer no survival advantage.
Natural selection should favor camouflaged, not colorful birds. Right?
Darwin’s color dilemma
Darwin and every birdwatcher know nature doesn’t work that way. He was exasperated. “The sight of a feather of a Peacock’s tail, whenever I gaze at it, makes me sick,” he wrote in a letter to Harvard University botanist Asa Gray.
Darwin later focused on “sexually dimorphic” bird species in which one sex (usually the male) is more brightly colored than the other. He said brighter and brighter feathers arose as a consequence of males competing with one another to be chosen by females. Singing, exhausting courtship battles and displays, and possibly high-quality nesting territory could also win mating rights. These males, Darwin wrote, have a “sexually selective advantage.”
Source of feather blues
The blue color in all feathers comes from the interior structure of the barbs. Under a sheet of keratin, the protein of human fingernails and hair, the interior of a barb is a highly ordered structure of alternating high-refractive-index and lower-refractive-index elements. Microscopically, this interior looks like nano-sized foam. Between air bubbles are nano-structured composite materials made of keratin or sometimes other materials. Many blue feathers have circular rods of keratin and circular air pockets or channels.
When sunlight passes through the nano-structured maze, something magical happens. In the Swallow Tanager, blue wavelengths of light bounce off maximally at a 65° angle. The reflection is called “coherent,” meaning the peaks and valleys of the waves of blue light are in-phase: the blue color intensifies. Other wavelengths of light are scattered, absorbed by an interior layer of melanin, or erased because the peaks and valleys of reflected light are out of phase.
However, at close to 0°, or straight back from the light source, green light is coherently and maximally reflected.
The Argentine and Canadian scientists said in their paper in The Auk: Ornithological Advances that the Swallow Tanager represents the first documented case of double scattering. “Few studies of avian coloration have considered variation in color in non-iridescent plumage coloration resulting from changes in viewing geometry,” the researchers said. They added that light reflected from other bird species with blue feathers should be measured at various angles to look for double scattering.
Male and female Swallow Tanager feathers also reflect UV light in all directions. This reflectance would be visible to each other, but not to humans because we can’t see UV light. Also, hawks and other raptors can’t see UV light, so the tanagers’ UV reflectance would be invisible to them.
Of course, a female Swallow Tanager looks for more than blue- and UV-reflecting feathers. She also may value indicators of a male’s health and vigor, which could possibly be indicated by the brightness of his blue feathers. She also may evaluate his singing, courtship displays, battles with other males, the quality of his territory, and possibly other factors.
Determining female birds’ preferences, in different social and ecological settings, is exceptionally challenging. Understanding a bird’s-eye view will help to explain those preferences and how they are shaping the evolution of the Swallow Tanager’s color.