SONIC ACTS RESEARCH SERIES #11
Infrasound and the navigating capacities of pigeons
By Liesbeth Koot and Menno Grootveld
Sonic Acts was very happy to welcome Jon Hagstrum to the 2015 festival ‘The Geologcic Imagination’. Before delivering his lecture on March 1st, Hagstrum talked to Sonic Acts' Liesbeth Koot and Menno Grootveld, publisher at Leesmagazijn.
Jon Hagstrum at Sonic Acts Festival 2015. Photo by Mayke Haringhuizen
In the interview Hagstrum elaborates on the amazing capacity of birds such as pigeons and the bar-tailed godwit to use infrasound to navigate and migrate. He talks about pressure waves from explosions in the atmosphere and the sources of seismic and infrasonic energy from standing waves in the deep ocean. Hagstrum provides inside into a fascinating part of the natural world and the way research develops in this scientific area.
Jon Hagstrum is a research geophysicist in California with the U.S. Geological Survey. In his lecture at the Sonic Acts festival Hagstrum talked about birds and how they might be using infrasound to migrate, navigate, forage and home over very long distances. Infrasound is very low frequency sound, below our hearing level in the atmosphere. This research is something that is not Hagstrum’s day job ‘and a bit of moonlighting’ he says, although has recently recceived some funding from the U.S. Air Force. Hagstrum works on it part time.
LIESBETH KOOT: Thank you for this interview. Can you start of by telling where your interest in the navigating of birds comes from? What has triggered you specifically?
JON HAGSTRUM: I started out as biology major at Cornell University in Upstate New York, but I did not want to be a medical doctor, which the training was leading me toward. Instead I wanted to be some kind of naturalist. So I switched over to geology and got into geophysics. In my senior year in college, a professor named Bill Keeton came over to the geology department to teach the Friday afternoon seminar. He was researching homing pigeons - how they find their way back to their loft. He gave a talk about it and I was absolutely fascinated and I remember thinking that his work would have been the kind of biology I really would have loved to do. This was my senior year as an undergraduate, so I ended up having a career in geophysics after all. But my interest was triggered and about ten years ago, I started seriously thinking about birds again and working on it.
LK: Where lies your fascination?
JH: I love the natural world, and the depth of time and the complications of evolution. How deep it all goes fascinates me. It is so interesting. It is something almost spiritual that I really want to spend time trying to understand and working with it. It is what I want to do; it is what I want to be around. And I want to be around people who are also working on it.
‘I love the natural world, and the depth of time and the complications of evolution.’
How does the subject fit in with your regular work as a geophysicist?
My background is in magnetics, and birds do use a magnetic compass as part of their navigational system. And the other thing is that there are some biologists who think they also use the geomagnetic field to find out where they are relative to where they want to go before they use their compass. As a geophysicist who knows quite a bit about the magnetic field, I realised that this cannot be right; it does not work that way. So it has to be something else, in my mind. That was one of the things that prompted me to start looking for another cue, another environmental factor they might be using. Infrasound is what I settled on and it is making a lot of sense, to me at least.
Jon Hagstrum at Sonic Acts Festival 2015. Photo by Mayke Haringhuizen
How did you come to think of infrasound? Did you stumble upon it somehow?
No, it had already been thought of. In the late 1960s, a man named William von Arx was at the Woods Hole Oceanographic Institution. He was an atmospheric physicist and he suggested it to Donald Griffin who was studying the way bats and birds were using sound. Bats use ultrasound, echolocation, to image insects. So, the possible use of infrasound was suggested in the 1960s, and some students at Cornell, students of Bill Keeton actually, did some experiments and showed that pigeons could hear down to .05 Hz. And that is phenomenally low. We can hear down to about 15 to 20 Hz. The .05 Hz is almost unbelievable. And once I heard about that capacity of pigeons, I started thinking: they can hear earthquakes and they can hear this whole range of sounds that we know nothing about. Indeed we did not even know anything about this until Krakatoa erupted in 1883 and it sent a pressure wave that went all the way around the world several times. All the meteorological barometers were picking up this pressure wave. That was the first time and it was amazing! We did not even know it was there; we had no idea. I think birds have been using it all along.
Can you tell us something about the sources of this infrasound, you mentioned earthquakes and volcanic eruptions, but I imagine that there might be other sources as well?
There would have to be because birds do not migrate only when there are earthquakes or when there are volcanic eruptions. There are sources that we can pick up on our rather crude arrays. But pigeons are far more sensitive than anything we can build at this point. What they are probably picking up on is sound coming initially from the ocean. There are standing waves in the deep ocean that are continuously moving. These produce sound in the atmosphere and seismic energy because at the same time the wave goes all the way to the sea floor. That seismic energy moves through the solid earth, comes through to the surface of the earth and moves the surface of the earth, up and down, ever so slightly. So right now, we’re going up and down every six seconds. But we are only going up and down a few microns. We can’t sense it at all. But that movement, like the way a stereo speaker moves, oscillates over a large enough area that creates a loud enough sound that pigeons can probably hear it.
‘…[seismic energy] moves the surface of the earth, up and down, ever so slightly. So right now, we’re going up and down every six seconds’
: Do changes in the world’s climatologic system, change infrasound? We all know that there are strange things going on in the biosphere because of pollution and human activity. It affects the way the currents are directed in oceans. Does that mean that this might influence the infrasound signals that birds fly on... Is it conceivable that they will get lost because of that?
It will certainly change things. Sound moves through the atmosphere depending on the temperature structure and the wind fields of the atmosphere. If those factors change it could have really dramatic effects on migration patterns and whether a bird can actually make it on their normal migration route. If they have to fight a headwind the whole way they might not be able to make it, so that might be the end of that species. But another important issue is that we produce infrasound. We have noise pollution. One of the first things I started to look at when I started thinking about infrasound, and birds using it... How do I prove this? How do I even test it? I do not have any pigeons. I do not have an experimental setup to do any of this. Then I just happened to read the newspaper about some pigeon races that were being disrupted and nobody could explain it. I thought of the possibility of some sort of acoustic disruption that was occurring. I actually told a friend who is a real internet surfer and he found a bunch of races in the U.S. and Europe where this was happening. Nobody could explain it. I started looking into it, and it took a while, but I finally figured out that it was the sonic boom of the Concorde. Sonic booms, shock waves, military jets... It is occasionally in the paper... A year or two ago there were some problems in Northern Scotland where the pigeons were all having problems. I thought of the British military flying jets and producing sonic booms off the coast. The thing about infrasound is that it travels very far in the atmosphere. Sonic booms could be generated off shore somewhere to then come on shore. The atmosphere absorbs all the higher frequencies, so we wouldn’t hear it. But this big infrasonic wave would come in and could disrupt the pigeons. You can measure it with a low frequency microphone. There are many ways to test it.
Model of acoustic propagation based on actual meteorological data between the Cornell loft in Ithaca, NY, USA (right edge of plot) and the Jersey Hill experimental release site (left edge of plot) on the one day that Cornell pigeons were well oriented there (inset circular diagram) and returned normally to the loft. Usually, Jersey Hill is in an acoustic shadow zone relative to the loft, but on this day infrasonic signals were transmitted directly from the loft area to this release site due to abnormal atmospheric conditions.
What kind of research tools do you have or do you use?
Well, this first study, finding out that it was the sonic boom off the Concorde, I published in 2000. But then it took me thirteen years until I published my next paper. Again, I am doing this not full time. But I found out that actually, Bill Keeton, the man who had introduced this to me at Cornell had died prematurely and his students and his colleagues took all of his data that he had not published, and put it online, so anybody could use it. All of a sudden, I had all this data to use. And I downloaded the program [HARPA: Hamiltonian Acoustic Ray-tracing Program for the Atmosphere] that modelled how sound moved through the atmosphere. In addition, I got the weather data to feed into that program so I could model exactly how the sound was moving on the days Bill Keeton had released his pigeons around Upstate New York. I could show that where there were the sound shadows the birds could not navigate... So it started to work out in a way that I could really demonstrate numerically that this might be happening.
The theme of the festival is the Anthropocene. Are there connections between the theme and your work?
Yes, many. But I am really directly focused on how the sound is moving through the atmosphere and how birds might be exploiting that. I think if it works for birds it may work for many, many other animals. Nobody has ever really shown what other animals are doing – sea turtles, whales, all of the animals that travel very far distances on Earth.... We do not understand it. But this is what they are using, this noise in the atmosphere. It is what they use to navigate with.
‘Nobody has ever really shown what other animals are doing – sea turtles, whales, all of the animals that travel very far distances on Earth.... We do not understand it.’
Is that also the case with the arctic tern?
The artic tern each year migrates from the Arctic to the Antarctic and back. So, it has a record for the farthest migration.
Probably also because it does not always fly in a straight line.
And it follows coast, and it lands, but – and this is going to be my first slide at the presentation – there is a bird called the bar-tailed godwit, and it flies from Alaska to New Zealand. Non-stop. In five days it flies in a straight line and the other thing that is absolutely amazing about it is it does not leave Alaska until the weather conditions along the entire route, from Alaska to New Zealand, are optimal. That bird in Alaska can tell what is going on in the Southern Hemisphere. It is almost miraculous. Before it even leaves, it knows what the weather is like along the entire route. And it flies in a straight line, non-stop. It is unbelievable. You need to think about acoustics. Sound depends on wind and temperature, so sound basically depends on weather. If the bird is listening to a signal, it has evolved an ability to listen to that signal and also interpret that signal to tell it what the weather is. Because if it had to fight a headwind for any significant portion of that route, it is over, it is not going to make it. It has to know what the weather is, and it does. That sort of thing for me is just incredible. That is why it is really so fascinating. I know I am a geologist; it is just that the natural world is so fabulous that I love learning that kind of thing. When I get some sort of insight, it is really exciting to figure something out, to understand it.
Jon Hagstrum out in the field along the Columbia River in Washington State explaining his sampling technique for paleomagnetism to a group of local students. The enormous lava flows that he is sampling can be seen cropping out across the river in the background
What is the length that infrasound waves can be at the maximum?
That is the thing, the lower the frequency, the longer the wave length. The sound that I was talking about, coming off the ocean, that is about .2 Hz. Birds, pigeons can easily hear that. The length of such a wave is 1.7 kilometres. It is very long, and if you are a physicist, you will wonder immediately: ‘The bird has ears only this far apart, and you are telling me it can localise where a sound is coming from with a wave length of 1.7 km? It is not possible’. And it is not. It is like when you buy a stereo, and you have a separate subwoofer unit. They will tell you to put that anywhere in the room because our ears cannot locate where it is, the wave lengths are too long. Our ears only tell immediately where the tweeters are, the short wave lengths. What pigeons do when you release them is to fly around in a circle and Doppler-shift that signal. When they fly toward the signal, the frequency goes up, when they fly away from it, the frequency goes down. Just like the sirens of an ambulance or a police car as it goes by you, you hear that drop as it moves away from you. Just turn it around: it is actually you moving and the source is stationary. If you moved toward it, the frequency would go up; the moment you moved away it would go down. Pigeons can tell where that sound is by moving. So they solved that problem. Actually it is a very standard process with people that are trying to have a small antenna and need to detect a large wavelength: they move around. Pigeons evolved to do that. It is amazing. People did not understand what pigeons were doing: were they looking around, gaining altitude? No they are not. I am in California and there is NASA Ames research center nearby. I told some people there, some friends of mine. They work with UAVs - unmanned aerial vehicles. And they said: ‘of course that’s what the birds are doing!’ Now it is so obvious to them. Because that is what they do with the UAVs, you do it with your iPhone, you move it around when you use the magnetic compass, and so it is exactly the same thing. It is just so cool.
‘Birds, pigeons can easily hear [.2 Hz]. The length of such a wave is 1.7 kilometres.’
Where are you heading with your work; to do more of this?
Oh, absolutely, I would love to do it full time. I would like to get rid of my day job and just do this. It is that much fun. And that is what it is all about; you do your best work when you are having fun.