Here is one of the sweetest bird songs you will hear. Nathan Pieplow even calls it more musical than most warbler songs. The "sweetness" comes from the long simple frequency sweeps in most notes. I might even go a step farther and say the average Indigo Bunting song is sweeter than those of the other Passerina buntings, such as Painted and Lazuli, although all are similar. Take a listen to this performance and see what you think. Then run the tape again and see if you discern the structure of each song, and of the entire serenade. Don't be distracted by the House Wren, Carolina Chickadee, and Northern Cardinal in the background.
Yesterday we spoke of the "monotony threshold" in birdsong. Indigo Buntings must have a high threshold, because each male only has one song-type. Each bird uses a handful of phrases from a species-wide repertoire. These phrases are recited in the same order, and each is usually produced twice. The bird may leave out some of the phrases, which does produce a little variety. Having the right song may be more important for a male bunting than variety. In some areas, a few males will share the same song. It's thought that dominant males are copied by yearlings and that a persistent song neighborhood results. The bird singing here was completely blue, and therefore an old male. He may not have invented this song, but chances are he will pass it on to his local successors.