Birdsong is music to human ears.
But have you ever wondered what birds themselves hear when they sing?
After all, we know that other animals' perceptions don't always match ours. Anyone who lives with a dog has probably experienced their incredible acute hearing and smell.
Do birds hear their songs as we do?
Psychologists Robert J. Dooling and Nora H. Prior think they've found an answer to that question — for, at least, some birds. In an article published online last month in the journal Animal Behaviour, they conclude that "there is an acoustic richness in bird vocalizations that is available to birds but likely out of reach for human listeners."
Dooling and Prior explain that most scientific investigations of birdsong focus on things like pitch, tempo, complexity, structural organization and the presence of stereotypy. They instead focused on what's called temporal fine structure and its perception by zebra finches.
Temporal fine structure, they write, "is generally defined as rapid variations in amplitude within the more slowly varying envelope of sound."
Struggling to fully grasp that definition, I contacted Robert Dooling by email. In his response, he suggested that I think of temporal fine structure as "roughly the difference between voices when they are the same pitch and loudness." Temporal fine structure is akin, then, to timbre, sometimes defined as "tone color" or, in Dooling's words, the feature that's "left between two complex sounds when the pitch and level are equalized."
Where do the zebra finches come in? In the Animal Behaviour paper, they're described as highly social songbirds who live in large groups. Males and females cooperate in parental care through what may be lifelong bonds. Males learn their song — and a complex one at that:
"The result of this sensitive period [of learning by males] is a single, highly stereotyped song consisting of an ordering of syllables, termed a motif, that is repeated several times throughout the song bout. Motifs are typically composed of five to eight notes or syllables. Each syllable is an acoustically distinct harmonic complex, which contains multiple cues that result in a unique sound."
Dooling stressed the key finding of the zebra finch research he has co-conducted over the years, and which he and Prior reviewed in their paper:
"Zebra finches have very harmonic vocalizations which are rich in fine structure (as opposed to whistles and pure tones). We know from perceptual tests they can hear these details and we cannot."
More specifically, compared to budgerigars, canaries and, yes, humans, zebra finches discriminate more finely the timbre-like features (related to temporal fine structure) in complex sounds played for them in the laboratory.
We know this because when the finches are trained to discriminate between two songs or song syllables by pecking a key in order to obtain food, Dooling told me, they respond to subtle variations in their song syllables that humans cannot hear.
This acute discrimination ability almost certainly means that zebra finch songs sound different to their ears than to ours.
I wrote at the outset that birdsong is music — that's my phrase, not Dooling and Prior's. And there's scientific skepticism on this point: It may be that we impose music we're familiar with onto the birdsong we hear in some cases, as a kind of default tendency in our brains.
Even if that's true, it's increasingly clear that as much as we construct meaning from birdsong, so do birds; we just don't know a lot yet about the fine details.
"Now whenever I hear birdsong," Dooling wrote, "I am constantly thinking that they are probably communicating much more to one another when they sing than we previously thought. Exactly what, I don't know."
Birdsong remains nearly as mysterious as it is beautiful.
Barbara J. King is an anthropology professor emerita at the College of William and Mary. She often writes about the cognition, emotion and welfare of animals, and about biological anthropology, human evolution and gender issues. Barbara's most recent book on animals is titled How Animals Grieve, and her forthcoming book is Personalities on the Plate: The Lives and Minds of Animals We Eat. You can keep up with what she is thinking on Twitter: @bjkingape