A global study by UW–Madison researchers reveal how habitat, geography, body size, and beak shape influence bird sounds.

Birds produce sounds to communicate—for attracting mates, deterring predators, or simply singing for enjoyment. However, the factors driving the vast diversity of these sounds remain poorly understood.

Researchers at the University of Wisconsin-Madison have conducted the first global study examining what influences bird vocalizations. By analyzing over 100,000 audio recordings from around the world, their study, recently published in the Proceedings of the Royal Society B, uncovers key patterns explaining why birds make certain sounds and how frequently they do so.

While previous studies have explored how habitat, geography, body size, and beak shape affect bird sounds, these investigations were limited to smaller regions. H.S. Sathya Chandra Sagar, a UW–Madison doctoral student working with Professor Zuzana Buřivalová in the Department of Forest and Wildlife Ecology and the Nelson Institute for Environmental Studies, aimed to test these hypotheses on a global scale.

Sagar analyzed audio recordings of bird sounds taken by people around the world and submitted to a bird-watching repository called xeno-canto. The analyzed recordings represented 77% of known bird species.

Key Findings from the Study

The study’s major takeaways included:

• Bird species’ habitat influences the frequency of the sound they may make, in unexpected ways. For example, in ecosystems with a lot of rushing water there is a constant level of white noise occurring at a lower frequency. In such cases, researchers found that birds tend to make sounds of higher frequency, likely so they wouldn’t be drowned out by the water.
• Bird species living at the same latitudes make similar sounds. Observing this pattern at a global scale is an important piece of the puzzle in the evolutionary story of bird sounds. It could inspire further research into the aspects of geographic location that influence bird sounds.
• A bird’s beak shape and body mass are important. Generally, smaller birds create higher frequency sounds while larger birds create lower frequency sounds. The global analysis not only proved this hypothesis correct, but it also added new information about the nature of the relationship between beak shape, body mass, and sound.
• Smaller bird species tend to have a wider range of frequencies at which they can make sound as a protection mechanism. Smaller, more vulnerable birds can benefit from being able to make a range of sounds. Higher frequencies can help them communicate with fellow birds of the same species, while lower frequencies can serve as a camouflage, tricking potential threats into thinking they are larger and less vulnerable than they actually are.
• The research also contributed to the broader understanding of soundscapes — all of the sounds heard in any particular landscape. Soundscapes are often used as part of conservation studies, but Sagar realized “there’s very little that we know about the forces that govern soundscapes.”
• He hopes this foundational work will provide a platform for future studies to improve conservation efforts by developing ways to monitor the health of an ecosystem through soundscapes.
• “In the tropics and all over the world, larger birds tend to be hunted for meat,” he says as an example. “Larger birds [tend] to call at a low frequency, and if we don’t find any sound in the lower frequency, we could [conclude] there may be more hunting in this landscape.”
• Next, Sagar hopes to use 24-hour soundscape recordings to understand if some birds modify the timing of their song in addition to their frequencies to communicate with their peers in a landscape crowded with noise. And he notes the important role that birdwatchers and citizen scientists play in discovering new insights about our natural world.