In the Hans Christian Andersen tale "The Nightingale," a songbird melts an emperor's heart with its singing, but flies away when the ruler forces it to sing duets with a jeweled, mechanical bird that warbles only waltzes. There's a moral here, a new study suggests. Although humans have long attributed musical qualities to birdsong, cold, hard statistics show that's all an illusion.

The birds we prize most for their songs sound most like the human voice, says Robert Zatorre, a cognitive neuroscientist at McGill University in Montreal, Canada, who was not involved in the study. The sounds they make have clear tones, repeat similar phrases, and are made of discrete notes. Despite these pleasing attributes, however, it has never been scientifically proven that the notes in birdsong follow the same organizational rules that govern most musical compositions. In fact, says ecologist Marcelo Araya-Salas of New Mexico State University in Las Cruces, author of the new study, no one has ever addressed the question using quantitative methods.

Billions of potential notes exist between the low and high notes in an octave. But for reasons that researchers only partially understand—the physiological limits of human hearing, for example, and cultural preferences that have evolved over time—most music is based on variations of only five to 12 notes. A baby grand piano, which has 88 keys, is tuned so that each octave is divided into twelve equal intervals, called half-steps, that form the 12-note chromatic scale underlying most of Western music. The seven-note diatonic scale, "do, re, mi, fa, so, la, ti (do)," is another familiar example, as is the ancient five-note, pentatonic scale used in Greek lyre music and nearly every riff played on the electric guitar.

Unlike a piano, a human or bird is capable of making seamless transitions between notes without paying heed to any definite scale, blurring intervals willy-nilly. For the most part, though, we don’t call such random swooping "music." As Zatorre puts it, "We don't sing by going, 'whoooeeeewwhoo.' " Although a jazz singer may add a vocal slide for stylistic effect, the notes themselves are stable and fit within a recognizable musical scale.

To determine whether birds also maintain these stable relationships between notes, Araya-Salas devised a statistical test that measures how often the successive notes in a bird's song adhere to musical scales. To give birds a fair shot, he tested a species known for the beauty of its song: the nightingale wren, which resides in forests between northern Costa Rica and southern Mexico. He analyzed recordings of 81 different birds, measuring the difference in frequency between each note and the next and looking for the same intervals used in diatonic, pentatonic, and chromatic scales. To make sure that his statistical analysis was working, he tested it on 24 musical recordings, including Johann Sebastian Bach’s Six Suites for Unaccompanied Cello and the jazz standard "Autumn Leaves." The recordings, he says, were from instruments that, like a bird's vocal chords, can generate seamless gradations between musical tones, such as cello, violin, and trombone.

The study, published in this month’s issue of Animal Behavior, shows that the resemblance between a nightingale wren's song and music is nothing more than a coincidence. Out of the 243 comparisons Araya-Salas made between nightingale wrens' songs and musical scales, only six matched harmonic intervals. Despite the beauty of birdsong, he says, when we call it music we’re projecting our own biases.

That's only natural, Zatorre says. Humans do the same thing when we listen to languages we don't know, picking up sounds that seem familiar and trying to match them to words we know. "A similar thing may happen when we listen to birdsong," he says. We hear a snatch of birdsong that happens to resemble a common scale and hear it as music. Although birdsong clearly has meaning for the birds, he says, it probably doesn’t mean what we think it does.

Adam Tierney, a neuroscientist at Northwestern University in Evanston, Illinois, who has studied the physical bases of human and birdsong, says he’s hesitant to make generalizations about all birdsong based on a study of one species. But the study has convinced him that nightingale wrens do not follow Western musical scales. That doesn't mean their songs are meaningless, he says, "it's just not human song."