The Factors That Make Bird Sounds So Diverse Across The World

An analysis of 140,000 vocalizations from 77% of bird species showed that beak size and geography play a big role in the way birds sound.

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• 0:52Ira Flatow?

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• 1:11Flora Lichtman?

  This is Science Friday. I'm Flora Lichtman. Today on the podcast, learning about bird song from chirps and cheeps captured by birders around the world.

• 1:20Speaker 04

  You could think of it, for example, if you had an orchestra with lots and lots of different types of instruments, really rich orchestra.

• 1:30Flora Lichtman?

  Birds are the virtuosos of the animal kingdom. I mean, they can make a remarkable range of sounds. But why is a barred owl more of a tenor

• 1:44Flora Lichtman?

  while a cedar waxwing is more of a soprano?

• 1:50Flora Lichtman?

  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. Susanna Boujevalova, Assistant Professor of Forest and Wildlife Ecology at the University of Wisconsin-Madison, based in Madison, Wisconsin. Welcome to Science Friday.

• 2:15Speaker 04

  Hi, thank you for having me.

• 2:18Flora Lichtman?

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

• 2:26Speaker 04

  So in this research, uh, which was actually led by a PhD student in my lab, Satya Chandra Zagar, we analyzed more than hundreds of thousand recordings that came from this amazing repository of bird sounds called Xenocanto. And this repository is where anyone can upload sounds of birds that they encounter in the forest, in their backyard, or anywhere.

• 2:52Flora Lichtman?

  Wow, so these are birders who, who made this dataset that you used?

• 2:55Speaker 04

  Yes, they're birders, they're citizen scientists, they're nature lovers. Anyone can upload recordings.

• 3:02Flora Lichtman?

  And they're from around the world?

• 3:04Speaker 04

  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.

• 3:19Flora Lichtman?

  So tell me what you found. Give us the bird's eye view.

• 3:22Speaker 04

  The bird's eye view, yes. So Zagar found a lot of different things in terms of testing the major hypotheses about what rules the bird's 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're able to confirm it across all these different species, showing that really the body mass of the bird rules how deep it can go.

• 4:01Flora Lichtman?

  Which is not that surprising, right? I mean, that's often true in other animals that we might be even more familiar with, like ourselves.

• 4:07Speaker 04

  Exactly. If you think of elephants having really, really low pitch 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're able to do is to really isolate this effect and separate it, for example, from even the role of the beak size and other, other things that turned out to be important, like the latitude and type of habitat.

• 4:39Flora 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?

• 4:46Speaker 04

  As 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.

• 5:17Flora Lichtman?

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

• 5:19Speaker 04

  So for a small one with a small beak, you might know, for example, the winter rein.

• 5:28Speaker 04

  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.

• 5:42Speaker 04

  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.

• 5:54Flora Lichtman?

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

• 6:00Speaker 04

  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.

• 6:21Flora Lichtman?

  Huh. Why would birds at the equator have a smaller range of sounds?

• 6:28Speaker 04

  This is one of the puzzles of bird songs. 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, smaller range of frequencies at which it can play so that the listeners can hear each instrument.

• 7:13Speaker 04

  Whereas at the temperate zones, we might have an orchestra with fewer instruments and each instrument can cover a broader bandwidth.

• 7:21Flora Lichtman?

  Hmm. 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.

• 7:34Speaker 04

  Yes, and this idea of species might be competing for the acoustic space was proposed already more than thirty years ago by Bernie Kraus. And we, 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.

• 8:19Speaker 04

  So it's still a little bit of a mystery why exactly this is happening across the latitudes.

• 8:26Flora Lichtman?

  You also found that, on average, smaller birds had a larger vocal range, like they could hit, hit low notes and high notes. Why, why would that be?

• 8:38Speaker 04

  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, other individuals might perceive, oh, I can hear this really low pitch sound. I am about to encounter a big scary bird, whereas actually it's a, it's a tiny bird.

• 9:10Flora Lichtman?

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

• 9:18Speaker 04

  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.

• 9:44Flora Lichtman?

  Where have you recorded?

• 9:46Speaker 04

  We have recorded at lots of different places. And currently, the lab's working on this soundscape baselands 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.

• 10:14Flora Lichtman?

  Do you go there yourself and place the microphones?

• 10:18Speaker 04

  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.

• 10:44Flora Lichtman?

  We have to take a quick break, but when we come back, can a forest soundscape amplify conservation?

• 10:50Speaker 04

  I consider it almost as a time capsule. We can record it now, because otherwise, we might lose it forever.

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• 12:14Speaker 05

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• 12:43Flora 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?

• 12:51Speaker 04

  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.

• 13:41Flora Lichtman?

  Okay, so what did you hear in that?

• 13:45Speaker 04

  Well, I got completely transported back to the rainforest in East Calamandan 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-rum shama in there. And then, of course, kind of omnipresent background of insect noise.

• 14:18Flora Lichtman?

  Can you use sort of computer analysis or AI to pick out the different animals in there?

• 14:25Speaker 04

  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.

• 15:03Flora Lichtman?

  Hmm, we need a Merlin for bugs.

• 15:05Speaker 04

  Yes, we do.

• 15:09Flora Lichtman?

  What has surprised you about some of these soundscapes that you've captured?

• 15:14Speaker 04

  Hmm, I'm 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 is the time that you just can no longer sleep, which is good. It motivates me to get up and and do research. And 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 that people might be familiar with are are birds that can mimic even chainsaws and one thing that surprised me, for example, in Gabon

• 15:59Speaker 04

  was the Patino's monkey that actually sounded really like gunshots and it even tricked our machine learning algorithms at one point. Uh, because their sounds got picked up as gunshots at first.

• 16:13Flora Lichtman?

  Really? Are you an audio junkie? Like, have you always been interested in sound?

• 16:20Speaker 04

  I have been always interested in rainforests, um, and I've been, I mean, I grew up in a in a temperate forest, but I think I've always used all my senses, uh, 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.

• 16:48Flora Lichtman?

  I was just gonna ask, are there advantages of capturing sound over video?

• 16:54Speaker 04

  Oh, absolutely. We have to listen. We can't just look because just using images such as from camera trips, uh, which is amazing, but it only picks up those animals that are big enough, um, 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 so images may not be great for insects or really quickly flying small birds up in the canopy, whereas with sound recordings, we can capture the animal's sounds.

• 17:36Speaker 04

  Essentially regardless of the direction.

• 17:39Flora Lichtman?

  What are the occupational hazards? Have you had an, have you had an animal walk off with your microphone before?

• 17:45Speaker 04

  The occupational hazards, definitely tropical forests are pretty tough environments for, 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, end up eating the microphone foam straps and pretty quickly, actually.

• 18:18Flora Lichtman?

  Do you have a soundscape wish list? A place you're dying to hear?

• 18:24Speaker 04

  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, I consider it almost as a time capsule. We can record it now because otherwise we might lose it forever.

• 18:48Flora Lichtman?

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

• 18:54Speaker 04

  Thank you for having me.

• 18:56Flora Lichtman?

  Dr. Zuzana Boujivalova, Assistant Professor of Forest and Wildlife Ecology at the University of Wisconsin-Madison, based in Madison, Wisconsin.

• 19:09Flora Lichtman?

  And that is about all we have time for. Lots of folks helped to make the show happen, including

• 19:15Speaker 06

  Annie Nero. Emma Gomez. Charles Bergquist.

• 19:18Flora Lichtman?

  Danielle Johnson. I'm Flora Lichtman. Thanks for listening.