The male Indigo Bunting in the breeding season is a brilliant blue. Indigo refers to the dark, purplish blue of a plant once grown in some of the southeastern states, especially South Carolina, or the dye of the same name. The natural dye was the original coloring for blue jeans (now synthetic). The Indigo Bunting is not valued for its color, however, but for its eating of insect pests. You would think it to be a good candidate for state bird, but it is not. The Lark Bunting, which is the state bird of Colorado, is actually a type of sparrow, once again showing how confusing common names can be. The Indigo Bunting is in the same family as the Northern Cardinal and Rose-breasted Grosbeak, and its scientific name is Passerina cyanea.
Blue color in birds is not due to a dye or pigment, but instead results from the physical structure of the feather barbs. These are the stiff shafts that look like they could be made from fingernails; both are keratin. The keratin is in very thin layers separated by air spaces, forming a multi-layered mirror. The layers are spaced such that they reflect and amplify short wavelengths particularly well, especially ultra-violet along with nearby wavelengths. (Think ROY G BIV; I represents Indigo between blue and violet.) The bright blue of the bird shows up on sunny days. On cloudy days, there is little blue to be reflected, and UV is invisible to us, appearing black. Fortunately for the birds, they can see the UV that is just beyond the violet in our visual spectrum. This may make them more attractive to the female which is brown. For someone interested in this topic and has access to academic databases, I recommend an article by Richard Prum, Staffan Anderrsson, and Rodolfo Torres, 2003, Coherent scattering of ultraviolet light by avian feather barbs, The Auk 120(1):163-170.
On a personal note, while growing up in rural Illinois, I remember being dazzled by the bright blue of Indigo Buntings at a particular place where the gravel road crossed a bridge by the woods where there was a patch of weeds. There atop the weeds was the male bird, most likely with the female close by. I was not aware at the time of their beneficial work in eating weed seeds and harmful insects. I only saw them in the summer; in the winter they were far to the south, mostly in the tropics. Many locals at the time called them bluebirds, certainly a correct description, but not correct identification. Good references for identification include National Audubon Society: Field Guide to North American Birds (Alfred A. Knopf, Inc., New York, 1998) by John Bull and John Farrand, Jr., and National Audubon Society: The Sibley Guide to Birds (Alfred A. Knopf, Inc., New York, 2000) by David Allen Sibley. I like the way Sibley, who tends to give precise renditions of songs, describes this bird’s song more colloquially as, “fire, fire, where, where, here, here.” It is described as a musical warble.
A pair of the sparrow-sized birds makes a concave nest of leaves and grass near the ground in thick vegetation where they tend 3-4 bluish eggs. They nest over the eastern and central United States and winter from Florida south into the tropics. The Lazuli Bunting is closely related, but the male has orangish brown on its breast. This species lives in the western states, and, where the two species sometimes meet in the central states, they may interbreed. These two species of bunting are among are most attractive birds.
