Red, orange, yellow, green, blue….INDIGO. What is that color, the “I” in ROYGBIV? According to Wikipedia, ”indigo” might refer to a number of hues in the range from blue to violet. But if you look at the jazzy birds singing in and around Ohio’s summer meadows, the answer is clear: Indigo buntings are blue!
The story of how birds achieve their blue color is often told and well known. Birds don’t produce blue pigments. Rather, their blue color is structural. Feathers are made of keratin, like our hair and fingernails, and tiny air pockets (“nanostructures”) within the keratin scatter light so that only blue wavelengths are reflected out. Change the lighting—backlight the feather, or put the bird in shadows—and the blue disappears. But while “blueness” might be their most noticeable feature, two other aspects of bunting biology, related to migration and to song learning, also have been the focus of substantial research.
Each year, indigo buntings fly from Ohio to wintering grounds in the Caribbean and Central America, then return to Ohio around April. Like many songbirds, indigo buntings migrate at night. Presumably that confers advantages like protection from predators, calmer airstreams, and cooler temperatures. But nighttime migration also raises an obvious question: how do the birds find their way?
It turns out that indigo buntings were the subjects of seminal studies into the basis for avian migratory navigation. Ornithologists had postulated that night-flying birds might orient using star maps. Nearly 60 years ago, Stephen Emlen (who’s still affiliated with Cornell University) set out to test that idea by studying indigo buntings. Emlen housed birds in a planetarium, then observed their directional orientation as he manipulated the overhead star patterns.
Those studies revealed that indigo buntings do indeed use the night sky as a compass. Most importantly, they learn star patterns in the north-polar region of the sky, which changes the least over the course of a night. (The entire sky appears to rotate above the earth, so it’s important not to be disoriented by changing star positions.) Those star patterns then provide directional information—just as humans might use the Big Dipper to find Polaris. Navigation by the stars is not an inborn ability; buntings that are deprived of viewing the sky as youngsters do not develop directional migratory behavior.
But what happens on cloudy nights, when star patterns are obscured? Emlen tested an idea about that, too, and he confirmed that indigo buntings have a second superpower. That is, they can detect and orient by the earth’s magnetic field. Even in starless conditions, buntings still could orient using their magnetic sense. Scientists are still unraveling the mechanism of magnetic sensing. Current ideas involve chemicals called cryptochromes that are found in the eyes of migratory birds and that form “radical pairs” sensitive to weak magnetic fields.
Note that the stars provide buntings with a compass (how to head north or south), but migrants also need a map (where exactly are they, and how do they know when they reach their destination?). The identity of that map is not fully resolved. Young buntings often migrate separately from adults, and so “the voice of experience” does not guide them. One possibility is, again, the earth’s magnetic field; its local orientation (inclination and declination) can provide positional information. Other potential cues include topographic features like mountains and rivers, familiar smells, or global wind patterns. Whatever the answer, the system works. Indigo buntings are potentially long-lived (the record in the wild is a bird recaptured in Ohio more than 13 years after it was originally marked), and they return to the same breeding area year after year.
When male indigo buntings do return to Ohio, they quickly get back to the business of singing to establish territories and attract mates. Indigos are persistent singers; they can sing one song per minute for hours on end, and even more frequently than that at the start of the day. Interestingly, though, not all indigo buntings sing the same song. Instead, they have local “dialects.” Wander through a park and one might notice that indigo buntings tend to sound similar for a while—maybe a few hundred yards—but then one enters a new “bunting neighborhood” and the song shifts.
This pattern of song variation results from a couple of processes. The first is that young male indigo buntings (only the males sing) can vocalize disjointed song elements even if raised in isolation. But there are a lot of individual phrases, and to coalesce them into a mature song requires hearing other singing males. That learning continues for about 18 months, well into the second summer of life.
Upon return to Ohio, first year buntings tend to settle nearby to where they were hatched. Their neighbors, which might well (but might not) include their father, then serve as models for song development. However, some young buntings end up settling in locales somewhat removed from their natal nest, where they learn from buntings with songs constructed from a different combination of elements. The result is that neighbors are likely to share songs, but there is always a bit of mixing as birds resettle after migration.
Indigo buntings are songbirds, and the color blue in song often sets a tone of melancholy. “Mood Indigo,” Duke Ellington’s jazz standard, is emblematic. But it’s another, slightly earlier jazz classic that seems more appropriate here. With their vibrant color and their amazing secrets and powers, indigo buntings might well be described as Rhapsody in Blue.
Article and photo contributed by Dr. David L. Goldstein, Emeritus Professor, Department of Biological Sciences, Wright State University.