Birds produce an impressive variety of songs and calls, but the biological and ecological factors that influence the pitch, or frequencies, that birds create hasn’t been well understood — until now.
Previous studies haven’t come to a consensus likely because they were small in scope, focusing on an individual species or a specific range. But in the largest study of its kind, researchers analyzed more than 140,000 recordings from 77% of bird species across 12 geographic regions worldwide to determine the physical and ecological factors that influence the acoustic frequencies of birds. They used citizen-science data collected on Xeno-Canto, a website dedicated to recording and sharing sounds of wildlife.
“It’s great to confirm and quantify the influences over such an impressive number of species,” Tom Bradfer-Lawrence, a senior conservation scientist with the U.K.’s Royal Society for the Protection of Birds, who wasn’t involved in the study, told Mongabay by email. The scale of the research is “impressive,” he said.
The study examined several factors that could influence bird call frequencies including body mass, beak size, habitat and density of vegetation, range size, and the latitude at which birds live. They also looked at acoustic competition from other birds, and if birds learned their vocalization from a parent or popped out of the egg with an innate song to sing.
The study found that body mass and beak size were the biggest factors contributing to a bird’s call frequency. Large birds in general tend to vocalize at lower frequencies, while smaller birds produce higher frequencies. Birds with larger beak sizes also seem to sing at lower frequencies compared to those with smaller beaks.
The findings of the study could have important conservation uses, says H.S. Sathya Chandra Sagar, the study’s lead author and a Ph.D. student at the University of Wisconsin–Madison, U.S.
“In the tropics and all over the world, larger birds tend to be hunted for meat. Larger birds are assumed to call at a low frequency and if we don’t find any sound in the lower frequency, we could say there may be more hunting in this landscape,” he said in a press release.
A popular hypothesis suggests that birds living in dense forests would vocalize at low frequencies, since low-pitched sounds can bounce past objects more effectively. The data collected in this study, however, don’t support that hypothesis.
Conversely, researchers did find that birds living near the ocean or rivers, a constant source of low-frequency white noise, “tend to vocalize at a higher frequency, which may not be drowned out by the sound of water,” study co-author Zuzana Burivalova, an assistant professor at the UW–Madison, told Mongabay by phone.
Editor’s note: Zuzana Burivalova has previously collaborated with Mongabay on the Conservation Effectiveness series.
Banner image of great hornbills that can weigh more than 2.7 kilograms (6 pounds) and are hunted for meat. Image by Natthaphat Chotjuckdikul via Macaulay Library
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