Subsequent studies supported this hypothesis. In 2016, a research team led by Timothy Genter from the University of California at San Diego reported that starlings (Sturnus vulgaris) shifted sequences can only be recognized if all fine details are removed from the sounds. The results underscore the importance of subtle elements for the animals when listening to their songs.

The waveform of a sound signal can be described on two levels: envelope and Fine Structure. The former is made up of slow fluctuations in amplitude, while the latter is made up of fast fluctuations in frequency and amplitude within the waveform (see “Fine structure”). The fine structure of a sound signal reflects changes in the millisecond range. Many ornithologists have ignored it in the past, partly because it is not immediately visible on spectrograms. However, if you look at the waveform of a single syllable in a detailed enlargement, its acoustic fine structure becomes visible in its details.

Robert Dooling of the University of Maryland pioneered the study of the fine structure of bird song. For decades he and his colleagues have been investigating the ability of birds to detect such acoustic details. In a landmark 2002 study, researchers tested the ability of birds and humans to differentiate between sounds that differed only in their fine structure. All bird species – zebra finches, canaries or budgerigars – performed significantly better: the animals were able to identify two to three times smaller differences in the fine structure than human subjects. The exact physiological mechanism of this high sensitivity is still unknown. It is thought to be related to characteristics of the birds’ inner ear, which, unlike humans, has a shorter and only slightly curved cochlea rather than a spirally coiled cochlea.

A seemingly simple performance

When I started comparing bird song to human speech in 2015 as part of my doctoral research at the University of Maryland, I did not attach any special importance to this fine structure. Instead, I looked for language-like grammatical competences of birds. But as I delved deeper into the question and conducted numerous experiments, it became increasingly clear to me that perhaps the key to understanding bird song might lie more in these fine acoustic details than in the sequences in which they occurred.

The champion bird Dooling tested in his 2002 study was the zebra finch (Taeniopygia guttata). This lively songbird, native to Australia, has established itself as the most popular species in experimental bird song research – not least because it sings avidly and reproduces successfully even in captivity. Moreover, its performance, performed exclusively by the males, appears quite simple: it consists of a single motif of three to eight syllables, which – usually in the same order – is constantly repeated. Because of its clear structure, the song of the zebra finch is better suited for scientific studies than other bird calls. Because the males learn both the syllables and the sequences that occur from a teacher—usually their father—it seems reasonable to conclude that both aspects of the song play an important role in perception.

We tested this hypothesis in 2018 by examining how well zebra finches could hear the difference between natural and manipulated song motifs, in which some syllables appeared either backwards or in reverse order (see “Distinguishing phonetic sequences”). To do this, we taught the birds to tell us the differences in sound they perceived: First, they were played repeated sounds. If they then pressed a button, an experiment was started in which the tones either changed or stayed the same. If the bird pecked a certain button with its beak while the sounds changed, this counted as a correct hit and the test animal received food as a reward. If, on the other hand, it pecked at the button while the sound pattern remained the same, the lights in the cage went out as punishment and the result was rated as a fault. With the help of this experimental arrangement, we were able to determine the birds’ ability to distinguish between the recurring natural song motif and new sounds (motifs with wrong or swapped syllables). For the birds, the experiment was all about earning tasty food.