[RADIOLAB INTRO]

JAD ABUMRAD: Hey, I'm Jad Abumrad.

ROBERT KRULWICH: I'm Robert Krulwich. This is Radiolab. And we're gonna begin today with a very simple question, a child-like question.

MARTIN WIKELSKI: That's good.

ROBERT: Okee-dokee.

ROBERT: Which was asked by a child.

JACKSON ROACH: And there's Robert.

ROBERT: Hey, Martin.

MARTIN WIKELSKI: Hi, Robert.

ROBERT: Who's now a man.

MARTIN WIKELSKI: Yeah, so I'm Martin Wikelski, I'm the Managing Director of the Max Planck Institute for Ornithology.

ROBERT: Anyway, reporter Jackson Roach and I ...

JACKSON: Hello!

ROBERT: Hello. He's gonna help me on this. We got together in the studio with Martin.

ROBERT: Where—where do you start? Like, what—when were you born? What was—what's your birthday?

MARTIN WIKELSKI: Oh that's in November, '65.

ROBERT: So November something, 1965?

MARTIN WIKELSKI: November 18, 1965.

ROBERT: And on November 1865—1965, where—where did you show up?

MARTIN WIKELSKI: Oh, that was in Bavaria.

JAD: Bavaria's right next to Germany. Is that where that is in the world?

JACKSON: Bavaria's in Germany, yeah.

JAD: In Germany! Okay.

JACKSON: It's like—it's like the German heartland.

ROBERT: It's the Iowa of Germany.

JACKSON: Yeah.

MARTIN WIKELSKI: Except that we have mountains. [laughs]

JACKSON: Anyway, so Martin's a little boy, growing up in this small town in Bavaria.

MARTIN WIKELSKI: It's a tiny little village. I mean, the entire village is only about 50 people or so.

ROBERT: Oh!

MARTIN WIKELSKI: My family was actually living in that village probably since it existed. 750 years old, the entire village.

JACKSON: And they're all farmers. They're farmers all the way back. His grandfather's a farmer, his grandfather's grandfather was probably a farmer.

MARTIN WIKELSKI: So I basically grew up on the farm.

JACKSON: So he gets to know all the animals of the farm, you know, goats and sheep and chickens.

MARTIN WIKELSKI: But there were also badgers coming or foxes coming.

JACKSON: Wild animals.

MARTIN WIKELSKI: Kestrels.

JACKSON: Barn swallows.

MARTIN WIKELSKI: And they are individuals and they are all different.

JACKSON: And even as a young kid, he could recognize that this kestrel that showed up in the tree at the edge of the field would always be there at certain times of day. And he could sort of figure out, like, oh that's—you know, I don't know if he gave them names, but ...

MARTIN WIKELSKI: You do get to know individuals. You know, like an old friend.

JACKSON: But one day, he's out helping out on the farm ...

ROBERT: He's about 11 years old.

MARTIN WIKELSKI: Walking the cows to the fields, to the pastures.

JACKSON: And suddenly he sees these birds.

ROBERT: Very odd birds.

JACKSON: They're tall and skinny and white. And they have kind of like orange dinosaur crests and long, sharp beaks. And Martin has never seen anything like them before, but ...

MARTIN WIKELSKI: I had my camera with me all the time to take pictures.

JACKSON: So he takes his camera out, and he starts taking pictures of them. He looks and tries to figure out if they're, like, zoo escapees, but they don't have bands or anything on them.

ROBERT: And they're almost dancing right with the cows. They're very, very close. Some of them going up to the cows with these really long sharp beaks and pecking at their eyes.

JAD: Wow!

ROBERT: And the cows let them. So for Martin, this is just totally weird.

JACKSON: And he's just thinking, like, what on Earth are these things? And—and where do they come from? Why are they here? And it was actually those questions that would set him off on a journey that would last kind of the rest of his life. First stop, his biology teacher.

MARTIN WIKELSKI: I had a really good biology teacher.

JACKSON: So he goes, he takes the pictures that he's taken of these weird birds, and his teacher's like, "Oh ..."

MARTIN WIKELSKI: "Those are the cattle egrets."

JACKSON: Cattle egrets.

JAD: Hmm.

MARTIN WIKELSKI: They're all over Southern Mediterranean, but also Africa.

JACKSON: And Martin is like, "I just saw a bunch of them here in Germany."

MARTIN WIKELSKI: You know, what are cattle egrets doing here? What could that be? And he said, "Well ..."

JACKSON: "I—I have no idea."

ROBERT: So his biology teacher doesn't know. But then someone says,"Well, there is a professor."

MARTIN WIKELSKI: A big professor in Munich. He just wrote two large volumes of the avifauna of Bavaria, which is each I think 500 pages.

JACKSON: Like, all of the birds that live in Bavaria. And you should just go and ask him. You should go and see—see what he says. So Martin ...

ROBERT: Who remember, is just 11 years old.

JACKSON: Gets on a train to Munich.

MARTIN WIKELSKI: I took the train into this big museum and met the guy.

JACKSON: Tells him all about the birds.

MARTIN WIKELSKI: And he said, "Wow, that's really interesting."

JACKSON: "But we don't know why they would be here."

ROBERT: So now Mr. 500-page bird book guy says, "I don't know."

MARTIN WIKELSKI: Then he said, "Well, go down to the—the Max Planck for Ornithology, the bird migration center and talk to those guys."

JAD: Oh, that's a big one.

JACKSON: Yeah, some of the biggest discoveries in bird science have come out of the Max Planck. So ...

MARTIN WIKELSKI: Maybe they know.

JACKSON: So Martin gets on another train and asks these guys, "Do you know why are these birds on my grandpa's farm?"

MARTIN WIKELSKI: And they didn't. Nobody knows where they come from. Nobody knows where they go to. Nobody knows what's happening to them. And I said, "Well, that can't be. I mean, you guys, you know, you're scientists. You have to—you have to know these things."

ROBERT: So you're a—you're a meandering 11 year old with a question that no adult could answer?

MARTIN WIKELSKI: Yup. And—and I realized that, you know—adults don't know anything about the world. It's just, you know, sort of a big enigma.

ROBERT: Over the next 40 years, Martin will find that the movement of birds—and not only birds but all kinds of animals all over the planet, which we see happening every fall and every spring, heading north, heading south, even though it seems sort of usual, this is one of those mysteries that only gets more mysterious when you look at it more closely.

ROBERT: And when do you go to Costa Rica? Like—like, tomorrow or the day after tomorrow or something?

LUCY COOKE: I'm going, no I've decided—I've switched my plans. Oh my God, I'm so goddamn excited, Robert! Guess what I'm doing?

ROBERT: What?

LUCY COOKE: I am going to the island of dwarf stoned sloths in Panama.

ROBERT: I have no idea what that means.

LUCY COOKE: Well basically, there's an island off the coast of Panama where—where the sloths have shrunk.

ROBERT: Oh!

LUCY COOKE: To half the normal size. And they live off algae in these mangrove swamps that's got alkaloids with a similar property to Valium. And so they're stoned dwarf sloths. It's an evolutionary cul de sac.

ROBERT: [laughs]

ROBERT: So before we get to Martin's lifelong search for the meaning and the mystery of migration, we start with our own investigation into the subject with our favorite untangler of animal mysteries, Lucy Cooke.

LUCY COOKE: I'm the author of The Truth About Animals.

ROBERT: And Lucy says people have been wondering about the comings and goings of animals across the seasons for a very, very long time.

LUCY COOKE: Ooh, yes. This was a very old question indeed.

ROBERT: And she starts with the Greeks.

LUCY COOKE: The great-grandfather of zoology, Aristotle.

ROBERT: Hmm.

LUCY COOKE: And he actually came up with three theories as to where birds disappeared to. The first one was migration. Brilliant, Aristotle!

ROBERT: Yeah! That's right!

LUCY COOKE: He should have just stopped there.

ROBERT: [laughs] But he went on, did he not?

LUCY COOKE: He went on. He went on, probably because the unlikely nature of—of tiny birds traveling thousands of miles on the wing, he came up with a couple of extra bonus theories.

ROBERT: So Aristotle's second theory?

LUCY COOKE: Transmutation.

ROBERT: Huh.

LUCY COOKE: Yeah. He thought that, much like Clark Kent and Superman, a lot of the birds weren't around at the same time and they were a little bit similar. So you had winter robins that are sort of small birds with a red breast were never around the same time as summer redstarts. So he thought that the robins were transmuting into the redstarts.

ROBERT: I see. They just change their clothes.

LUCY COOKE: Yeah. Yeah. But I mean, which is sort of—it seems silly to us now obviously, but if you think about it, back then, you know, we didn't even know that caterpillars turned into butterflies, which is just as ridiculous and fantastic.

ROBERT: Okay, there's a third idea as well?

LUCY COOKE: Ah, now the third idea was considerably more enduring. Hibernation. Now at the time, it was known that bats hibernated and bats were often classed as birds because they were fly-y things. So why not birds?

ROBERT: And this idea stuck around for hundreds upon hundreds of years.

LUCY COOKE: Well into the 17th century ...

ROBERT: Scientists were debating if birds hibernated in trees or in small nooks or crevices.

LUCY COOKE: And the crazy rumor that was doing the rounds was that swallows hibernated underwater like fish.

ROBERT: [laughs]

LUCY COOKE: And—and one of the—the staunch anti-hibernation, pro-migration theorists was a chap called Charles Morton, who was an Oxford-educated physicist and a very logical chap. And he was like, "Well, don't be ridiculous. Of course the swallows can't be hibernating underwater. How would they breathe? No, no, no, no, no! They migrate—to the moon!"

ROBERT: [laughs] With all of the destinations to choose from, how did the moon enter this?

LUCY COOKE: Well, this was the 1600s, and the telescope had just been invented. So people were able to observe the moon for the first time ever. And previously, the moon had just appeared to be this sort of marble in the sky, and then thanks to the telescope it could be observed. And it was found that it had mountains and craters and valleys and looked like a, you know, according to Charles Morton, a perfectly reasonable place to holiday if you're a bird. And he came up with this—this sort of wonderfully thought-through theory about—he'd worked out, he estimated how far the journey was and, you know, he wasn't far off. I mean, he wasn't right but he wasn't wildly wrong.

JAD: I mean they—they wouldn't have known at that point there's no air in space.

ROBERT: That's right, or gravity.

JAD: Or gravity. So, they were like yeah, they might just fly up to the big glowing thing up there.

ROBERT: And—and that idea lasted a really long time, until finally ...

LUCY COOKE: Well ...

ROBERT: ... there was a breakthrough.

LUCY COOKE: May, 1822. So on May 1822, there was a German chap called Count Von Bothmer, and he was out in the grounds of his estate.

ROBERT: In northern Germany, hunting.

LUCY COOKE: A venerable pastime for a German count.

ROBERT: And while he's walking around, gun in hand, above him he saw a flock of birds. So he takes his rifle, his big rifle, points it up to the sky. And then falling from the blue ...

LUCY COOKE: A stork. A white stork.

ROBERT: Now the Count had shot down plenty of white storks on his property.

LUCY COOKE: But this time he shot down a stork that was an unusual stork. And what was particularly unusual about this stork was that it had already been shot.

ROBERT: But not by a gun.

LUCY COOKE: It had been lanced by a spear.

ROBERT: Sticking through this stork's neck ...

LUCY COOKE: The bird's spaghetti neck ...

ROBERT: ... was this spear.

LUCY COOKE: Sort of half on one side and half on the other side like a kebab.

ROBERT: And the Count had no idea what to make of this.

LUCY COOKE: So he sent the stork off to a local university, and one of the professors there deduced that the spear in question had been, and I quote, "Thrown from the hands of an African."

JAD: The spear is African?

ROBERT: Yeah.

JACKSON: The spear is of African make.

JAD: Where in Africa did the spear—did he say specifically where it came from?

JACKSON: He didn't say specifically. I think the European perception of Africa is very limited at that point.

ROBERT: But yeah, what's important though, is that they knew the spear came from somewhere in Africa.

LUCY COOKE: And so it was assumed that the stork must have traveled to Africa in order to have been speared.

JAD: Wait. So a stork would have flown from—from somewhere in Africa with a spear through its neck?

JACKSON: Yeah!

JAD: What a—what a valiant, heroic stork!

ROBERT: Well, until you find out that it's not the only one.

JAD: What do you mean it's not the only one? There are others?

ROBERT: Well, a few years after the first stork got shot down, another one also got shot down.

JAD: Same deal? Thrown—spear thrown by someone in Africa?

ROBERT: Yes, with a spear and everything. And then another one, and then another one.

LUCY COOKE: There's actually 25 that have been collected over the years.

JAD: Wow!

LUCY COOKE: They're called—I'm gonna pronounce this totally wrong, but—Pfeilstorchs?

ROBERT: And these storks seemingly flying all the way from Africa with spears stuck in their necks ...

LUCY COOKE: Helped solve one of the greatest mysteries in biology, and that is the question of where birds disappear to over winter.

JACKSON: So these storks were the thing that finally convinced people that when birds vanish in the winter, they're not hibernating, they're not flying to the moon, they're not transforming into other birds, they're just migrating.

LUCY COOKE: And then what they did most crucially was that they inspired the idea that you could tag birds.

JACKSON: Like, scientifically track their migrations by just putting little metal bands on their legs or in their ears. And so this whole new field of science starts up.

[ARCHIVE CLIP: One of America's most spectacular birds is the sandhill crane...]

JACKSON: They start tracking all kinds of birds.

[ARCHIVE CLIP: ..the water fowl ...]

JACKSON: And then mammals, fish, whales.

[ARCHIVE CLIP: Many species of large shark and giant manta rays ...]

[ARCHIVE CLIP: Game rangers capture animals in just about every way possible—with nets, ropes, tranquilizer guns, even with our bare hands.]

[ARCHIVE CLIP: Each collar contains a tiny radio transmitter that sends out a powerful signal through this antenna ...]

[ARCHIVE CLIP: They'll swim in three months nearly 5,000 miles to calving grounds in Mexico.]

[ARCHIVE CLIP: The caribou.]

[ARCHIVE CLIP: The giant leatherback turtle.]

[ARCHIVE CLIP: The wildebeest and zebra in this part of Africa have followed the seasonal rains.]

JACKSON: And they start to realize that it's not just like this isolated little quirky phenomenon in a couple of species. Migration is everywhere. And so this is when we come back to Martin, the boy from the beginning of the story with the question. He grows up, but he stays fascinated by this question and he starts chasing answers to it. He goes to school, he studies migration science in birds.

MARTIN WIKELSKI: I learned how to band birds myself.

JACKSON: He starts chasing birds tagged with radio transmitters around the Midwest. He has to capture them every night, so he drives up to random people's farms in the middle of the night.

MARTIN WIKELSKI: Because the, say a thrush just landed there after a night's flight, somewhere in the middle of Iowa or northern Wisconsin or somewhere. Knock on the door and you want to talk to the guys and say ...

JACKSON: So, um, there's a—there's a bird. Uh, it's—I need to—can I come into your backyard?

MARTIN WIKELSKI: And then, inside you just hear [clicks], so then you say so well maybe ...

ROBERT: That's your version of a door slam?

MARTIN WIKELSKI: No, that's the gun being loaded.

ROBERT: [laughs] That's the gun being loaded.

MARTIN WIKELSKI: And then you think, "Well, maybe I don't need to catch this animal again."

ROBERT: Did you ever get shot at or arrested or ...?

MARTIN WIKELSKI: We did get arrested but that was in, well, different places. Algeria once and a few other places.

JACKSON: But as his career went on, the tracking devices he was using got more and more high tech.

MARTIN WIKELSKI: Nowadays, it's really almost like your cell phone. You have a little—you have a GPS receiver that gives you a GPS point every second.

JACKSON: Coming from a tiny transmitter that's attached to the bird's back.

MARTIN WIKELSKI: It's a little tiny box that's like about 25 grams. It's like a chewing gum box or so.

ROBERT: A chewing box, we can do better. Like, can you think of ...?

MARTIN WIKELSKI: Let's say it's about a quarter of a Snicker bar.

ROBERT: Okay, all right. So and that is—that is sending you all this information, but where does the information go to? To an antenna or to a ...?

MARTIN WIKELSKI: Well, it's going to a cellphone tower, and via the cellphone network it's going to Movebank, which is this database for animal movement.

JACKSON: That collects migration data from thousands and thousands of animals.

DAVID WILCOVE: So it's only relatively recently that we've had the technology that enables us to really track these species.

ROBERT: This is David Wilcove, biology professor at Princeton University.

DAVID WILCOVE: So we're only now beginning to piece together the journeys that these species have been making for millennia, and that have been fascinating us really for as long as people have been looking up and seeing a flock of birds flying over or watched a butterfly drifting by.

JACKSON: And when you look at what we now know about these journeys, some of them are just unreal.

ROBERT: For example, there's a bird called the Arctic tern, which is born up, of course, in the Arctic in the spring and the summer, where it's just sunshine all the time, almost 24 hours a day. But when the—when the fall comes, like the sun starts to vanish, and this bird needs sunshine. So it has to chase an endless day, which means it has to go all the way to the other side of the earth, to the Antarctic to find the same conditions.

JAD: Wow!

ROBERT: And then when the Antarctic begins to lose its sunshine, it has to come all the way back again. So this is a bird that has to transect the globe, chasing the sun. Or ...

DAVID WILCOVE: A Monarch butterfly, flapping its way from New York state to Central Mexico. That's really a piece of Kleenex, flapping its way to this particular site. And it's never been there before. Its parents have never been there before. Its grandparents have never been there before. By virtue of the weird, inter-generational migration of Monarchs, it's going to a place that its great-grandparents left.

ROBERT: Or here's one. There's a dragonfly that is born in a pond, is sucked up by evaporating air, hits a monsoon cloud, goes into a jetstream, goes across India, across the Indian Ocean, down the East African coast. The clouds reverse and it goes back in the other direction. What amounts to a 10,000-mile journey, always in the cloud. And the cloud when it rains dumps it onto the ground. It has a new baby and the baby takes off and is flown up in the air. So there's this weird pogo-stick existence.

JAD: Wow!

JACKSON: And when you put all these journeys together, they can actually kind of visualize it, they can put it on a map and animate it.

JAD: Okay, here come—come closer. All right, we are opening up a YouTube video here?

JACKSON: And Movebank has published videos of this, which you can just search and find on YouTube.

JAD: Okay, here we are. We're starting now. It's winter for the top of the globe. So you start to see these purple veins appear on the bottom, bottom of Africa. Now the veins are flowing up ...

ROBERT: Flowing is the right word, you know?

JAD: ... toward, toward—where—what is this region right here? It looks like Scandinavia.

ROBERT: Scandinavia, yeah. So there's—like, this is filling with birds, probably.

JAD: Wow!

ROBERT: And fish.

JACKSON: What you'd see is basically all these different points of light, each one is representing a tag on an animal. And as the clock ticks forward and the seasons change, all of these little points of light start moving.

JAD: It's just like this entire, like, rush of all of these, like, lines suddenly shoot down to the bottom of the—of the planet.

ROBERT: Right.

JAD: It does remind you of, like—of like, blood flow in a body.

AMANDA SUBALUSKY: Yeah, the animal movement paths look like these, you know, arteries.

ROBERT: This is Amanda Subalusky, a University of Florida professor. She studies the wildebeest migration in Africa. And she said, you know, this is really more than just a visual metaphor. All these animals moving around the planet really do form ...

AMANDA SUBALUSKY: Kind of a global circulation between different regions of the world. Sometimes really vast regions.

ROBERT: Take, for example, the wildebeest.

AMANDA SUBALUSKY: They can get swept downstream and miss the ...

ROBERT: Amanda says thousands of them will cross these rivers during the migration, and they don't all make it. So in the months that follow ...

AMANDA SUBALUSKY: Up to half of the fish diet is comprised of wildebeest carcass.

ROBERT: So as these animals move around the planet, they move what they poop around, what they eat, when they die how they decompose. They move not just themselves, but bits and pieces of the world. It does give you the sense that these animals really are, like, the lifeblood of the planet, flowing away from their home and then coming all the way back again. And then going out again and then coming back again. And out and back. And out and back. But when you zoom in on this global flow to all the individuals who actually made these journeys?

MARTIN WIKELSKI: That's not trivial. I mean this is—this is halfway around the world.

ROBERT: You find yourself asking ...

MARTIN WIKELSKI: Why are they doing that, how do they go there, how do they find back to this nest?

ROBERT: And that's when things get really messy and beautiful. And totally unexpected. As you will discover in just a moment or two.

JAD: Okay.

[LISTENER: My name is Chipora, calling from Seattle. Radiolab is supported in part by the Alfred P. Sloan Foundation, enhancing public understanding of science and technology in the modern world. More information about Sloan at www.sloan.org]

JAD: Jad.

ROBERT: Robert.

JAD: Radiolab. We are back with Robert and Jackson Roach and migration.

JACKSON: Right. So before the break, we had scientists that were gathering up all this migration data and turning it into a kind of global story of animals moving from place to place across the Earth.

MARTIN WIKELSKI: Yes. I think we're getting a lot more knowledge about the collective behavior of these individuals, giving us a completely new emergent knowledge about life.

DAVID WILCOVE: We—we are just entering the—what I call the golden age for studying migration.

ROBERT: But even with all their new gizmos and their measurements, people like Martin and David will tell you that the most basic facts about migration still remain deeply mysterious.

DAVID WILCOVE: Yeah, I find the whole phenomenon of migration so mind boggling that ...

ROBERT: What is it that most boggles your mind about the whole thing? The mind boggler?

DAVID WILCOVE: That they pull it off. That they do it!

ROBERT: And really, the simplest question you could ask about all this is: why? Why migrate at all? I mean, you know, for some animals it seems sort of obvious.

DAVID WILCOVE: It makes sense that if you like to eat insects or float on freshwater, and you happen to be up in Maine where it's gonna get snowy and cold, makes sense that you're gonna wanna get outta Dodge—or Portland.

ROBERT: [laughs]

DAVID WILCOVE: So that part makes sense. The part that's a real puzzle is: Why do you go so far? Why does the blackpoll warbler that was up in Maine go to the Amazon? Couldn't you have stopped over in Fort Lauderdale or Cuba or Guatemala? I don't think anyone really knows what's going on there.

ROBERT: The best anyone can say is that over thousands and thousands of years, individual creatures would go just a little bit further somewhere, and then a little further still, looking for greener grass or more insects in the late afternoon, until they ended up thousands and thousands of miles away from where they started.

JACKSON: But that raises an even harder question, right? Like, if you're a warbler that made it from Maine all the way down to the Amazon where it's warm all year, why not just stay there?

BEN WINGER: Yeah, so the hard part is always why come back?

JACKSON: This is Ben Winger. He's an evolutionary biologist at the University of Michigan.

BEN WINGER: The amazing thing about migration is not so much that these organisms accomplish this journey, but that they're doing it just to come back to this one place.

JACKSON: Ben says the craziest thing is that so many of these animals come back not just to the same general area, but to the exact same spot.

BEN WINGER: Yeah. For birds it's often the exact same tree where they built a nest the year before.

ROBERT: That's one tree of a trillion trees in the world.

JACKSON: And all kinds of animals do this.

AMANDA SUBALUSKY: Yeah. The alewife will do that. You know, they'll go to the ocean ...

JACKSON: Amanda Subalusky told us that the alewife, foot-long little silvery fish, is born in a freshwater lake then spends its life swimming around the open ocean, all just to come back to that same lake.

AMANDA SUBALUSKY: Imagine a small alewife in the vast ocean [laughs] returning to the very inland lake where it was born to spawn again.

KEN LOHMANN: On the face of it, some of these migrations are extremely illogical.

ROBERT: Consider the sea turtle. According to University of North Carolina biologist Ken Lohmann ...

KEN LOHMANN: There are turtles that have feeding grounds in Australia within, oh, five or 10 miles of a really good nesting beach that other members of their own species will use. But instead of nesting at this location that's just 10 miles away, they'll swim literally 700 miles to get back to their home area.

JACKSON: To exactly the same beach!

ROBERT: And if you think about that, a turtle could go to any European beach, any African beach, any South American beach or any North American beach if it's an Atlantic turtle, and yet for some reason they choose the one where they were born.

JAD: Wow!

AMANDA SUBALUSKY: That's just astonishing to me how that can even happen. And the fact that it does happen does suggest that the drive for it to happen is very, very strong.

ROBERT: Nobody really knows what drives that drive, but one of the ideas floating around has to do with the fact that getting born in the wild? Very, very hard thing to do. A female sea turtle for example, will lay roughly 2,000 eggs in its lifetime, and probably only two of her babies are gonna survive.

KEN LOHMANN: Yeah. And if you think about what is required for a good nesting beach for a turtle, there's really a very long list of requirements.

ROBERT: Just to name a few: the beach has to have the right slope.

KEN LOHMANN: The turtle has to be able to get out of the water.

ROBERT: Once the turtle's on the beach, the sand has to be loose enough for it to dig a nest.

KEN LOHMANN: The temperature has to be exactly right. It can't be too wet. It can't be too dry.

ROBERT: It has to have the right kind of vegetation and it can't have too many egg-eating ants.

KEN LOHMANN: In addition, there—there can't be too many predators. And so from the mother turtle's perspective, the only place in the world that is absolutely certain to have everything exactly right is the beach that she herself hatched out on.

JACKSON: It's always the best bet.

BEN WINGER: So it's probably the case that many animals have a kind of a home sweet home sense.

JACKSON: This, like, hardwired compulsion to go home, no matter how far away it is, no matter how risky it is to get there.

BEN WINGER: You know, it doesn't matter how long you study migration, the fact that is built into this creature is endlessly fascinating.

ROBERT: No, it isn't. It's like it's a toaster or something. It's like sorta—it's like a machine thing. I would be rooting for creativity.

BEN WINGER: So that's the whole thing with ...

ROBERT: So at this point, this is where I—where I dig in on this. I just always wondered when you watch all these animals moving back and forth across the Earth, are you watching just, you know, puppets on the end of some kind of genetic code? Is it all just rote?

DAVID WILCOVE: Well ...

ROBERT: Or is there some kind of will and some kind of play and some kind of improvisation? Isn't that one of the questions? Or is that already known?

DAVID WILCOVE: Oh, it's one—it's one of the most important questions. You know, can they adjust their migratory behavior in the face of climate change or habitat loss? And to answer that question, we're gonna need to be able to track individuals over multiple migrations, and we're gonna need to be able to track lots and lots of individuals in a given migration.

MARTIN WIKELSKI: And I think that's something that we can do now for the first time on a global scale.

JACKSON: So again, this is where we come back to Martin. He's now the director of the Max Planck Institute for Ornithology, which is the place he visited as a kid, looking into the cattle egrets. And while on the one hand, the data they're gathering about all these different migrating animals is giving them a lot of info about their collective behavior ...

MARTIN WIKELSKI: But then on the other hand, we're getting much more information about individuals.

JACKSON: They're gathering an astonishing amount of data about particular animals.

MARTIN WIKELSKI: We get acceleration behavior, so it's really—we know when it sleeps and when it's active and all of that. If they are pecking, if they are walking, if they are swallowing.

ROBERT: Oh so you can read—from these monitors you can read, now he's on the ground. Now he's pecking, pecking, pecking, pecking.

MARTIN WIKELSKI: Exactly.

ROBERT: Now he's up in the air, now he's on the ground again.

MARTIN WIKELSKI: These recreations of behavior. These sort of little movies that you can make.

ROBERT: And when you—once you get to those tiny little stories, then a whole 'nother—a whole 'nother world opens up to you.

MARTIN WIKELSKI: And we —we had a stork, you know, that went to an area where no other stork was at the time.

JACKSON: So Martin now lives in this little town called Radolfzell where the Max Planck Institute is. And every year, he and the other migration scientists go out and tag all of the storks in the area so they can watch them move around town, and eventually come fall, migrate.

MARTIN WIKELSKI: They—they circle up to the clouds and fly around and test their flying abilities and so on. And then they go on this big voyage.

JACKSON: And one year, when all of the storks take off, Martin is sitting there at his computer watching these dots, these clouds of dots move off in different directions. And he's watching one cloud of dots which is going east, sort of through Eastern Europe, and then start to curl around the eastern edge of the Mediterranean to go south into Africa.

MARTIN WIKELSKI: We had a bunch that went to Africa, a bunch ...

JACKSON: And then he sees that one of the dots, one of these storks just sort of peels off.

MARTIN WIKELSKI: Yes. So—so that actually was Hansie.

ROBERT: Hansie.

JACKSON: Apparently, they name the storks when they tag them. So Martin actually knew specifically which one this was. Anyway, Hansie ...

MARTIN WIKELSKI: He was in an area where no other stork was at the time.

JAD: Oh, so they saw like a little blip of purple peel off?

ROBERT: Yeah.

JACKSON: Yeah, peel off from the group.

MARTIN WIKELSKI: In the southeast of Turkey, close to the Syrian border.

ROBERT: And he drops down into a patch of what seems to be utterly ignorable ground in the Middle East all alone. Like, there's no other storks there.

JAD: Wow!

ROBERT: And ...

MARTIN WIKELSKI: So we wanted to know why did he choose to, you know, stay—spend his winter close to the Syrian border in an area where usually no stork winters.

ROBERT: Who's the 'we' in this story?

MARTIN WIKELSKI: Oh, it's just—well, in that case, it's my—my partner. She's—she's also a scientist.

USCHI MŰLLER: My name is Uschi.

ROBERT: Uschi.

USCHI MŰLLER: Uschi Müller.

ROBERT: Uschi Mull ...

USCHI MŰLLER: But a 'oo' you know?

ROBERT: Yeah, an umlaut.

USCHI MŰLLER: The German 'oo?' Yeah.

ROBERT: Okay. And do you—like, do you remember Hansie?

USCHI MŰLLER: Yes, I do. They told me that there is a—a stork. He tried to—to go the south, and then he decided to stay in Turkey and ...

ROBERT: And you're thinking, "What's with this guy? He's, like, not—didn't he get the memo you're supposed to go south?"

USCHI MŰLLER: Yes.

JACKSON: You know, migration is a survival strategy. So if you're not migrating, you're probably in trouble.

ROBERT: And Martin and Uschi are thinking "Well, what happened here? Did he get hurt, badly hurt? Or maybe he made a really stupid decision and is now going to starve to death."

JACKSON: So they figured, we know exactly where he is, let's go see him.

USCHI MŰLLER: Yeah. I wanted to come along. I wanted to see that, because I'm so interested. And it's also kind of an adventure to follow the bird, to see what he's doing.

MARTIN WIKELSKI: I mean, it's actually an interesting way to have not your local travel office guide you to a place, but animals. They—they guide you to interesting places.

USCHI MŰLLER: We flew into the Turk—to Istanbul, rent a car and then try to—to find the bird.

JACKSON: So they hop in the car, they start driving around. And they have their phones or their laptops or whatever out, and—and they're watching this little dot, which tells them basically exactly where Hansie is.

MARTIN WIKELSKI: Down—down to the closest two meters or so, so ...

ROBERT: Oh, wow!

MARTIN WIKELSKI: But—but only at a specific time.

JACKSON: Turns out the way this device works is that it sends out its data to a cell tower if it can find one, but only once a day.

MARTIN WIKELSKI: Only at basically noon or noon plus three minutes.

ROBERT: [laughs] So you—so you have to drive. So that means you get close and then you have to wait a day to take your next move, I guess.

MARTIN WIKELSKI: Exactly, yeah.

ROBERT: Oh, God!

MARTIN WIKELSKI: Yeah. So you just go to a place and there was nobody.

ROBERT: Oh.

MARTIN WIKELSKI: And then you wait for the next day and you realize, oh, he left an hour before you were there and flew another 20 kilometers. So you had to go, you had to pack up your stuff and go to a different town and find a different hotel and different place to stay and—and try to do the same thing over again.

JACKSON: Now they know if they can get close enough, they can pick up a radio signal from the tracking device too. So each day, they're coming into some town that they think Hansie was nearby at least, and driving around the area, sweeping for a signal.

USCHI MŰLLER: We followed him for I think two or three days.

ROBERT: Days?

USCHI MŰLLER: Days.

JACKSON: And then, finally ...

USCHI MŰLLER: We—we get—we got a signal in the—in the early, early morning. It was dark. And then ...

JACKSON: They get up, start driving around.

USCHI MŰLLER: We tried to find, to—to come closer to the bird with the antenna, we hold outside of our car. And then the signal was—became louder and louder.

JACKSON: And eventually, they get to a field where the signal is really strong.

USCHI MŰLLER: And—and stayed there in the car 'til it—the daylight comes a little bit more.

JACKSON: And just as the sun was coming up ...

USCHI MŰLLER: We saw him feeding on a—on a field.

MARTIN WIKELSKI: Yeah, in an old field with a little ditch next to it.

JACKSON: This tall, white bird, all alone, no other storks in sight. And he looked fine.

MARTIN WIKELSKI: He was just walking around, feeding on frogs and snakes and whatnot. So he was having a good time there.

ROBERT: So are you thinking, "Good for you? You've actually discovered a—a restaurant on the outskirts of town that none of the other storks know about?"

USCHI MŰLLER: Yeah. Yeah.

ROBERT: Frog City or something.

USCHI MŰLLER: [laughs] Frog City!

JAD: So what do you make of the fact that this—is this just an errant, rogue stork getting lucky? Or is this the beginning of something?

ROBERT: Well, it may be just Darwin. It may be that, you know, when you get a population group and they all on average do one thing, that nature kind of requires that someone on the edge do something else.

DAVID WILCOVE: There's probably selection for at least some portion of the progeny to wander farther afield.

ROBERT: So that just in case, there's a creature around who can handle a new environmental challenge.

MARTIN WIKELSKI: It's actually probably those—those innovative individuals that really in the end are really important for that entire species to thrive, but—because they have—they explore novel ways to do things.

ROBERT: Wow, so there are—this is not Robotland where animals, you know, awake to a particular instinct and just fly as best they can to a goal. This is, like, full of—"I think I'll go east this year. I think I'll go south next year." Like somebody taking a vacation.

MARTIN WIKELSKI: Exactly. In the old days we always talked about sort of an average animal, you know, that storks they fly there and they go here and whatever. And—but none of the individuals really do that. They all do totally different things. But if you average it out then yeah, you have sort of the average German goes to the beaches in Spain in—in summer. But yes, there are a few people that do that, but there are so many others that don't do it—or do different things, or do it one year and go to some totally different place the next year.

ROBERT: There's the one who goes to Norway and you think, "What's with that guy?"

MARTIN WIKELSKI: Exactly [laughs].

JAD: I did notice when we were looking at the flow patterns of migrations, you have broad strokes. Big aggregate flows from one spot in the globe to another. But all along those routes you see little—like, little wisps shooting off in opposite directions.

ROBERT: Right. And there's a larger thought here, which is if all migrations were just instinctual, and if the world is going to change in any dramatic fashion, then all these animals would be in the wrong place in the wrong season and be dead.

JAD: Yeah. Yeah.

ROBERT: But if, in every population, there's somebody sitting there who's like Hansie, who didn't follow the rules and didn't do what everyone else was doing, then Hansie gets to be the one who's fed and has the children and keeps the species going.

JAD: I mean, do you see Hansie writ large, though? Do you see a lot of these birds—like, do you ever see a moment where Hansie says to the other birds, "Hey guys, this field is great. We don't have to go all the way over there. Let's just come here. It's so much closer. There are all these frogs."

ROBERT: Jackson asked that very question.

JACKSON: Could the other storks hear about Hansie's weird trip and say, "Oh. Maybe I should try that."

MARTIN WIKELSKI: Yes, because we know that they learn socially. They communicate and they learn from each other. We still need to understand the extent of that and how exactly they communicate and what they tell each other, but they do follow each other. And if there's somebody who knows exactly where to go and is totally determined to go there, it's easy to imagine that others follow because they realize something good has happened there, must have happened there.

JACKSON: Hmm.

JAD: So maybe Hansie's the beginning of a chapter, a chapter of storks.

ROBERT: Maybe.

JACKSON: But most storks that are changing their migrations, instead of going to a cool field full of delicious frogs, are going to garbage dumps in Spain.

ROBERT: Mm-hmm.

JAD: Oh.

JACKSON: And quitting migrating entirely.

LUCY COOKE: Yeah. No, they are. They're—so they ...

ROBERT: Lucy Cooke, our mistress of the history of animal mystery actually did follow an individual stork.

LUCY COOKE: You know, you can get this app where you can—you can follow the birds that have been tagged yourself. And I followed one for six months, and he basically never really left. In fact, I don't think he even bothered going back north again in the—in the spring. Just stayed by the dump the whole time.

JAD: Oh, that is depressing.

ROBERT: Yeah.

JACKSON: Yeah. And the truth is what we heard from pretty much every scientist we talked to is that for lots and lots of animals, land, sea, water ...

DAVID WILCOVE: Migrations are declining and in some cases disappearing because we are altering their habitats or creating all sorts of barriers to those journeys, and we're changing the climate.

ROBERT: And what the future brings is anyone's guess. But the coolest thing of all really, is that as we speak, Martin has placed a box in the space station above the Earth, and that box will very soon begin to collect the data from all of the animals, species across species across species, crawling, swimming, sailing, whatever across the Earth. And they're all gonna report to this box at the same time. And then we—we who have computers all over the world can watch the full ballet ourselves in real time.

JAD: Oh wow. That's ...

JACKSON: Or ...

JAD: Or?

JACKSON: ... we're gonna watch this ballet that's been happening on the planet for millions of years and has been part of human existence forever, disappear. But I mean, however the story goes.

ROBERT: However the story goes [laughs], we'll have a front seat in what will either be a very sad show or maybe one where we'll be cheering for Hansies in the whale community. Hansies in the plover community.

JAD: Hmm.

ROBERT: Yeah.

JACKSON: This story was reported by Robert and Jackson Roach.

ROBERT: And it was produced by Jackson with Pat Walters and Matt Kielty. And I should also give a special thank you to Joel Berger, Bert Heindrich, Bill Cochrane and Isabelle Houton.

JAD: Special thanks to our yodelers, Allie DeNeen and Gregory Corbino, to Jeremy Bloom for his mixing and for his original music. And of course, to Mr. Bobby K and Jackson Roach.

JACKSON: Yeah, thank you. Can I just—I just want to say one more thing, because migration is, like, something you can obviously look at in this kind of like science-y way, and—and that's great. It's really cool. But it's also like a—kind of like a basic human experience, to look up at maybe the geese in North America, and just think, like, "What is it like up there, to be totally free of all of our stuff down here, borders and all the things that get in our way?" And so Martin, while he's amazing at the hard science side of things, he's also curious about that experience of, like, what it feels like to be a migrating animal. And at a certain point in his life, he decided to sort of try to find out.

MARTIN WIKELSKI: I bought a hang glider and went up to the mountains. And—and I wanted to understand what the birds experience in the air. I mean, learn things about the air that usually you just don't know. I mean, in the hang glider, you are the bird. You are the body of the bird, and you have your wings that you fly left or right, and you just fly with your hands, with your—just moving your body around. We had days when we couldn't even get down to the ground because the—the uplift was so strong. We had days when, you know, we flew into the night because over a swampy area late at night, there's an updraft. And I wanted to know anytime, you know, when it was a little misty, foggy in the valley and I mean, you are—you're immediately feeling the air. It's—you know, if there's an updraft, you get—you feel it immediately. And you can—you can turn yourself directly into an updraft, fly up in a thermal. You're up at four and a half thousand meters and there are people climbing those mountains, they're up on the glacier, and you fly over there and you sort of shout down to them and say, "Hi guys." And—and, you know, because you're just 50 meters above them, that's—that's absolutely amazing. And then you take off and fly to the next mountain peak. And it starts to drizzle and it starts to rain and it starts to snow up there. You know, you're in the—you're in the realm of—of the weather. You're the closest to a bird you can ever get.

[LISTENER: Hi, this is Reuben from Passaic, New Jersey. Radiolab is created by Jad Abumrad with Robert Krulwich and produced by Soren Wheeler. Dylan Keefe is our Director of Sound Design. Suzie Lechtenberg is our Executive Producer. Our staff includes: Simon Adler, Becca Bressler, Rachael Cusick, David Gebel, Bethel Habte, Tracie Hunte, Matt Kielty, Annie McEwen, Latif Nasser, Sarah Qari, Arianne Wack, Pat Walters and Molly Webster. With help from Shima Oliaee, W. Harry Fortuna, Sarah Sandbach, Malissa O'Donnell, Marion Renault and Russell Gragg. Our fact-checker is Michelle Harris.]

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