Summer days in Washington, D.C. are usually humid, with an opalescent air that slouches low. This day was different. Rains from the previous night cleared the air, giving way to an endless cornflower blue sky arching over the Anacostia marsh.

A young couple and their boisterous children were approaching me on the boardwalk, just where it emerged from the forest edge. The birds had fallen silent, retreating to the safety of thicket and reeds. Moments after the family passed by, though, the chattering of a pair of wrens and the ronk-a-ree of red-winged blackbirds marked the re-emergence of the birds. A dozen yards ahead, a solid blue songbird popped out of the undergrowth. After hesitating in the shadows, he headed for the nearby sycamore, transforming from twilight blue to brilliant cyan in the process.

In the sunshine, the male indigo bunting (Passerina cyanea) was an arresting turquoise blue. The upper part of the short, heavy bill is black, which yields to a hint of deep purple on the face before quickly turning into indigo. The wings and tail transition from the bird's dominant color into black.

In the shadows, he looked much darker. It wasn't the usual lack of color clarity that comes with poor lighting-the indigo bunting isn't blue at all. The bird's feathers contain no blue pigment. The structure of the feathers diffracts light so that we see only the primary color blue. The amount of light doesn't just illuminate the male bunting; it changes our perception of the bird's color.

Female indigo buntings display no such optical tricks. They are a dull brown, as are the young males.

Indigo buntings are small, averaging a bit less than 5 inches from beak to tail. They weigh half an ounce.

In winter, the birds live in Central America and the Caribbean. Every spring, they migrate north, looking for open fields and forest edges. These stunning songbirds can be found during the summer from New England to North Dakota and south to Texas and into the U.S. Southwest.

Like many other birds, indigo buntings migrate at night. Researchers long wondered how birds accomplish the feat. A father and son team from Wisconsin, John and Stephen Emlen, used indigo buntings to test their theory that night migrants use the North Star Polaris as a compass point. In a series of inventive experiments, the Emlens put captive indigo buntings in a large planetarium. The birds consistently oriented themselves toward the northern stars, regardless of where the Emlens projected the constellations on the planetarium's ceiling. The researchers went so far as to wink out Polaris, but the birds maintained their northerly orientation. It wasn't until the team erased about a third of the stars that appear to revolve around Polaris that the birds lost their sense of direction.

The male that I saw was not engaged in feats of celestial navigation. He was foraging in the sycamore for insects. That's a typical feeding practice. These birds glean bugs from branches, bounding from branch to branch in search of prey. Indigo buntings also eat seeds, berries and the tender buds of trees.

They build their nests in low scrub. Females lay one to four eggs in a cup-shaped nest of twigs and leaves held together with spider webs. The mother incubates the eggs for up to two weeks before the chicks hatch. The young birds are helpless at birth. In just 9-13 days, the chicks are ready to fledge.

The parents may produce a second brood that same summer.

Male indigo buntings learn their characteristic mating song in their second summer. Subtle variations in song delineate different avian territories. If a young male tries to establish himself in an area away from his original nest site, he will learn to sing from the other fellows in the neighborhood, and it will not be the same sequence of notes that his father is singing in his different territory.

The male I watched was too busy eating to do any singing. He emitted an occasional call note that sounds like "spik," but mostly just kept dazzling me with that captivating blue as he hopped from spot to spot, gobbling up insects.

In the distance, I heard talkative teens approaching and knew that the bunting would soon retreat to the underbrush. The bird perceived the youths as a threat when in reality they were simply enjoying a beautiful walk.

I had my own misperceptions, as I stood there admiring a gorgeous "blue" bird that was actually black. We live in a physical world; but that young family, the approaching teens and this solitary birder were having three distinct experiences on this short stretch of boardwalk.

Our reality, it seems, is colored by the slant of light and where we stand.