On May 16 a new bird began singing in the meadow behind our cabin. The weather was warm, and the windows were open. Sitting inside, I could hear him hundreds of yards away, and he went on for hours through the heat of the day, after most other birds had gone silent. Indigo buntings are famous for this. They often sit on power lines under the midday sun, belting out their rich, exuberant, paired phrases with their heads thrown back and their bills wide open.
My local bunting sings something like “too-wee, too-wee, too-werr too-werr, too-wah too-wah,” but that’s just his version. When they are juveniles, individual buntings learn their songs from other males in the neighborhood, but never from their fathers. In this way, regional “dialects” develop and persist for decades, but slowly evolve into something different through innovations by more inventive birds. What makes every indigo bunting song recognizable is the reliable repetition and consistent tone that is somewhere between the ringing quality of the more musical sparrows and the liquid sound of some finches, like the purple finch.
The indigo bunting is a member of the sprawling Cardinalidae family, which includes many, but not all cardinals, and many, but not all grosbeaks, and many, but not all buntings. The indigo bunting (Passerina cyanaea) is related to the lazuli bunting (P. amoena) of the western U.S. but not the snow bunting (Plectrophenax nivalis), which is part of an Old-World family distributed through the high latitudes of the Northern Hemisphere, or the lark bunting (Calamospiza melanocorys), a New World sparrow.
The tropical origins of the genus Passerina are evident in their bright colors and their generally southern distributions. The closely-related indigo and lazuli buntings (they often interbreed) are the only two species to range into southern Canada. The blue grosbeak (P. caerulea), which was recently reassigned to this genus, ranges into the upper Great Plains, but other Passerina species are tropical or subtropical. The painted bunting (P. ciris) reaches North Carolina, and the varied bunting (P. versicolor) gets into southern Arizona and Texas.
Sexual dimorphism is pronounced in this genus. In some icterids, like cowbirds and red-winged blackbirds, the females look like a different species, and that is the case among the Passerina species as well. While the male indigo bunting is entirely dark blue, the female is a stunningly drab light brown. Her only field marks are that her upper mandible is darker than the lower and her breast and sides may be slightly streaked. When she flies you may see flashes of light blue on her wing coverts.
Drab females are an adaptation to remaining hidden on the nest. Hole-nesting birds like woodpeckers and swallows show much less or no dimorphism. Female belted kingfishers are actually more brightly colored than the males. In the orioles, icterids of tropical origin, the females can be quite brightly colored, but they also build a sack-like nest in which they can hide as they sit on their eggs. This isn’t a hard and fast rule: female robins, who will build their open-cup nests on your front porch, are just a bit paler than the males.
Over the course of about a week, a female indigo bunting builds an open cup of leaves, stems, grasses and bark and wrap it in spider webs. The nest is within four feet of the ground and placed in a fork of a branch. She lines it with soft vegetation, thistle down, and sometimes animal hair. It is hidden in dense vegetation, but it is more vulnerable than something covered or placed higher in the canopy or in a hole.
The male takes no part in building the nest, which is likely more about keeping it hidden than chauvinism. While he never incubates the eggs, he does feed the female while she sits and brings food to the nestlings, while she builds another nest nearby for the second brood.
While you will almost never see the secretive female, the male is fond of sitting on exposed perches while he sings. The other day I listened and watched as the local male made his circuit around the entire meadow, singing the whole way.
While he is often seen in the bright sunshine, it can be very difficult to get a fix on his exact color. No blue birds have blue pigment in their feathers. Instead, the color is produced by the refractive structure of the beta-keratin protein molecules. Consequently, the angles between the sun, the bird, and the viewer affect the hue. It appears to range among cerulean, ultramarine and azure or suddenly to gray or black. In the best light, you can see that the head is a darker blue, the only part of the bird that may actually appear indigo.
Like the orchard oriole, indigo buntings arrive in New Hampshire relatively late in the spring and depart by September. They molt during both their spring and fall migrations and undergo a partial molt in late summer, an unusually complex annual cycle. Second-year males may not be entirely blue, even during the breeding season, and fully mature males have varying amounts of brown in their non-breeding plumage. The juvenile (“hatch-year”) birds resemble the females, although the juvenile males may have more blue patches.
These birds have the tendency to migrate directly southward, so our indigo buntings end up in the Caribbean, and Midwestern birds go to Mexico and southward. Their migrating “software” is still not understood. They don’t use the Earth’s magnetic field, but instead appear to use the arrangement of the stars, but no one has discovered which ones or how they do it. It seems though that about 2 out of 25 birds complete two round trips and live to molt into fully blue plumage.
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