In Our Time - Bird Migration

Episode Date: July 6, 2017

After 27 years, Melvyn Bragg has decided to step down from the In Our Time presenter’s chair. With over a thousand episodes to choose from, he has selected just six that capture the huge range and depth of the subjects he and his experts have tackled. In this fourth of his choices, we hear Melvyn Bragg and his guests discuss bird migration. Their topic includes why some birds migrate and others do not, how they select their destinations and how they navigate the great distances, often over oceans. For millennia, humans set their calendars to birds' annual arrivals, and speculated about what happened when they departed, perhaps moving deep under water, or turning into fish or shellfish, or hibernating while clinging to trees upside down. Ideas about migration developed in C19th when, in Germany, a stork was noticed with an African spear in its neck, indicating where it had been over the winter and how far it had flown. Today there are many ideas about how birds use their senses of sight and smell, and magnetic fields, to find their way, and about why and how birds choose their destinations and many questions. Why do some scatter and some flock together, how much is instinctive and how much is learned, and how far do the benefits the migrating birds gain outweigh the risks they face?With Barbara Helm Reader at the Institute of Biodiversity, Animal Health and Comparative Medicine at the University of GlasgowTim Guilford Professor of Animal Behaviour and Tutorial Fellow of Zoology at Merton College, Oxfordand Richard Holland Senior Lecturer in Animal Cognition at Bangor UniversityProducer: Simon Tillotson In Our Time is a BBC Studios ProductionSpanning history, religion, culture, science and philosophy, In Our Time from BBC Radio 4 is essential listening for the intellectually curious. In each episode, host Melvyn Bragg and expert guests explore the characters, events and discoveries that have shaped our world

This is the BBC. Thanks for downloading this episode of In Our Time. There's a reading list to go with it on our website, and you can get news about our programs if you follow us on Twitter at BBC In Our Time. I hope you enjoyed the programs. Hello, for millennia, bird migration was a complete mystery to humans. Today, while what we know is remarkable, much of that mystery remains. There was an idea in ancient Greece that birds turn into fishes when they were not around.

In later folklore, some were thought to turn into barnacles. others to hibernate in cliffs or at the bottom of lakes. And perhaps those ideas are less extraordinary than what we now know. For example, that birds weighing less than a cup of water can fly across oceans non-stop from New Zealand to Alaska, breed and return. How birds know where they are going is not yet fully understood, but may include some combination of internal clocks, of ways of detecting magnetic fields and of heightened sense of smell.

With me to discuss bird migration are Barbara Helm, reader at the Institute of Biodiversity, Animal Health and Comparative Medicine at the University of Glasgow. Tim Guilford, Professor of Animal Behaviour and Tutorial Fellow of Zoology at Burton College Oxford, and Richard Holland, Senior Lecturer in Animal Cognition at Bangor University. Tim Guilford, can you define migration for us, and it isn't only for birds, can you give it as an overall view of migration? Yes, well, I think migration means different things to different people, But the sort of classical definition of migration would be something which,

a bird, for example, or a turtle, which makes a very long journey repeatedly from one season to the next, from one year to the next, between a breeding site in one location and an overwintering site somewhere else on the planet. And so it sees these very long distance repeated return movements, which are usually seasonal between breeding and, overwintering, so the iconic definition of migration. But of course, people do refer to migration of plankton and fish on a daily basis in the marine water column up towards the surface at night and down deeper during the day.

And some people would regard that as migration. So it's different things to different people, but it always involves this kind of cyclical, surprising movement. So birds are not alone in this, that's what I'm trying to establish. Absolutely right. Some of the greatest navigators, some of the greatest migratory species on the planet are turtles, for example, or whales. Now, what were the earlier, I mentioned one or two,

what were the earliest ideas of what was happening to birds until the late 19th century, really? Well, I mean, one of the characters, I think, is iconic in the history of this debate, if you like, is Gilbert Weiter, a naturalist from Southern England. and in the late 19th century, he was still puzzling. He was a great observer. He was the first person to watch Swift's mate on the wing, for example, without a pair of binoculars. But despite this, he still puzzled as to what happened to swallows

and house martins when they finished breeding and appeared to disappear from the countryside. And he, in the end, came down on the idea that they hibernated in the mud in riverbanks, at the bottom of pond. So that was a less surprising idea to him than the idea that they fly all the way, well, we now know, to sub-Saharan Africa.

And so it wasn't really until, I suppose, till the start of the 20th century when systematic ringing studies started. Can you explain ringing, please? Yes, of course. So, well, at the beginning of the 20th century, people started to place little metal, bands around the lakes.

It did it? With a chap called Mortensen. That's correct. So he started it and he put his own rings on birds before they disappeared for the winter and then the same birds would come back and he'd know they were the same birds because they carried the same unique identity number on the metal ring. And that has now become a systematic way

of studying long distance migration across the room. Sorry. Did you know where they'd been? I mean, they came back but where from? not from the bottom of a leg, presumably, but still from somewhere, rather. Absolutely right. So that's right. The rings don't tell you until, of course, somebody spots or catches or finds a dead bird somewhere else on the globe. And it's the reports of these individually numbered rings

found on a dead bird, say, in Senegal, which will tell you that this is the same individual that is at one time breeding in Denmark, the next it is found dead on migration in Africa. This presupposes a global network of bird spotters though, doesn't it? Was it anywhere in existence at the end of the 19th century? No, not really. I mean, that's something that has grown and grown during it. So how did he know then?

I mean, this is waiting for somebody from Ethiopia to say, dear, dear Mr. Mortensen in Denmark, I found the bird with your ring on it. And really, what happened? It's interesting. So I think it was individual anecdotal occasions of people in southern Europe who found a bird, a swallow. I'm not sure that it was a swallow, but a swallow, for example. An individual bird with a ring on was spotted.

And it's just, you only need one example, actually. You only need one documented example of the same individual being rings in a breeding site in northern Europe and being spotted alive or dead in Spain to know that it must be something to do with long distance movement. That's only an anecdote, but it tells you that it's possible. And once you know it's possible... And there's also the fact that Europeans are exploring Africa

in the 19th century and they were finding birds that looked very like birds that left behind in Europe and that entered into the curiosity, the nest of curiosity. Barbara Helm, people experimented with or looked at or studied caged birds because at least they wouldn't fly to South Africa, as it were. What did they find when they studied at them and how was it useful? Yeah, just to sort of add on to what Timter said.

So this kind of puzzlement about where birds went It was just one part of the debate, but there were also, at the same time, and even centuries before, quite a few people who never believed that stuff about turning into fishes and so on. And they had done visual observations, but also on wild birds, but as well in cage birds. And there's this whole culture of bird fanciers that has been around for a very long time, where people observed and reported from the early 1700s onwards that just during the times, when wild conspicuids would migrate, there was an odd change in behavior in caged birds.

The Nightingale, for example, was one of the first examples of that. There was a French report. There were simultaneous reports also from Germany and the UK. And what happens is that these birds, they become really activated and restless. They also become kind of plump. They put on a lot of extra weight for that time. And then they change, they extend their behavior

from being active in the daytime to being really, really active in the night time to the extent that bird fanciers often had a problem that the birds would hit their heads because they would fly against the cages and so on at night. So what you're saying is that they began to notice that when wild birds were migrating, cage birds were acting out the process of migration

inside their cages. They were getting fat and they were being restless. They wanted to fly. They'd go in a certain direction. They put marks on the wall, and it was one direction that marks on their fore, one direction that they hit. So they knew something big was going on here. Exactly. And they talked about things like instinct already like in the 1700s. Their reports, sometimes people say that was the beginning of actually animal behavior studies,

like in more scientific ways when people said this is like a drive because they realized that the birds were showing this behavior also when they were, for example, inside a house, not just when they were like in an aviary outside where they saw their conspicuics. There's a defining and magnificent, extraordinary illustration of a stork, 18, 20, a dead stalk, with an arrow going, a long arrow, going right through it, but it came to Europe, Germany, wasn't it? It came to Germany from, from, from, from, from, from, from, from, from Africa. Was it Bantu, Scots? Yeah, and they knew that something was going, did that help for people, lodge people?

That, that helped a lot, actually. as Tim said earlier, I mean, one example already proves the point in some ways. And so that Bantu's spear in 1822 was establishing that. And then there was actually in Germany alone 25 occurrences of birds, storks coming back with spears. It's amazing that they could actually carry out that long migration. It's a big spear, like through the neck, basically. and that one famous kind of like in some ways game-changing stork then of course Medet said fate when it returns to the breeding grounds

and then it was shot because it was such an oddity and it is now visible for inspection as a preparation in the museum. So when can you date the earliest tracking developments in this game? You mean going beyond rings? I think that would probably be the late 80s. I would think the first studies that I became aware of were actually also on large species like white stork,

like the migratory bird studies. And there was satellite transmitters that informed us about it. We're a long way from satellites at the moment. We're not quite getting to satellites. We'll stick with the stalk with the Bantu spear going through and the ringing. And turn to Richard Holland. Let's go back a bit.

Why does some birds migrate and not others? That's a good question. So the classical explanation for why birds started migrating was that there was competition in their home areas, we tend to think of in the tropical regions, and that some birds would move away from those regions to try and escape this competition and discover... Competition for food?

Competition for food, yes, exactly. And as anybody who's been particularly to Northern Europe will know, there are these very, very large emergencies of particularly insects. If you've ever been to Finland, then you'll experience a lot of mosquitoes, a lot of small biting insects. But for birds, this was fantastic. It was this very intense emergence of food resources that allowed them to move north, exploit these resources,

breed very successfully, feed their chicks very rapidly, and their chicks could grow very quickly and then retreat away back down to the tropics as the winters came in. But of course in biology, nothing is ever quite that simple. So it's a very beautiful, elegant explanation and I'm sure in many cases it is true. Why do some migrate?

I mean, some robins migrate and some robins don't. Now why do some do and some? Is there any way of getting to the heart of that? So there's a number of different theories to propose why some birds might migrate and others might not. We call this partial migration. So in England, in Britain, sorry,

the robin is an iconic winter bird for us, but as you say, in Europe, many robins migrate. But also within some species, some individuals within the same population migrate and others don't. So we need other explanations to just simply... What are the other explanations? So the other explanations,

one is that it's a body size that larger animals are able to survive and fast even through the winter. Another explanation is that it's, a dominance issue, that there are limited resources that they can survive in the temperate regions using, but that more dominant individuals out-compete subordinates. And the lesser-black-back gull is an example that we think may explain this. In the UK, older lesser-black-back gulls are less likely to migrate than younger ones. The younger ones tend to move away. And this is

something we actually know from ringing. Another argument is actually something called the arrival time hypothesis, that particularly small songbirds require a breeding territory they do. defend a territory in the summer to attract females and defend resources. And there's an argument that birds would arrive earlier and earlier to get the best territories to the point where some would never leave that that was the best way of actually making sure you've got the best territory was to not migrate, to throw away the migratory behavior. If you think about it, that's quite similar to the phenomena we see at Spanish holiday resorts around the pool in battling for sunbeds.

So getting there earlier and earlier to defend that. How far, I mean, this is obviously fascinating. Is it speculation? Is it science yet? Where are we? Some of it is speculation. Some of it is science. So we do, for instance, as I mentioned, we know with the lesser blackback gull from ringing studies and more recently even from tracking studies that younger birds are more likely to migrate than older ones,

and this feeds into the idea that older birds are more dominant. sometimes we can't distinguish between several of the hypotheses so generally older males are larger so the three different potential explanations are difficult to disentangle from each other in that case so we can't say we can't pin down for certain why in the same place some birds of the same species stay and some go Tim Gilford

are we at an early stage in the development here. It seems as if there's almost a sort of silicon valley of technology in the minutest form possible, especially in these tiny songbirds which do amazing things. And it's very risky, isn't it, for migration? Migration is very risky for some birds. Yes, it is. I mean, we're at a fantastic stage in the study of migration. I mean, I don't think there has been a more exciting time, really, because of the march of miniaturized technology, as I'm sure we'll come on to, we can now observe migratory, individual migratory movements on an astonishing scale and with astonishing precision and this is teaching us all sorts of new things

about where birds migrate to, some amazing journeys that we didn't know about before. Such as? Well, the northern wheat here, for example, is one of my favourites. This is a small songbird weighs about the same as a bag of crisps. they breed all across the northern hemisphere, but in North America, there are populations breeding in Alaska

and populations breeding in eastern Canada, let's say Baffinland, but both populations migrate to sub-Saharan Africa, and they do it in the opposite routes around the globe. So an eastern Canadian breeding northern wheat here will head on a 7,500 kilometre journey across Greenland, 2,000 kilometres across the open ocean. This thing weighs 20 grams, all the way down to sub-Saharan Africa and back to breed again.

An Alaskan bird will go the other way, a much longer journey, 15,000 kilometres, in fact, but less risky, requires less fuel because it can stop over and feed on the way. Now we know all of these facts now because of these emerging technologies which have allowed us to observe movements with much greater precision. And one of the things that I think we are learning is that the risks and costs associated with migration vary depending on conditions and depending on the different routes that birds take. In the case of the northern wheat here, nearly half of the year is spent on migration. So it's not surprising that that's probably where the greatest mortalities occur. for some species like my favourites, the pelagic seabirds, such as shear waters and albatrosses,

these long-distance movements are almost effortless in some cases. An albatross can... A wonderful, albatross, scarcely flaps its wings in its whole existence, isn't it amazing? It sticks its wings out. Great thing to think about it, just soaring around on the... It's wonderful, isn't it? No wonder if Coleridge got intoxicated by it. It's not clear that it was an albatross, actually.

It might have been a giant pet... We're not here to distress that. But anyway, similar creatures. But my point is that for small flapping flight birds that make these huge journeys, migration is extremely costly. And a we tier migrating from Canada will need to double its mass before it heads out on that migration in order to fuel that flapping flight. But for a sheer water migrating between the Welsh coast and South America,

it's a much less costly journey. And so there's huge variation. So let's, the next session, let's try to get into the, if I make or probably miscall, the technology that is involved, which is, it's quite extraordinary because you're all written about it so well, but it's still bewildering. Barbara, Barbara, how do birds know when it's time to migrate? Yeah, that's one of the great wonders of migration. I think that people in all times have observed. They noticed that some birds were returning to their breeding grounds, for example, so timely that the air. actually use them as an agricultural calendar,

sawing seeds when the brown strike arrived, for example, on Borneo and so on. So there was an observation of absolute reliability in some of these species, but then as Tim said, migration can take different forms. There's also some species that go more by the weather. So people had distinguished in older times between calendar birds and weather birds, depending on what they did. Whether they're calendar birds or weather the birds, how do they know? Yes.

So the calendar birds that carry some sort of an indigenous clock, an internal clock that tells them when it's time to come back. Because imagine a bird crosses the equator and winters in sub-Saharan, southern Africa, for example. Days are actually increasing. Birds down there are starting to breed. And yet, instead of just staying and, you know, breeding along with them, sometimes with the same species, they just take off.

And the further... They come back. you mean? Come back, yes. Come back. And the longer the distance, the more they have to rely on their internal clocks, because if you're an Arctic breeder, you have just a few weeks, and then someone's gone. If you don't, if you miss that, then you've just forgotten your chance to breed. But have you any idea how this clock works? So they've got that, they've got South Africa, or let's leave it at that. That's a good enough example. From northern Europe. And they've got to the same place. But now it's time to go back. How do they know?

precisely when to go back. Because as you said, with the calendar birds, some of them are almost as regular as calendar as need. So what's going on? So that clock, basically, we still don't know very much exactly how, where the clock is located in the brain and so on, but we know that it kicks off sort of a cascade of events

that prepare that bird to go. It basically transforms it into sort of an athlete, basically that goes from membranes in the cells to enable fast use of oxygen for example fast passage of fat deposition of fat so that they overeat they become totally like eating machines but that doesn't sound like athletes but it's actually

what they want which is to have a lot of fat so they can go uninterrupted flying. Yes exactly if it's just a few grams I mean it's amazingly efficient as it is to fly across an ocean with a few grams of fat but they have to put that down. And their bodies shrivel away, the liver, the reproductive organs, they shrivel right down. Yes, exactly. Basically, they turn into a different bird in many ways.

And that takes a little bit of preparation. Some birds renew their, they have their flight feathers for migration, that also, like, for a mold. They can also take up to a month. So they have to be like really anticipating the time of breeding very long in advance. But do we actually know yet? I mean, it's fascinating what you know. It's awkwardly fascinating to me what you don't know. Do you really know why they know what they know?

Why? You mean in terms of evolution? In terms of why they're doing it? Can you say they're going now because of X? Or is it because of X, Y, Z and A, B and Cs? Because of a lot of things that aren't quite defined yet. Well, I think Richard's explanation was really well. I mean, there's such, especially like if you're going to extremes,

like high Arctic breeders or so, there's a huge pulse of resources. that would otherwise not be used because nobody really survives the winter there. So that is a strong driver. Let's go up to Richard then. So obviously you'll go along with what Barbara says. There's anything you want to add.

Before I ask you the next big question, is how do they choose their destination? Yeah, well, I think what Barbara was saying was interesting and your point, do we really know what it is that makes a bird take off in any given moment? I think the honest answer to that is actually currently no. and Barbara and I actually did an experiment where we were tracking...

We've got an objection from Tim. I want to challenge Melvin in a way. Don't challenge me, I don't know anything. Challenge the other two. They know what they're talking about. My point is this. I think scientists are often misjudged. Our work is not just about facts.

It's about the unknown. That's where we are busy. We're doing science because of the things we don't know. And that's where we live and that's where we work. And so not understanding things is what we concern ourselves with most of the time. I get that. I understand that. And proceeding by failure and so on, which is a very good method. I agree with all that.

But I'm just trying to define what you do know. So that's my job. And if there's a lot you don't know, that's very interesting too. But I'm going in my little track, and it isn't a challenge. It's going to help. Thank you very much, Tim. Back to Richard. How do you think they choose their destiny?

So you don't know why they do it. You're saying. I think we have some indication of the... Tim didn't challenge that. You're saying you don't know. But the question to you is, how do we know how they choose their destination? Right. So there's two possible ways in which we think birds could do this. One is that a lot of large birds, so we've talked about the storks, we think of swans, geese, a lot of large waterfowl that migrate as groups, sometimes as family groups. They're following experienced birds who've been to their winter destinations before.

But that's not true for a lot of songbirds. Songbirds, adults usually leave the breeding area before their offspring. So what we call the juvenile birds, the birds that were born in the breeding grounds that year, they are having to head to somewhere they've never been to before, and we don't think that they follow other birds. We think that they're heading there of their own volition, and that what they have is an inherited compass direction, and an inherited way of judging the distance that they've flown,

whether that's through a clock and an amount of time that they should fly, or whether some people have even suggested it may be a number of wing flaps that they count or something like that. I'm not sure that's true, but... This is absolutely fascinating. So the adults have left. These are juvenile birds. They've never been to South Africa.

Let's keep going with that. They've never been from Northern Europe to South Africa before. They set off and they get there. Yes. And do they get to? the same place as they're, sorry about the parents, or do they go to, or do they get just somewhere in South Africa, which becomes their place?

I guess what we know is that the ones that survive and make it back get to a place that they can survive the winter, and that usually is the place that the adults that are returning to, yes. So it really must go back to the egg, mustn't it? You tell me, please. But they got to hop out of the nest, they're next to nothing, they weigh less than a bag of crisps or something like that, And they set off.

And flight night, fly the oceans, there's predators, there's all sorts of strong winds knocking them about all over the place. They're flying extra kilometres and so on and so. But they get there. Some of them. I mean, and I think going back to the question... Yeah, but those who get there are those who are interested in. Yeah, the ones that get them.

Are they heading for a destination, a particular destination? We think... Or heading just to get to somewhere. So we think that they just have a program that gets them somewhere, and then there's a certain amount of chance as to whether they get to a place that they can survive. And going back to the discussion of whether migration is costly, there's actually a few studies suggesting that juveniles are much more likely to die on migration.

There's even quotes as much as 80 to 90% of mortality happening to juveniles on migration. That's just one study, and we're not 100% sure if that's always true, but it shows that juveniles are at the greatest risk. But enough of them get somewhere to survive the winter and come back. Now, what's interesting is having made it there, once they then can return to that same location time and time again in subsequent years. So they learn, returning to the same twig even in their breeding areas sometimes. And based on that first journey where they don't know where they're going,

they learn that that's where they return to. There was a classic experiment done by a ditch ornithologist called A.C. Perdek. And he captured 11,000 starlings. He put rings on them, and he took them to Switzerland. He captured them on the coast of the Netherlands. He took them to Switzerland, and he released them. And over the next 10 years, people in various parts of Europe would report recoveries of the rings.

And they found that the juvenile birds, that normally they're migrating to France and England from the Baltic region, these birds that he'd taken to Switzerland ended up in Spain. But they had been then migrating back to their same breeding grounds that they were born in, but then migrating back to Spain again. So they'd established a new area to migrate to based on the displacement. Tim, how do they find their way? How do they navigate? What are the most important tools here?

Well, that's the big question, and it's why many of us are engaged so actively in this area. because we know some things and there are some really interesting things we don't yet know. I think if we were to say what are the major tools, I think we can focus on two things. First of all, a bird that's any animal, actually, that's needing to make a long-distance journey, must need to know approximately where it's going

in relation to where it is now. So it needs some sort of map sense. And secondly, once it knows where it's trying to get to from where it is now, It needs something to guide it through the entirety of that journey. And that's a compass sense. So generally, the problems of navigation are thought to be solved by reference to these two processes, a map and a compass. Now, if we think about, let's take just a compass, for example, based on the sun. This is one of the classic compass systems for diurnal animals, particularly birds.

The sun is a big prominent object in the sky. you can use it to give you direction, except, of course, that the sun moves during the day. So it's useless unless you have a clock. And as Barbara has been telling us, clocks are important not just in determining the timing of migration, but in compensating for the sun's movement across the sky during the day. And it's astonishing just how accurately birds and indeed insects

can compensate for the sun's movement during the day using a clock in order to maintain a compass direction based upon, the Sun's position. I'm still completely bowled over by how much technology and knowledge is going into the tiny places there which are, anyway, let's get on with and it's me stop thinking it's amazing.

Barbara, how much of the navigational toolkit is learned and how much you think is inherited? So yeah, I think we haven't quite emphasized enough there. Also the inherited component. So in some

experiments when birds were crossbred, for example, species that were population members that would go northwest, say, and others would go southwest or so, then the hybrids would actually take an intermediate course of that. And that's been shown even in free-flying birds now, that the hybrids actually inherited direction intermediate to their parents. So, as Tim said earlier, that, you know, you have different ways in which migration is, is passed on. In some species you observe your conspicuers or indeed your family members. So some species

for example geese travel in family groups so they establish their own cultures or traditions but as in these individually migrating birds that are programmed actually that directional sense is part of the inherited program

and so these clocks and so we can say without risability that is in the genes. Yes, as you said, it starts in the egg to some extent. And then I think many species have a really large distribution area, but then, for example, a Siberian population would have to go very different, longer ways to come to a winter quarter than maybe, say, a central European population. And so this program needs adjusting, and the case of the wheat here that we heard about.

Tim's favorite example is really amazing, because that traditional roots of the Alaskan birds to actually cross all of Russia to go down to Africa makes actually no sense in terms of geography. But it is just understandable as an extension of that inherited program of that population. So they colonized further and further away areas, but always kind of retract the inherited root of the population.

Apart from all of that, Richard, and the sense of smell has come into the argument, hasn't it? It has, and it's been a very interesting 40 years or so of discussion on the role of olfactory cues, so the sense of smell in navigation. Now, most of this work has been done on homing pigeons, which of course aren't migratory birds, but they've been viewed as one of the tools that we have. They very reliably, usually, return back to their home loft

when you take them away from the loft and release them. a number of groups have done experiments trying to work out what cues they use. And there's been something of a competition between olfactory cues and magnetic cues as an explanation for the navigational map, this ability to fly these long distances. But a large amount of data is suggested if you remove the sense of smell and they've done this from fairly light-touch ways to cutting the olfactory nerve, that the birds don't return home. Tim

How do they know when they've arrived? Well, that's a good question. I mean, we've already heard from Richard's earlier explanation that for the first-time migrant songbirds, let's say a willow warbler migrating all the way to sub-Saharan Africa, they probably don't have a very clear idea of when they've arrived. They just have to be in roughly the right kind of habitat So for those first-time migrants, it's presumably some rough expectation about what sort of temperature, what sort of light regime or what sort of food availability.

But on subsequent migrations, or indeed on shorter distance journeys, it's clear that the precision with which birds will stop in the same place year after year means they must have learnt something about the particular place that they're going to. And it's that role of memory in structuring these long-distance journeys, whether it's destinations or the stopover sights en route, is that role of memory that I think is becoming one of the sort of hot topics. We haven't brought in magnetic fields. No. So magnetic fields are, there's no doubt that Earth's magnetic field is a ubiquitous and important cue in the long-distance movements of birds in particular,

but also other animals. indeed the magnetic sense was discovered in birds by the behavioural phenomenon of change in orientation with respect to a manipulated change of the magnetic field in the cage in which the bird was orienting. So we didn't know about the fact that animals could sense the magnetic field until orientation was used as a measure if you like. Now birds can use the Earth's magnetic.

field in principle for two things. They can use it as a compass, and we've already referred to this idea with respect to the sun's position, but the Earth's magnetic field provides a compass direction for long distance movement. We think the birds sense the Earth's magnetic field for a compass in a different way to the way that we use a compass. They have what's called an inclination compass rather than a polarity compass. That means that they can tell the difference between going polewards and going equatorwards, but they can tell the difference between going equatorwards, but they can't tell us between north and south. But birds can in principle, anyway, also use the Earth's magnetic field to measure differences

in intensity and other factors in the field to give them position. Now that's a much more controversial area. Barbara, Barbara Helm, has there any clear evidence as to why some migrate in groups and even in V-shaped groups and some assist on going on their own? Yeah, it partly has to do with their style. of flying. We mentioned the albatross and the gliding before. So, for example, in soaring birds, there is a very clear advantage in having this kind of V-shaped transport mechanism when they alternate leadership roles because the one in the front has the sort of the hardest, the aerodynamically

most challenging task, and then generates vortices that actually help the other ones, the following ones, to just fly with less energy requirement. So for a flapping fly, a bird, small bird, that there isn't any equivalent advantage. Richard, Richard Holland, why do some birds go at night? It's a good question. We think that there's two possible explanations. One is that to avoid predators,

that a lot of day migrants are followed by birds of prey that are also migrating and are following these day migrants and picking them off as they go. Some bats even eat migratory birds. But another possibility is that a recent study has shown that there is much less turbulence and environmental conditions are much better at night. And this could also be an explanation for why birds choose small songbirds, particularly flapping songbirds, choose nighttime to migrate.

This is a very naive question. But does it imply that sight isn't as important? as we think it might be? Not necessarily. If you think about it, it's very rarely completely dark at night. If you've ever been up in a...

I've been up in a light aircraft actually flying around New Jersey at night and there's lots of visual cues of the terrain and things. So it's not necessarily the case. Tim, is there any generalisation you could make about bird migration? If we say it is normal for this,

because there are so many different examples, songbirds, arbitroses, Arctic turns, stalks, they're all. And is there any way you could say, well, what they're in common, well, what they're in common, is they migrate? But apart from that. Well, I think we've referred to two common mechanisms, if you like, by which migration is. So I'm not going to give you one, I'm going to give you two,

two common mechanisms by which migration is controlled. I mean the real mystery is how do these journeys work, how do they work? And we've looked at two mechanisms which are a common either to long-lived big birds like geese and stalks and cranes in which individuals learn their migratory roots from others in their population, often their parents. So they migrate and they form these traditional roots. And those memories then persist and their children inherit them, so to speak. On the other hand, for shorter-lived birds, which have to make long journeys for the first time without their parents,

we've been hearing about songbirds like the Willow Warbler, for example, that's not possible. And we know that they have inbuilt genetically programmed directional and time-based, what we call clock and compass algorithms. And these two mechanisms for controlling migration are thought to be common to most, most migration. tree phenomena. Barbara Helm,

excuse me, how flexible are they? Migration birds in dealing with environment. We're told that they can arrive within two or three days. Back to the beginning of the conversation, we said there are calendar birds. People expect them in Borneo. When they turn up, they start to put the seeds in the ground and so far.

But is it a flexibility as well, because of storms, because of changes, and so on. Yeah, that again, depends on this, like whether you're on the side of the calendar bird. spectrum that Tim just explained, like very much programmed or whether you're like on the weatherbird side. If you go very far, there is relatively limited scope to modify because if you need to go

from the southern hemisphere to your Arctic breeding grounds and you should not be late by a few days, you can really not very much sit out bad weather conditions. And there is a problem now with our changes to phenology to seasonal timing. across the world because birds may just get it wrong but if you're more like a weather bird you're by definition responding more directly to the weather around you

when you're sitting in France for example you have a better idea of what's going on in the northern hemisphere for example could we end with an assessment of a thing that's fascinated me most of when I was getting ready for this program is the complexity and the technology and the

and you as you see failing is getting there, but even so what you know is amazing and what you've said that you don't know, it seems to be even more amazing. Does it amaze you? Am I the only one in this quartet amazed? Every few months we read working in this field, you read

an article which has come out in a journal, you think, oh, I'd love to have discovered that. Sorry, I'm asking probably a question that's a bit... Anyway, look, is it... is the complexity of it and the delicacy of the discretion about it, something like

the technological changes that have been made. I'm using Silicon Valley. Is it on a par with that? No, I don't think so. I think that there's no doubt that the migratory phenomenon is various and complex, and the more we dig into it,

the more questions we can ask about these extraordinary journeys which dominate the lives of, well, probably 50% of all bird species and many other taxa as well. But there are simple principles that run through it. First, Richard, then you, Obama. I think essentially the more that technology comes into it, the more we can understand about bird migration that we don't know.

We tend to have thought of bird migration to some extent as getting from A to B, but as we start to track, for example, the BTO's cucko's reveal to us that it's A to B to C to D to E to F to G back to A again, these migration routes are much more complicated than just a straight line between two points but still the orientation cages that Barbara talked about at the start of the programme are vital and actually in understanding the mechanisms that they use and we still use those

and we know more about how birds navigate because we have that tool than some other species that we don't have that tool for. I'm sorry, very briefly Barbara, we're nearly the end. I'm on your side. I think it's dauntingly complex and I think it does measure up to Silicon Valley. Oh, thank goodness for that. So please.

Well, thank you very much, Barbara. Barbara Helm. Thank you, Tim Guilford. Thank you, Richard Holland. And we're... Sorry, yeah, I'm not going to say we're migrating. We're off air until the end of September. Thanks for listening. Bye.

And the In Our Time podcast gets some extra time now with a few minutes of bonus material from Melvin and his guests. What did we miss out? Well, I think we could delve a little more into the cues and mechanisms that are used in navigation itself. We did touch on the issue of olfactory cues used in navigation, and we touched very briefly on the debate about whether magnetic cues are also used in navigation.

But there have been some really interesting experiments, from my perspective anyway, recently, on what for me is the big unknown, and that is oceanic navigation. if I may, we know now that many birds learn a great deal about their underlying visual landscape when they're making long-distance movements all the way up to these return migrations, not the first one, but... And so the complex visual landscape is very important to moving birds, but you don't have that in the ocean. And so these long-distance movements that Albatross make just to go and forage, you know,

2,000, 3,000 kilometres away. We don't know how that's controlled. Maybe the surface of the sea is more complicated than we think. Yes. I think that may well be right. And just because it looks featureless to us, there are actually probably lots of queues in there which provide sufficient information.

It doesn't have to be precise. And I think that's one of the misses in this problem, really. We think that because the bird gets back to the same twig, that the cues that it needs must be that. precise for the whole journey. And of course that isn't the case. You only need, in a sense, to have a funneling process in which you get to approximately the right place and then a different scale of queue comes in. And eventually you can end up coming back to the same tree. And I think that is one of the things that the field is coming round to, that it's not just a polarisation

between one queue or another, that there are different levels and there are different hierarchies of queues depending on where you are and how accurate they might be. Although interestingly, while the debate goes on over whether olfactory cues are really part of the navigational system for homing pigeons, nobody thought even those who worked on homing pigeons really thought that it would be relevant to migratory birds who migrate much vaster distances and over which it's really hard to explain how odours and smells would distribute. And yet recently some experiments that I've been involved in, and actually also that you Tim have been involved in, have suggested that the sense of smell is even important for these birds making these larger distances. I have no good explanation for the mechanism by which that may be working right now.

Yeah, and I just wanted to explain why I'm on your side saying that it's at least the level of Silicon Valley. So I think we've only scratched on the very surface of what these migrations entail. And to give you an example, so if I recover a bird ringed in my hand and it has a ring, for example, from Africa. So look at this tiny bird, and it's done so many things. For example, if we go to a tropical country, we probably just get our vaccination pass out and check for the country, advise, you know, you need chaps for this or that disease. These birds just like casually cross hemispheres,

and they're still coping with all of these environments, with all of the pathogens, all of the channels, maybe different predators, and so on, they still make it back. And actually, talking about the risk of migration, there's a pattern that microbe birds actually are more long-lived once they've survived their first migrations than the resident birds.

They're easily, like a tiny willow warbler travels back and forth over 10 years, 12 years or so for a 10 grand bird. And I think we are far from building microcomputers that can achieve such performances. But you don't agree.

No, no. Barbara's on my side. I like the way Barbara's put it. I mean, I think what the choice challenge for us a scientist really is to understand how you can how you can use relatively simple machinery to solve such a staggeringly, a staggeringly complex task. And I think probably this sort of hierarchical approach to the solution is

part of that resolution. So nature has this way of finding relatively simple solutions to problems which look unbelievable, unbelievably complex until you dig down. into them. And it's, it's, you know, what we talked just now, Richard's just been talking about this. We talk just now about how scale, changing scale, can help you to solve getting back to exactly the same place over a very long distance. If you don't need to know where you are in, with that precision the whole way. So you can have a system, a compass system which will take you hundreds of kilometres without you really knowing where you are and then you focus down

with greater precision to solve that final part of the task. Yeah, I think so. I agree that the bird isn't necessarily actively navigating at every moment and doesn't necessarily need to be able to constantly know where it is. And we do think that when you look at magnetic cues, the Earth's magnetic field gives some information about your position. So it's stronger at the poles and weaker at the equator, which means that it's an approximate cue to your latitude and all position.

But it's very messy. And there are places where it's actually a longitudinal cue rather than the way it kind of moves around, changes the direction it's moving. So it's a very coarse cue. And the earth's magnetic field is very noisy. So a bird cannot look at the earth's magnetic field at the point it flies and look at it 10 kilometres later

and know that it's in a different position. The magnetic field is just bouncing around as what we call DL variation in the strength of the magnetic field. So only over larger distances does it become a relevant cue. Barbara, do you like to say something else? I mean, I want to hear as much as you've got to say

because... She's on your time. Absolutely. Not on my side, no. It's such a relief to think that Silicon Valley is not as smart as a Robin. Yeah, I mean, I'm being a little bit polemic to my dear colleagues here, but that is just an event.

which is awesome in itself. But there's just so much more that's required. I mean, just even the physiology, the energetics, and, as I said, coping with diseases and so on. So I think if you wanted to build a computer that can solve all of these problems simultaneously, you would need a formidable supercomputer still. Let me come round to this view.

Imagine if we could... We're not. I know, you're teaching. I mean, we're speaking to a lot of people, but we were live. I do apologize. I'll write it up, I'll find somewhere. I mean in a church magazine or something. But imagine if we could build an aircraft, a passenger aircraft,

which turned its wheels into something, it turned its wheels into fuel whilst it was in the air and then back into wheels when it needed them to land and did away with different parts of its superstructure in order to save weight to make the journey more efficient as the journey was progressing. Now that's what migratory birds, are capable of doing. They'll digest their internal organs so they don't have to carry as much weight.

It's a staggering business and we are nowhere near understanding how it work or indeed and of course nowhere near being able to exploit that kind of flexibility. And I guess continuing with that theme and maybe also coming around to your way of thinking, as we now starting to really look at the genetic control of migration, what seems remarkable we don't understand much yet, but what we have seen is that there seem to be very few genes involved directly in migration. You would think there'd be this large array of genes

coding for proteins that are translating. All of these, as I say Barbara says, very complex behaviours. And yet, there seems to be relatively few genes actually directly involved in migration. You disagree with me, Barbara. Yeah, I don't think we know that yet. There is an extent to which I agree. But so far, there doesn't seem to be any

obvious huge battery of I think the migrant, this extreme migrant that we all talk about is a complex of lots and lots of different capabilities that are just taken to an extreme but that in some form probably every bird has and so you probably have

variants just on the extreme ends and then maybe the few genes possibly that you're thinking of might just be kind of switches that activate parts of what all these birds carians out themselves. Well, thank you very much.

I think you're going to be refueled if I produce his way. And then he comes. Yeah, and it's like tea or coffee. There are many more science and discussion programs from Radio 4 to download for free. Find these on the website at BBC.co.com. UK slash Radio 4.