You can hear an indigo bunting singing from a block away. Not only hear it, but see it as well. Indigo buntings seek the highest branches for their song perches, and they prefer dead branches with no leaves to obstruct the world’s view of their bright blue plumage.

Their song–like a human singer’s interpretation of an old standard–are individualized renditions of the general pattern of the species. Passing through an area where several males sing, you can fairly quickly learn to recognize each separate singer’s version of the standard. The feature they all share is a strong tendency to repeat each of the two- or three-note phrases in the song.

I have had a chance to get to know several indigo buntings this year while working on a survey of breeding birds at Somme Woods, a Cook County forest preserve at Dundee and Waukegan roads in Northbrook. They seem to be the most common nesting birds at Somme. One morning late in May, I counted eight singing males in one portion of the preserve. Numbers have declined since then. Some of those males could not find mates or could not hold their territories against rivals and so moved on without breeding.

The sweetness of bird song and its undeniable connection with mating lead us to think of it as love song. It is in a way, but it is not the sort of love song that showers extravagant compliments on the lady. It is more like an ad in the personals, a catalog of the singer’s virtues, an advertisement for a mate. It is also a literal expression of machismo, a warning to other males that they had better stay away from this territory and any females that choose to live in it.

The closest human equivalent to this fusion of sex and aggression is the old Bo Diddley song “Who Do You Love,” which begins: “I walk 47 miles of barbed wire / Use a cobra snake for a necktie / I got a brand-new house on the roadside made from rattlesnake hide / Got a brand-new chimney up on top made out of a human skull / Now come on take a little walk with me honey and tell me, Who do you love?”

The aggression in the indigo bunting’s song is clearest along the borders where the territories of two males meet. Two birds will sing at each other for several minutes at a time, and then one will launch himself at the other, precipitating a high-speed, twisting chase through the trees–two furious blue bullets heedless of consequence. “Got a tombstone head and a graveyard mind / Just 22, and I don’t mind dying. Who do you love?”

Male indigo buntings in breeding plumage are the only all-blue birds in the eastern United States. Actually, they aren’t quite all-blue. There is some mixture of black in the wings and tail. And in one sense, they aren’t blue at all. Bluebirds, whether they are bluebirds, blue jays, indigo buntings, or cerulean warblers, have no blue pigment in their feathers. The physical structure of their feathers, in ways that are not exactly understood, reflects blue light while absorbing the rest of the visible spectrum. Cardinals–whose color comes from carotenoid pigments–look red even under an overcast sky, but when the day is gray, indigo buntings look gray, too.

Female buntings look brown no matter what the weather. They are shy creatures that would rather lurk in the brush than call attention to themselves. The females handle most of the work of producing a new generation, from nest building through incubation to feeding the nestlings.

The nest is built near the ground in dense brush at the edges of woodlands. Somme Woods, which is being gradually converted to a combination of prairie and savanna, has a lot of this kind of edge habitat, which is why indigo buntings have become the most common breeding bird in the preserve.

Somme Woods was the site of my second encounter this year with large numbers of indigo buntings. The first came in February in the Yucatan where I saw them hanging around the ruins at Chichen Itza. Their behavior was quite different in Mexico. The males were as subdued as the females, neither singing nor chasing each other about. Their plumage was quieter, too. They were just molting out of their brown winter feathers and into their bright blue breeding plumage, and they had the mottled and slightly raggedy look that birds get at such times.

Seeing indigo buntings in southem Mexico is disappointing in a way–why go that far to see birds that might land in your backyard?–but I love the sense of the world’s small size and great unity that such sightings give me. Indigo buntings breed in the eastern U.S. and southern Canada. They reach their western limits in the riverbottom forests of the Great Plains, where they sometimes hybridize with their western counterpart, the lazuli bunting.

They winter from southern Mexico to Panama, performing long flights across the Gulf of Mexico and the Caribbean on their way to and from the south. The flights of migrating birds are one of the wonders of animal life. Creatures weighing an ounce or less not only travel thousands of miles a year but navigate so skillfully that they can return to the same breeding ground each spring and to the same wintering area each fall.

We know from experiments that birds use a considerable array of navigational clues to achieve these epic flights. They guide themselves by the sun and the direction of the sunset, by the direction of winds aloft, by changes in the earth’s magnetic field, and by major topographic features, and they also rely on the stars just as human navigators do.

An ornithologist named Stephen Emlen has studied stellar navigation in the indigo bunting. Using captive birds, he carried out a series of experiments that both proved the birds use the stars and showed just how they use them.

Songbirds usually migrate at night, and captive birds display migratory restlessness in spring and fall, staying awake all night and trying to fly. Emlen confined restless buntings–one bird per cage–in funnel-shaped paper cages with transparent tops. He placed the cages so the birds could see the night sky but no terrestrial landmarks of any kind. He put an ink pad at the bottom of each funnel.

Standing in their cages, the birds got ink on their feet, so every time they tried to move they left tracks on the side of the funnels that showed which direction they were trying to go. On clear nights in spring, the birds repeatedly tried to go north. In fall, they tried to go south. On overcast nights, they made an equal number of starts in all directions.

Emlen used a planetarium for the rest of his experiments. Inside the building, he could manipulate the night sky to change the season or the time of night and see how his birds reacted. He discovered that they didn’t have an internal clock. They did not change their flight direction when the artificial sky varied from local time.

Then he began a series of experiments involving blocking out portions of the sky and testing the birds’ reactions. He learned that the birds were responding to the patterns of stars in the sky just as humans orient themselves by finding familiar constellations, and that all the patterns they used lay within 35 degrees of the North Star. The major constellations in this region of the sky are the Dippers, Cassiopeia, Cepheus, and Draco.

The birds knew many different patterns from this part of the sky, so the removal of one, such as the Big Dipper, didn’t affect their ability to stay on course. Presumably, this redundancy in the system would be helpful on partly overcast nights.

The northern sky is most helpful for navigation because the North Star appears to stand still and the stars near it appear to move in tight arcs around it. The birds use these northern stars in both spring and fall, flying toward them in spring and away from them in fall.

Emlen was also able to determine that the choice of direction was under hormonal control. By altering day length–the principal stimulant of molting, migration, and many other sorts of behavior–he was able to put indigo buntings in a fall condition during spring. These birds listened to their bodies and not the calendar and tried to fly south.

The most intriguing of Emlen’s discoveries was that young indigo buntings learn the sky during their first summer of life. Birds raised from infancy under diffuse lighting with no visible point sources had no navigational sense at all. Another group was raised in the planetarium where the “sky” was manipulated to make Betelgeuse, the red giant in the constellation Orion, into the pole star. All the other stars rotated around it rather than around Polaris. In the fall, when the young birds’ migratory restlessness appeared, they set courses 180 degrees from Betelgeuse, flying in the direction their experience had told them was south.