How do animals know where to go?

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It's Unexplainable.

I'm Noam Hasenfeld.

Last fall, journalist Sonia Shah headed out to a forest in Illinois with a team of scientists.

They were there to catch a bird.

I thought they would just put up the nets and kind of wait for the birds to kind of fly into them by mistake, but actually it was more like hunting because they were looking for a a specific bird.

One specific yellow-billed cuckoo, this slim white and brown bird with a long, beautiful tail.

And they're like, Okay, it's five in the morning, like, he's probably gonna be in this one, you know, general location.

So, that's where they set up their nets.

And they just waited.

You know, they had these little sort of radio stereo things like hung up in the trees to play recorded bird song, you know, just little calls that the yellow-billed cuckoos do.

And then they'd listen,

and the bird would call.

They'd play more coups and they'd listen.

They were communicating with the bird, they were calling it.

And as he got closer and closer, you know, like the excitement kind of grows, and then he suddenly was in the net.

At this point, they move quickly.

They glue a little device on his back, and they have a little harness that they put on him, so it looks like a little bird backpack.

And then they set him free, and he immediately just flew off into the trees and was gone.

That yellow-billed cuckoo is one of over a thousand species that have been tagged so far as part of a new global project.

After years of planning, the project started scientific operations last September and it's already teaching scientists some fascinating things about animal movement.

It's called International Cooperation for Animal Research Using Space, or just Icarus.

Icarus is a global animal tracking system.

It was devised by scientists at the Max Planck Institute of Animal Behavior in Germany.

And basically, it streams data from tiny little solar-powered tags that are attached to animals moving around the world up to a receiver on the International Space Station and then back down to Earth so that scientists and the public can see where creatures from over a thousand different species, where they're moving across the planet continuously over the course of their lifetimes and nearly anywhere they may go on Earth.

Until now, scientists have only had short-range tools.

You know, there's these historical studies of scientists like following birds around on little planes and driving through the rainforest trying to pick up signals on their handheld receivers.

But with Icarus's receiver all the way up at the International Space Station, scientists will finally be able to tackle a pretty basic question.

How far do animals move?

Believe it or not, scientists still don't have a great answer.

We just don't know really the fundamentals of animal movement.

I mean, when you think about it, you know, we glimpse the movement of other animals very episodically.

We make assumptions that, well, I saw the ducks at this lake, you know, every day at two o'clock when I feed them, so they must be there all the time.

Well, you know, what scientists are figuring out now is if you actually track those birds, they're moving hundreds of kilometers away and back.

These are movements that are completely obscure to us unless we track them.

So Sonia, what are the major types of questions that scientists are trying to answer with this massive Icarus tracking project?

Okay, so I could see three ways of thinking about it.

One is we want to know where animals go because we want to understand animals.

You know, we want to understand their biology and their behavior.

We want to know where animals go because it helps us understand the patterns of disease.

And then we want to know where animals go because they know stuff that we don't know.

So we want to know where they go so we can know what they know.

Okay, let's start with the first one, animal behavior.

I imagine this is pretty important for

endangered species, for conservation.

Yeah, it's the main conservation question right now, since we're living in this era of mass extinction, is where are animals dying?

We know we're losing 150 species a day.

We're living in this era of mass extinction.

But what exactly is killing them?

And where is that that happening?

Is there any specific example here for conservation that you can point to?

So the yellow-billed cuckoo, you know, their numbers are declining.

People knew that, you know, they migrate to South America and then they come back.

But that was sort of the extent of what was known about where they went until some scientists started tracking them and putting these little solar-powered tags on these birds.

And they discovered that, well, they kind of, you know, fly over to the Caribbean, they hopscotch across the islands and they end up in this forest called the Grand Chaco.

And that particular forest is under extreme stress from the expansion of agriculture.

So their trees are getting cut down.

And they think that this is why the yellow-billed cuckoos are declining so quickly because they're, you know, the forest that they go to is basically disappearing.

So you can imagine if we do that for, you know, all the other species, how much more we could learn.

Okay, how about the second one?

What are scientists hoping to learn about the spread of disease?

Well, what we know is that microbes take advantage of the movement of animals.

So where bats go, that might determine where Ebola outbreaks occur.

Where birds fly might determine where outbreaks of avian influenza occur.

And what some of the tracking studies have already revealed is that in some of the cases where we thought the disease outbreaks were following wildlife movements, that they're not related at all.

So there's actually something else that's driving those outbreaks, not the movements of animals.

So it's sort of opposite of what you'd think.

Like, if we track animals, then we'll know where all the diseases are coming from.

But actually, it's like, no, they're not the ones.

At least in these couple cases we've seen so far, there's some other factor.

It's more about movement of us, maybe?

Maybe, or just some other factor.

We don't know, climatic or immune changes.

I mean, it could be anything.

Okay.

And the third thing that scientists are hoping to learn from tracking animals is what animals themselves know about the world.

Is there a good example that scientists have learned from other tracking projects that Icarus might be able to expand on or deepen?

Yeah, so white storks arrive in Europe and then they migrate over to Africa.

But where they go, how far they go, all of these things were kind of unknown.

And so there's this tracking study of white storks and they found that they flew to these very obscure places in the middle of the desert in sub-Saharan Africa.

And when they further started looking into where those locations actually were, these were places where desert locusts were emerging from their hidden spots underground.

This is knowledge that humans have been trying to pin down since biblical times.

Locust swarms emerge out of remote desert locations in places that we can't find, right?

Then they form the swarms and they come and take over farms and move, you know, thousands of miles across farmlands and devastate, you know, all crops and cause famines.

But once you have a big swarm of locusts, it's very hard to control.

What you want to do is figure out where are they coming from in the beginning when they're still little tiny things and they haven't formed a giant swarm.

And let's try to address it at its source.

But nobody knew where those locust swarms emerge.

Well, it turns out the storks have known all along.

By tracking white storks, it seems possible that we could start to understand where those locusts are coming and use that to predict outbreaks of you know locust plagues um and even to prevent them and control them

what happens to the storks if we go and kill all the locusts like they go and next time they're expecting their meal are the storks in trouble then there's no immediate danger of that because I don't think you would have to kill all of the locusts in order to control swarms and also storks eat lots of other things.

Okay.

Storks are going to be fine.

Yeah, the storks are probably going to be fine.

But I think you're right.

You're touching on sort of an ethical issue, which is like, okay, you're going to surveil all these animals.

Are you going to then interfere in some way with like their livelihoods?

And I think that is something that would need to be worked out.

I mean, with the Icarus project, they'll have a whole kind of ethics committee.

But, you know, the hope is that the Internet of Animals would be a way for us to connect with the non-human world in a more fruitful way.

I know the Icarus project is still in its early stages here, but is there such a thing as full results?

Like, is there a time when all the results are going to be done and in?

Or is this just something that scientists are going to be tracking for the foreseeable future?

Yeah, this is not like an on-off switch.

This is data that is accumulating and will continue to accumulate as more scientists tag more animals will get more real-time tracking data.

And those tracks will kind of fill out and get more intricate and layered.

And people can see this.

There's a public database at movebank.org, and there are already something like 2.5 billion location points on there from over a thousand different species.

And there's actually some beautiful visualizations that you can see on YouTube if you look up Movebank of the planet kind of crisscrossed with all of these tracks.

Yeah, I'm seeing.

I looked it up just now and I can see sort of these topographical maps of the world with these various colors.

snaking across the landscape.

It's really cool.

It looks like a subway system or something like that.

Yeah, yeah, you just get the sense of like the planet kind of encased in this sort of intricate filigree of tracks of all these different creatures kind of connecting us all.

You know, they're connecting continents across oceans, across deserts, across mountains.

It's almost like we're right at the beginning of this entire

new way of conceiving animal movement.

Is that fair to say?

I mean, it's just huge when you could just scratch a little bit of the surface of this project and there's just so much potential.

I mean, what we're already finding is that a lot of the creatures who we thought we knew where they were, we thought we knew what their habitat was.

Well, turns out they are moving well beyond that, which then, you know, brings up other questions like what kind of cognitive tools do they have that allow them to do that?

You know, we long thought of animal movement as really difficult, you know, that it's hard to move against the wind or to swim a really long distance in the sea or to, you know, travel across a desert.

Like those things are just really, really physiologically difficult.

Well, it turns out now we're finding that it's actually a lot easier to move across these geographic obstacles than we once thought.

There might be sort of environmental highways that kind of connect all these different places over the mountains or wind currents so that it's not so physiologically taxing to move long distances.

It's more like you have to know when, you have to know where, and then you get on the highway and you take the 95 and you go all the way down it, you know?

And that changes our whole idea about the kind of habitats that we live in, too.

I guess it's sort of like saying the default state of animals might be motion.

Yeah.

You know, movement is critical to how animals survive on a changing planet in a changing environment.

And they have much more capacity for it and cognitive power, navigational power, physiological capacity to do it, and more knowledge of how sort of mobile the planet already is, you know, how dynamic it really is.

Whereas, you know, our idea that everything is sort of settled in one little place, inscribed on a map with like borders around it, is actually, you know, we're the ones who kind of got it backwards.

Coming up after the break, how in the world do these animals know where they're going?

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What happens when your birds start to fly?

I want to get away.

Why are they doing this?

The birds?

Unexplainable, we're back.

In the first half of the show, science writer Sonia Shah was explaining everything we can learn about animal movement from Icarus, this massive new initiative that's starting to track animals constantly all around the world.

And thinking about all these birds going island hopping or targeting locusts from hundreds of miles away, I couldn't help but wonder just how?

How do they know where they're going?

So I called up David Berry.

My most recent book is Supernavigators, and it's all about the science of animal navigation.

And even though he wrote an entire book about animal navigation, David says there's still so much scientists don't know about how how animals get around.

I mean, one of the things in my book that I tried to do was to constantly insert lots of small examples of things that remain mysterious for precisely this reason.

So, David, just to narrow down this kind of enormous topic here, I wonder if we can focus on one animal in particular, like what we know and what we don't know about how it gets around.

Okay, so the homing pigeon is

very, very well studied.

People have been looking at homing pigeons for 100 years or more, trying to work out how they do their thing.

The most puzzling thing about homing pigeons

is that you can take a homing pigeon maybe 300 kilometers away from its home loft to a place it has never ever been near before.

And you can take it under anesthesia.

so it has absolutely no consciousness of where it's been taken and you can release it in this unfamiliar location and very often the bird will be able to find its way home.

Now when you pause to think about that, what could the pigeon possibly be using to determine what the relationship is between that new location and its home loft?

Well there is some argument about this, but over the last 40 years or so, there's been increasing support for what I think is a truly bizarre hypothesis, which is that they do it by their sense of smell.

Is the idea that they can somehow smell their home from 300 kilometers away?

No, no, that isn't the idea.

The idea is more peculiar than that.

The idea is that when they're in their home loft as young pigeons, they are constantly sampling the smells brought to them on the wind from all different points of the compass, and that they learn to associate particular bouquets of scents with each wind direction.

That reminds me of the the Greek gods of the wind, you know, it was like each direction of the wind had its own specific personality.

Yeah, no, exactly.

I think the pigeons would definitely understand all of that.

But when they are released, the idea is they sample the air and they think, ah, there's a little bit of that bouquet I used to get when the wind was blowing from the southwest.

So I'm going to have to fly northeast to get back to where I came from.

I'm almost imagining this like

an interstate highway map or something that, like where you're born, you kind of know what like Route 1 smells like and Route 4 smells like and you can kind of follow them.

Yeah, yeah, it is roughly like that.

I mean,

the essential notion is that there's a reasonably stable gradient of different mixtures of smells that characterize the landscape around the home loft, wherever that happens to be.

And because the gradients are reasonably stable, each wind coming to the loft will tend to carry the same,

roughly the same mixture of smells to the bird.

Now, I've got to emphasize this is still hypothetical.

This has not been comprehensively demonstrated.

It's now looking quite likely that smell is pretty critical.

However,

let's not for a moment imagine that pigeons rely exclusively on smell.

Because one of the things that

unsurprisingly has been revealed by animal navigation research is that pretty well every animal uses a multiplicity of different navigational systems.

And it makes sense because at any given moment, one system may fail you.

They've got a sun compass, they've got a magnetic compass, they're really good at recognizing landmarks.

They're going to use whatever they can get to find their way home.

With smell or with recognizing landmarks, I kind of get how that works at a basic level.

But what kind of organ can sense like magnetism?

Nobody knows.

Talking of mysteries, you've put your finger straight away on perhaps the biggest puzzle that is really exercising scientists right now.

People all over the world are struggling to clarify exactly how the magnetic sense works in a whole host of different organisms.

All kinds of animals seem to have a magnetic sense.

One idea is that there would be particles of this magnetic mineral called magnetite dotted all over the body of the animal, or they may be concentrated in a small area.

And as the animal moves within the Earth's magnetic field, the magnetite granules are subject to mechanical stresses produced by their movement in the field.

And those stresses are linked to the central nervous system of the organism and enable it somehow to interpret the direction it's headed in.

So that's probably that mechanism that is used by some of these animals that navigate magnetically.

But it doesn't look as if all the birds that have a magnetic sense use that because it's been found that

they actually appear to be using a system that depends on light.

Meaning that whatever birds are using to sense the magnetic field, it wouldn't work in the darkness?

That's right.

And the idea is that they probably have a molecule called cryptochrome, which is a light-sensitive molecule.

When a photon of light hits it, it will be influenced by the orientation of the surrounding magnetic field.

So the bird, as it's flying along, actually,

through this mechanism, can actually see the Earth's magnetic field.

So you're saying this magnetic bird sense needs to be sort of activated by light?

Yeah, they can't successfully use their magnetic compasses in the absence of light.

Now,

this is still

no more than a hypothesis.

There are dozens and dozens of research teams working on this right now.

And,

you know, the evidence is gradually accumulating, but it isn't yet

So these homing pigeons might be using smell to sense wind directions.

They could have this magnetic sense, which might need light to work.

What other kinds of navigational systems do they have?

Well, homing pigeons actually may be using infrasound,

very low frequency sound, to help them navigate.

And

even more bizarrely, it may be that the infrasound they're detecting

takes the form of tiny micro-syisms.

Like little earthquakes?

Little miniature earthquakes.

And the theory is that the pigeon has got used to a particular

almost like a tambour of micro-syism.

So the pigeons can get this sort of sonic map by listening to little earthquakes that are constantly happening?

Yeah, this is continuous, but the theory is that that where the pigeons live, they listen to the infrasound in that location.

And it has a very, very particular quality, like the note of a bell or something.

So for all of these things, sight, smell, sound, even this weird magnetic sense, is any one thing the main system of navigation?

Or is that the wrong way to think about it?

Aaron Ross Powell, well, I think it's probably better not to think in terms of a a main system of navigation.

Different elements in the toolkit will be used depending on the need that the pigeon faces.

So, obviously, if it's got access to visual landmarks that it recognises, it'll probably make use of them.

If it's taken to some totally unfamiliar place, that's the time when it may start to kind of sniff the air or listen for infrasound.

It just depends what the animal needs at the time.

How certain are we on all of these ways that birds get around?

I mean, you were pretty clear that a lot of these things are just hypotheses, right?

So if we're still uncertain on a lot of this, why haven't we figured it out yet?

I mean, I think some things are certain.

We know for sure that they are fantastically good at landmark recognition.

There's also no doubt they have access to a magnetic compass sense.

But whether they smell or listen to little earthquakes to get around is less certain?

Exactly.

And I think one of the reasons why it's not settled is because it's actually very hard to prove.

There have been dozens and dozens of experiments trying to kind of pin this down and make absolutely certain that it's smell and not something else.

And that's hard to do.

You've got an animal that has quite an extensive navigational toolkit at its disposal.

And designing an experiment or a series of experiments that absolutely eliminate the possibility that anything else is going on is not easy.

It's as simple as that.

You know, David, in the first half of the show, Sonia laid out all these reasons why tracking animals is super important.

But this conversation is less about knowing where the animals go than it is about knowing how they know where to go.

So aside from just being interesting, why do you think it's so important to know how animals do this?

I mean, my own feeling also is that if we really, really want to protect the environment effectively, we need to understand our fellow creatures better and we need to recognize that we are not,

as we have so often imagined,

the kind of princes of creation.

We're not

the most extraordinarily gifted animals of all.

We are indeed extraordinarily gifted, but so are many, many, if not all, of our fellow creatures.

David Berry's latest book is called Super Navigators: Exploring the Wonders of How Animals Find Their Way.

Sonia Shaw's book is called The Next Great Migration.

And if you missed last week's conversation with Sonia, you can go back and check it out for more on animal migration and different ways to think about invasive species.

This episode was produced by Noam Hassenfeld and me, Mandy Nguyen.

I handled the fact-checking and Noam wrote the music.

We had editing from Brian Resnick, Jillian Weinberger, and Meredith Hodenon.

Christian Ayala did the mixing and the sound design and Bird Pinkerton wrote an entire screenplay that didn't make it in.

Lauren Katz heads up our newsletter and Liz Kelly Nelson is the VP of Vox Audio.

You can sign up for our newsletter, read our articles, and find our show transcripts at vox.com/slash unexplainable.

Unexplainable is part of the Vox Media Podcast Network, and we'll be back in your feed next Wednesday.