What Birds Really Listen for in Birdsong

When we humans hear birdsong, which many have appreciated more than ever during the pandemic, we can’t help but think about parallels to human music and language. We discern distinct melodies linking the clanks and buzzes of Song Sparrow songs, sentencelike structure in the Red-winged Blackbird’s pronouncement of conk-la-ree! and a cheery whistle in the wide-open-beaked songs of the White-throated Sparrow.

Birdsong, which has intrigued scientists since Aristotle’s time, is traditionally defined as the long, often complex learned vocalizations birds produce to attract mates and defend their territories. Modern researchers categorize it in contrast to bird calls, which are usually shorter, simpler, innately known and used for a more diverse set of functions, such as signaling about predators and food. These definitions are by no means clear-cut. For instance, some species have songs that are simpler than their calls. But when I refer to birdsong, I mean those longer, more complicated sounds as opposed to the short cheeps and peeps.

The very terminology researchers and laypeople alike use to talk about birdsong reflects the musical and languagelike way it strikes our ears. Getting deeper into the lingo for a moment, when researchers analyze birdsong, we usually break it down into smaller units, termed notes or syllables. We then group the syllables into sequences called phrases or motifs that have characteristic rhythms and tempos. In this way, we can measure potentially important aspects of a song, such as the number of syllable types in a bird’s repertoire or the patterns in which phrases are arranged. These descriptions also parallel the ways we mark the relations among words in human syntax or among notes in musical compositions.

But what do the birds think about all these features? How does birdsong sound to them? Recent research that my colleagues and I have conducted, along with work from a growing number of other scientists around the world, has revealed that birdsong sequences do not sound to birds like they do to us. Moreover, birds appear to listen most closely not to the melodies that catch our ears but rather to fine acoustic details in the chips and twangs of their songs that lie beyond the range of human perception.

Beyond Melody
Birdsong researchers have known since at least the 1960s that birds hear songs differently than we might expect. One of the classic ways to test perception in birds in the wild is through so-called playback experiments, in which investigators play songs to birds and measure their behavioral response. Many birds respond to playback of a typical song of their species as if a territorial intrusion were occurring—they approach the speaker from which the song is playing, fly around the sound’s source to look for the intruder, and emit their own threatening calls or songs. By comparing responses to natural and manipulated songs, researchers can learn which features are important in perception. In the predigital age, they would capture songs on tape recorders and literally splice together the magnetic tape to create manipulated songs with, for example, rearranged syllables or shorter silent intervals between notes. Today digital recording equipment and sound-editing software make such manipulations much easier to create.

The vibrantly blue males of this species deliver songs consisting of syllables that they almost always utter two at a time. Ornithological field guides often call attention to this pattern of paired syllables when describing the song, and it is easily seen in a spectrogram, a visual depiction of song that shows the frequency and amplitude of its signal over time. (The perceptual equivalent of frequency is pitch, and that of amplitude is loudness.) Despite the prominence of the paired pattern to human ears and eyes, when Emlen played a modified song with unpaired syllables to the birds, they reacted with the same intensity of territorial response they exhibited when they heard the natural paired song. This result means that, despite its salience to us, the pattern of paired notes is not significant for the birds in terms of recognizing fellow species members. If the Indigo Bunting were to write a field guide description of its own song, it would differ considerably from our assessment.

Testing how birds perceive songs in the wild is important, but it has its limits. A bird could be out of earshot looking for food when you want to start your experiment, for instance. In the laboratory, researchers can test hearing in birds with more precision and control. When you go to the doctor’s office and have your hearing checked, you are instructed to raise your hand or push a button to indicate that you’ve heard a sound. Researchers use a similar approach to probe auditory perception in birds. Because we can’t explicitly ask the birds, “Did you hear that?,” we train them to peck a button on the side of their cage if they detect a sound or if the sound they hear fits into a particular category or differs from another sound.

Lab studies have found many similarities in auditory sensitivities between songbirds and humans, including the thresholds for hearing differences in pitch or detecting gaps between sounds. But they have also revealed surprising differences between the abilities of birds and humans to hear sequences of sounds and acoustic details.

One pivotal finding from such work is that birds perform surprisingly poorly on recognizing a melody shifted up or down in pitch. This is something humans do naturally: we still recognize the tune of “Happy Birthday to You” if it is played in higher or lower registers on a piano. Classic lab experiments in the 1980s and 1990s by Stewart H. Hulse and his colleagues at Johns Hopkins University showed that for birds, when the pitch of a sequence changes, the tune sounds different, even though the underlying pattern remains the same. Thus, the melodies we hear when we listen to birdsong may be very different from the birds’ perceptual experiences.

Subsequent studies have bolstered that hypothesis. In 2016 a team led by Micah Bregman, then at the University of California, San Diego, reported that European Starlings can recognize transposed sequences but only when all the fine details in the sounds are removed. That work highlights the importance of those fine details to birds when listening to songs.