Video: Thrush’s song fits human musical scales

Once described as the finest sound in nature, the song of the North American hermit thrush has long captivated the human ear. For centuries, birdwatchers have compared it to human music – and it turns out they were on to something. The bird’s song is beautifully described by the same maths that underlies human harmonies.

To our ears, two notes usually sound harmonious together if they follow a set mathematical relationship. An octave is a doubling of frequencies. Tripling the frequency of sound produces a perfect fifth, quadrupling is yet another octave, and quintupling produces a perfect third.

These relationships define the most common major chords – the ones that, across human cultures, we tend to find most pleasant to listen to.


Early studies sought to determine whether these mathematical relationships also governed the notes in bird song. Studies in the white-throated sparrow and the northern nightingale-wren failed to find the same musical intervals as those used in human music, and deemed birdsong to be something different entirely.

Making tweet music

The song of the hermit thrush challenges that conclusion. Tecumseh Fitch of the University of Vienna in Austria and colleagues analysed recordings taken in the wild of 70 full songs from this species. They isolated the frequencies corresponding to each note, and calculated the relationships between pitches appearing in each song.

Lo and behold, the vast majority of songs used notes that fitted the same simple mathematical ratios as human harmony. What’s more, Fitch says the thrush can produce other notes – meaning it must choose to use these harmonic chords.

The study shows a natural bias in the thrush towards certain harmonies, similar to those found in humans and some other birds, says Martin Braun of the Swedish organisation Neuroscience of Music in Karlstad, who says the study is an important contribution to the field.

Others remain cautious. Dale Purves of Duke University in North Carolina points out that it concerns just one species, and one component of music – pitch. “What does it all mean? That’s unclear,” he says. The study may explain why the hermit thrush song sounds melodious to our ear, but the debate over whether or not animals have music, and whether theirs is similar to ours, remains very much open.

Journal reference: PNAS, DOI: 10.1073/pnas.1406023111