The Factors That Make Bird Sounds So Diverse Across The World

FLORA LICHTMAN: This is Science Friday. I’m Flora Lichtman. Birds are the virtuosos of the animal kingdom. They can make a remarkable range of sounds. But why is a barred owl more of a tenor–

[OWL HOOTING]

–while a cedar waxwing is more of a soprano?

[WAXWING WARBLING]

My next guest analyzed 100,000 bird sounds from all over the world to try to peck out some kernels of knowledge about what determines a bird’s vocal range and the sounds that they make. Joining me now is one of the study authors, Dr. Zuzana Burivalova, assistant professor of forest and wildlife ecology at the University of Wisconsin-Madison based in Madison, Wisconsin. Welcome to Science Friday.

ZUZANA BURIVALOVA: Hi. Thank you for having me.

FLORA LICHTMAN: OK, so you analyzed 100,000 bird sounds. Where did they come from?

ZUZANA BURIVALOVA: So in this research, which was actually led by a PhD student in my lab, Sathya Chandra Sagar, we analyzed more than hundred of thousand recordings that came from this amazing repository of bird sounds called xeno-canto, and this repository is where anyone can upload sounds of birds that they encounter in the forest, in their backyard, or anywhere.

FLORA LICHTMAN: Wow, so these are birders who made this dataset that you used.

ZUZANA BURIVALOVA: Yes, they’re birders. They’re citizen scientists. They’re nature lovers. Anyone can upload recordings.

FLORA LICHTMAN: And they’re from around the world?

ZUZANA BURIVALOVA: Yes, they’re from around the world. They are from every single continent, biome. Obviously, there are some areas that are better represented than others, but because so many people participate, there’s a pretty good coverage, actually.

FLORA LICHTMAN: So tell me what you found. Give us the bird’s eye view.

ZUZANA BURIVALOVA: The bird’s eye view, yes– so Sagar found a lot of different things in terms of testing the major hypotheses about what rules the birds sounds. One of the most important one– and that one has been known for quite a while– is that the bigger the bird, the bigger its body size, the lower the sound it makes. We found that, yes, this relationship is true, and we were able to confirm it across all these different species, showing that really the body mass of the bird rules how deep it can go.

FLORA LICHTMAN: Which is not that surprising, right? That’s often true in other animals that we might be even more familiar with, like ourselves.

ZUZANA BURIVALOVA: Exactly, if you think of elephants having really, really low-pitched sounds as low as infrasound and then if you think of mice making sounds at high frequencies, it’s not something that’s surprising. But the novel thing we were able to do is to really isolate this effect and separate it, for example, from even the role of the beak size and other things that turned out to be important, like the latitude and the type of habitat.

FLORA LICHTMAN: Well, let’s talk about beak size. How did beak size seem to influence or correlate with the types of sounds a bird made? As

ZUZANA BURIVALOVA: You can imagine, the bigger the bird, the bigger the beak in general, but even when we take this kind of correlation into account, still some birds might have disproportionately large beak for their size. So the beak size itself was also related to the frequency at which they make sound, again with bigger beaks having lower frequencies and smaller beaks being able to make sounds at higher pitch.

FLORA LICHTMAN: Can you give me an example of a bird I might know?

ZUZANA BURIVALOVA: So for a small one with a small beak, you might know, for example, the winter wren.

[WREN CHIRPING]

And for a big one, for example, all the hornbill species that are common in tropical regions are pretty big animals with even larger beaks.

[HORNBILL CRYING]

Coincidentally, a lot of these hornbill species are actually threatened because the beak is being targeted by poachers who use it for all sorts of different things.

What about geography? How did that play a role in the sounds that birds are making?

Specifically, the latitude at which the species occur played an interesting role. What we found was that at the equator, the bird species making sound had a smaller range. So they were, in a way, occupying a narrower bandwidth than birds at northern latitudes.

FLORA LICHTMAN: Well, why would birds at the equator have a smaller range of sounds?

ZUZANA BURIVALOVA: This is one of the puzzles of birdsongs. One potential explanation is that at the equator there are many, many more species, and so the bird species might have to carve for themselves out a narrower niche, narrower bandwidth that they will be using so that they could effectively communicate with each other.

You could think of it, for example, if you had an orchestra with lots and lots of different types of instruments at the equator, a really rich orchestra, each instrument might be given a smaller range of frequencies at which it can play so that the listeners can hear each instrument, whereas at the temperate zones, we might have an orchestra with fewer instruments, and each instrument can cover a broader bandwidth.

FLORA LICHTMAN: So basically, if you’re in an environment where there’s a lot of sound, you don’t want to compete across a lot of different frequencies. You want to carve out your own little sound niche, frequency niche.

ZUZANA BURIVALOVA: Yes, and this idea of species might be competing for the acoustic space was proposed already more than 30 years ago by Bernie Krause, and we looked at this question, whether this is the case, at several different ways. We looked at the latitude, and we know that at the equator there are many species. And we did find this relationship.

But then Sagar also calculated for every single point for where we had recordings. He calculated how many other species are there likely to be both for birds and frogs so that we could see whether the species richness actually correlates with this bandwidth. And we did not find that that was the case. So it’s still a little bit of a mystery why exactly this is happening across the latitudes.

FLORA LICHTMAN: You also found that, on average, smaller birds had a larger vocal range. They could hit low notes and high notes. Why? Why would that be?

ZUZANA BURIVALOVA: This is something really interesting, and we didn’t really expect this. One of the reasons that this might be the case is that smaller birds might want to, on occasion, pretend that they’re actually much bigger than they are, and the way to do this might be by lowering the frequency, so lowering the voice, so that other individuals might perceive, oh, I can hear this really low-pitched sound, I am about to encounter a big, scary bird, whereas actually it’s a tiny bird.

FLORA LICHTMAN: Now, you don’t only study birds. I understand you lead something called the Sound Forest Lab. What is that?

ZUZANA BURIVALOVA: In the lab, we’re interested in, actually, all animal sounds, not just bird sounds. We are using the sounds that animals make across tropical forests but also in temperate zones to better be able to monitor biodiversity to understand how healthy forests are, what is the impact of different things that humans do, and all that using sounds.

FLORA LICHTMAN: Where have you recorded?

ZUZANA BURIVALOVA: Oh, we’ve recorded at lots of different places, and currently the lab is working on this soundscape baselines project where we’re trying to record soundscapes at the world’s most intact remaining forests. So some examples are in Gabon in Central Africa, in Ecuador, in Peru, also in Brunei, and also at one forest in Wisconsin and in Germany.

FLORA LICHTMAN: Do you go there yourself and place the microphones?

ZUZANA BURIVALOVA: Usually, what it looks like is that I go– for example, I went to Ecuador and Gabon and Peru, but there I work very closely with local scientists or community conservation practitioners or people who know the forest really, really well. And together we place the microphones and decide where should we be recording.

FLORA LICHTMAN: We’ve got a clip of one of your soundscapes from Borneo, the Borneo rainforest. Before we play it, what should I be listening for?

ZUZANA BURIVALOVA: Oh, you should be trying to spot as many species as you can or at least, especially, different groups of animals. One example you might listen out for is the hornbill that we mentioned earlier, a bird with a really big beak, and you might be even able to hear its wingspan. I think in this recording you can also hear some gibbons.

[BIRDS CHIRPING]

FLORA LICHTMAN: OK, so what did you hear in that? Well,

ZUZANA BURIVALOVA: I got completely transported back to the rainforest in East Kalimantan in Borneo, and I could hear the hornbill right in the middle, this kind of almost dinosaur-like sound. You can also hear the gibbons singing because this is early in the morning just when they are waking up. There are lots of different bird species. I think I heard the white-rumped shama in there and then, of course omnipresent background of insect noise.

FLORA LICHTMAN: Can you use computer analysis or AI to pick out the different animals in there?

ZUZANA BURIVALOVA: We are using a lot of artificial intelligence to just sift through these recordings and be able to pick up different species. So for example, for bird species and some mammals, it is quite easy to train these AI algorithms to identify them, but unfortunately, there is no machine learning approach yet to identify every single species. There is just not enough known about the different species of insects, how they sound, to be able to teach a computer to recognize them.

FLORA LICHTMAN: We need a Merlin for bugs.

ZUZANA BURIVALOVA: Yes, we do.

[LAUGHTER]

FLORA LICHTMAN: What has surprised you about some of these soundscapes that you’ve captured?

ZUZANA BURIVALOVA: I am always surprised by the soundscapes we capture. I think one surprising thing is just quite how noisy it is in the morning, and this is something that’s consistent everywhere where I’ve been. The dawn is the time that you just can no longer sleep, which is good. It motivates me to get up and do research.

But I’ve also been really surprised by some of the animals that just seem to be mimicking other sounds. So some of the famous ones that people might be familiar with are birds that can mimic even chainsaws. And one thing that surprised me, for example, in Gabon was the putty-nosed monkey that actually sounded really like gunshots, and it even tricked our machine learning algorithms at one point because their sounds got picked up as gunshots at first.

FLORA LICHTMAN: Really? You an audio junkie? Have you always been interested in sound?

ZUZANA BURIVALOVA: I have been always interested in rainforests, and I have been– I grew up in a temperate forest, but I think I’ve always used all my senses, hearing. Also, I love the smells of forest, so not just the sounds. But I think sounds are something that is not yet fully explored, and it can tell us a lot about the biodiversity that we just can’t pick up by just looking.

FLORA LICHTMAN: I was just going to ask, are there advantages of capturing sound over video?

ZUZANA BURIVALOVA: Oh, absolutely. We have to listen. We can’t just look because just using images, such as from camera traps, which is amazing– but it only picks up those animals that are big enough that are going to pass through the field of vision of the camera, so, for example, a chimpanzee moving on the ground rather than sitting up in the canopy.

And also, the animal has to be moving slowly enough to trigger the camera, so images might not be great for insects or really quickly flying small birds up in the canopy, whereas with sound recordings we can capture the animal sounds essentially regardless of the direction.

FLORA LICHTMAN: What are the occupational hazards Have you had an– have you had an animal walk off with your microphone before?

ZUZANA BURIVALOVA: The occupational hazards– definitely tropical forests are pretty tough environments for recording devices. We have had elephants attack or take away the recorders from trees. We don’t know if it’s because it was upsetting or if they were just curious. But surprisingly, the biggest challenge is termites and ants. They just end up eating the microphone foam, straps and pretty quickly, actually.

FLORA LICHTMAN: Do you have a soundscape wish list, a place you’re dying to hear?

ZUZANA BURIVALOVA: I have a long wish list. I just wish that we can record soundscapes in all of these intact forests because there are so few remaining. For example, in Madagascar, those forests are disappearing pretty fast. It’s just something I considered almost as a time capsule. We can record it now because otherwise we might lose it forever.

FLORA LICHTMAN: Well, I wish you luck, and I hope to get to hear Madagascar sometime. Zuzana, thanks so much for coming on the show.

ZUZANA BURIVALOVA: Thank you for having me.

FLORA LICHTMAN: Dr. Zuzana Burivalova, assistant professor of forest and wildlife ecology at the University of Wisconsin-Madison based in Madison, Wisconsin.

Copyright © 2025 Science Friday Initiative. All rights reserved. Science Friday transcripts are produced on a tight deadline by 3Play Media. Fidelity to the original aired/published audio or video file might vary, and text might be updated or amended in the future. For the authoritative record of Science Friday’s programming, please visit the original aired/published recording. For terms of use and more information, visit our policies pages at http://www.sciencefriday.com/about/policies/