What makes birdsong beautiful? Answering this question could help us understand how soundscapes make the natural world relaxing for humans, opening the door to improving parks, upgrading the design of stressful places like waiting rooms, and promoting conservation through birds with especially lovely songs. By asking people to listen to birdsongs and rate them for pleasantness, then cross-referencing this with information about the calls' acoustic characteristics, the scientists identified key aspects that make some birdsongs seem more pleasant than others. But although people who valued birds more rated songs more highly, knowing more about birds didn't influence perceived pleasantness.
"You don't have to be a 'bird person' to find bird sounds beautiful," said lead author Dr Nadine Kalb of the University of Tübingen, author of the article in Frontiers in Bird Science. "Being able to recognize birds by their appearance or song did not affect how people rated calls. This suggests that enjoyment of birdsong is connected to something more fundamental than just knowing what you are listening to."
Music to the ears
Pleasantness is a subjective, perceived measure which is affected by personal taste. The scientists suspected that some acoustic characteristics, like a high amplitude — perceived as a loud noise — would be associated with a lower pleasantness rating. But they also suspected some personal characteristics, like individuals' knowledge about birds, would influence pleasantness ratings.
"I have been a birder since I was 10, although at that age I didn't know about ecosystem services or acoustic analyses," said Dr Christoph Randler of the University of Tübingen, senior author. "I'm very interested in human-bird relationships. Bird sounds are often considered one of the most enjoyable aspects of spending time in nature, and previous research has shown that they can contribute to psychological restoration and well-being. However, not all bird sounds are perceived equally positively."
The scientists recruited 84 people and played them short clips of 123 songs from wild birds commonly found throughout Germany. Participants were then asked to rate each clip on a scale from one to five — the higher, the more pleasant. Participants were also asked questions which measured their birding skills and their perception of birds.
The scientists then analyzed the clips, measuring aspects of each song's amplitude, frequency, complexity — the number and variety of elements in the song — and bandwidth, which captured the range of frequencies a song used. A song with a big difference between its highest and lowest frequencies would have a wide bandwidth. This data was then cross-referenced with participants' pleasantness scores for songs and their pre-existing interest in birds.
The scientists identified 21 species among their samples which had also been included in a study about perceptions of birdsong as restorative — helping people to recover from mental fatigue or stress. They compared the ratings these birds received in that study with their pleasantness scores.
Blackbirds over barn owls
A blackbird's song was considered most pleasant while a barn owl's was considered least pleasant. Interestingly, blackbirds' songs were also considered highly restorative. A narrower bandwidth and greater complexity were perceived as more pleasant, as were birdsongs at a higher frequency or with a relatively low amplitude. The songs that scored highest also avoided a key frequency range where human hearing is highly sensitive. People who liked birds more tended to rate them more highly, but their level of bird knowledge had no effect on ratings.
"We might be wired to prefer more variable sounds with moderate amplitudes and frequencies because they signal a safe, healthy, and restorative environment where we can relax," said Kalb.
