In Our Time Page 11
First, Melvyn turned to Tim Guilford for an overview of migration. While it means different things to different people, there is a classical definition in which something such as a bird or a turtle makes a very long journey, repeatedly, from one season to the next, between a breeding site in one location to an overwintering site somewhere else on earth.
TIM GUILFORD: That is 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.
There is always this cyclical movement, even within these different meanings.
Long before modern tracking techniques, naturalists have shown a lot of interest in migration across the centuries. One of those mentioned was Gilbert White (1720–93), vicar of Selborne in Hampshire, who made repeated observations, but the first methodical approach to working out where birds went when they disappeared was in Denmark in the early twentieth century. There, Hans Christian Cornelius Mortensen (1856–1921) started to place little metal bands around the legs of birds before they disappeared for the winter. When birds appeared in the spring, he could tell if his birds were among them and had returned, as they carried the same unique identity number on the metal ring.
MELVYN BRAGG: Did he know where they’d been?
TlM GUILFORD: 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.
While many birds were ringed, Mortensen only needed one documented example of the same individual, ringed in a breeding site in northern Europe and later spotted alive or dead in Spain, to know that migration must be something to do with long-distance movement. Besides, as Melvyn said, the Europeans exploring Africa in the nineteenth century were finding birds that looked very much like ones they had seen in Europe, and that piqued their curiosity.
The puzzlement about where birds went was one part of the debate, Barbara Helm said, but there were quite a few people who had never believed stories about birds turning into fish and who had learnt from what they had seen of wild birds and caged birds. From the early 1700s onwards, just when wild birds would migrate, bird fanciers reported that there was an odd change in the behaviour of caged birds if the same migrating species.
BARBARA HELM: These birds become really activated and restless, they also become plump, they put on a lot of extra weight for that time, and then they change their behaviour from active in the daytime to being really, really active in the night-time, to the extent that the birds would hit their heads, because they would fly against the cages at night.
The owners talked about things like ‘instinct’ as they realised that the birds were showing this behaviour when inside a house not just when they were in an aviary outside and could see birds of the same species flying away.
This prompted Melvyn to recall ‘a defining and magnificent, extraordinary illustration’ he had seen, a photograph of a stork that had been captured in Germany in the 1820s with a long Bantu arrow or spear piercing its neck. That, we heard, helped with ideas a lot.
BARBARA HELM: One example already proves the point in some ways. That Bantu spear in 1822 was establishing that. And then there were actually, in Germany alone, twenty-five occurrences of storks coming back with spears. It’s amazing that they could actually carry out that long migration carrying a spear …
MELVYN BRAGG: Yes, it’s a big spear, isn’t it?
BARBARA HELM: Through the neck. And that one famous, in some ways game-changing, stork then, of course, met its sad fate when it returned to the breeding grounds. It was shot because it was such an oddity and it is now visible for inspection as a preparation in a museum.
Why, though, do some birds migrate, when others do not? The classical explanation, Richard Holland said, was that there was competition for food in their home areas, and some birds would try to escape this competition and discover new sources.
RICHARD HOLLAND: 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. There 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 back down to the tropics as the winters came in.
In biology, though, he cautioned, nothing is ever quite as simple as that classical explanation. In Britain, the robin is an iconic winter bird but, in Europe, many robins migrate. Even within some species, some individuals from the same population migrate and others do not. It could be a matter of body size, where larger animals can fast through winter so don’t have to find new food sources, or where older and more dominant ones stay and the younger migrate, as with lesser black-backed gulls. Another argument is called the arrival-time hypothesis. In that category, examples can be found in small songbirds that require a breeding territory, and the males defend their territory in the summer to attract females and defend resources.
RICHARD HOLLAND: There’s an argument that birds would arrive earlier and earlier to get the best territories, to the point where some would never leave. If you think about it, that’s quite similar to the phenomenon we see at Spanish holiday resorts around the pool, battling for sunbeds, getting there earlier and earlier to defend that.
Sometimes there is potentially a choice about whether to stay or go, for some birds in a species, depending on age, size or dominance. Sometimes, though, the environment, so full of insects in summer, becomes too harsh to stay over winter and, while it is impossible to stay, it is also risky to leave. The northern wheatear, Tim Guilford said, was one of his favourites, a small songbird that weighs about the same as a bag of crisps. These breed all across the northern hemisphere but, in North America, there are populations breeding in Alaska and populations breeding in eastern Canada – for example, Baffin Island – but then both populations migrate to sub-Saharan Africa, and they do that by following opposite routes around the globe.
TIM GUILFORD: An eastern Canadian breeding northern wheatear will head on a 7,500km journey across Greenland, 2,500km across the open ocean (this thing weighs 20g), all the way down to sub-Saharan Africa and back to breed again.
An Alaskan bird will go the other way, which is twice as far but less risky and requires less fuel stored up as it feeds on the way. A wheatear migrating from Canada will need to double its mass before it heads out on that migration in order to fuel that flapping flight. In the case of the northern wheatear, nearly half of the year is spent on migration and, given the time spent and the risk of flight, that is probably where the greatest number of mortalities occur. For a shearwater, migrating between the Welsh coast and South America, it is a much less costly journey. For some of the pelagic seabirds, such as shearwaters and albatrosses, these long-distance movements can be almost effortless.
MELVYN BRAGG: Yes, there’s a wonderful albatross – scarcely flapped its wings in his whole existence, isn’t it amazing? Just soaring around, it’s wonderful, isn’t it? No wonder Coleridge got intoxicated by it.
TIM GUILFORD: Well, it’s not clear that it was an albatross actually, it might have been a giant petrel.
MELVYN BRAGG: We’re not here to discuss that.
One of the great wonders of migration, said Barbara Helm, is one that people have long observed. This is that some birds return to their breeding grounds so punctually and regularly that they could be used as an agricultural calendar, as in Borneo, when seeds would be sown as the brown shrike arrived. While that was a reliable event, other birds go more by the weather, which led to a distinction between calendar birds and weather birds.
BARBARA HELM: The calendar birds carr
y some sort of an endogenous clock, an internal clock that tells them when it’s time to come back. Imagine, a bird crosses the equator and winters in sub-Saharan or southern Africa, for example, [where] days are actually increasing, birds down there are starting to breed, and yet, instead of just staying and breeding along with them, they just take off.
The longer the distance they have to fly to their breeding grounds, the more these birds have to rely on their internal clocks. An Arctic breeder has just a few weeks, and then summer is gone and, with it, the chance to breed. We still do not know how this clock works or where it is located in the brain, but it kicks off a cascade of events that prepare that bird to go, transforming it into an athlete, facilitating the fast use of oxygen and conversion of fat, and the accumulation of the right levels of fat.
MELVYN BRAGG: And parts of their bodies shrivel away – the liver, the reproductive organs, they shrivel right down.
BARBARA HELM: Yes, exactly. They turn into a different bird in many ways, and that takes preparation. Some birds renew their flight feathers for migration, that can take up to a month, so they have to anticipate the time of breeding a long time in advance.
Preparation is essential, but there is then the question of how the birds choose their destinations.
Richard Holland suggested there are two ways in which birds could do this, as far as we think. One is that a lot of large birds, the storks, swans, geese and other large waterfowl, often migrate in groups, sometimes as family groups, where the young birds who haven’t been to their winter destinations before follow the experienced ones. That, though, is not the case for a lot of songbirds.
Kittiwakes fly over the frozen Chukchi Sea in Alaska.
RICHARD HOLLAND: Songbird adults usually leave the breeding area before their offspring, so the birds that were born in the breeding grounds that year 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.
The ones that survive these journeys and make it back have made it to a place where they can survive the winter, which is usually the place that the adults have returned to. It appears that these juveniles have an inherited compass direction and a way of judging the distance they have flown, perhaps through a clock or even, some suggest, the number of wing flaps. Barbara Helm mentioned experiments where birds were cross-bred, with some that usually went north-west mating with some that went south-west, and the resulting hybrid birds would take an intermediate course between north-west and south-west. Some experiments with homing pigeons have suggested that sense of smell may have a role. There is, though, a certain amount of chance.
RICHARD HOLLAND: There’re actually a few studies suggesting that juveniles are much more likely to die on migration. There’s even [one suggesting] as much as 80–90 per cent of mortality happening to juveniles on migration. We’re not 100 per cent sure if that’s always true, but it shows that juveniles are at the greatest risk.
Enough of them get somewhere to survive the winter and come back and, having made it there once, they can then return to that same location time and time again in subsequent years. Tim Guilford said that these birds would need some sort of map sense and another compass sense. The compass might be based on the sun, a visual prompt for direction, except that, from the perspective of the earth, the sun moves during the day.
TIM GUILFORD: It’s useless unless you have a clock, and 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 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.
First time out, whether a bird knows when to land may be a matter of having an expectation of the right temperature or how much daylight there is or what kind of food is available. On subsequent flights, though, the birds will stop in the same place, so they must have learnt something about their destination, and the role of memory is another thing that is being studied. The earth’s magnetic field is an important cue in the long-distance movements of birds in particular, but the birds appear to have an inclination compass rather than a polarity compass, which means that they can tell the difference between going polewards and going equatorwards, but not the difference between north and south.
Birds can do some things to mitigate the risks of migration, such as flying at night to avoid predators or to avoid the turbulence of daytime. The decision to fly may not coincide with the best weather conditions, though, and Barbara Helm recalled being on an island in the Baltic watching calendar birds take off into storms.
BARBARA HELM: 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, you should not be late by a few days. You cannot really sit out bad weather conditions. There is a problem now with changes to seasonal timing across the world, because birds may just get it wrong.
Melvyn asked his guests to end with an assessment of something that fascinated him when he was preparing for the programme, which was the complexity of the technology available to birds.
MELVYN BRAGG: Does it amaze you or am I the only one in this quartet amazed?
TIM GUILFORD: No, every few months you read an article that has come out in a journal and you think, ‘Oh, I’d loved to have discovered that.’
As for whether the technology really was complex, Tim Guilford thought that the migratory phenomenon was varied and complex but that there were simple principles running through it. Barbara Helm, answering in the short time left, said she was on Melvyn’s side and that migration was dauntingly complex and measured up to Silicon Valley, which pleased Melvyn very much.
In the studio afterwards, Melvyn’s guests talked about the visual cues that the birds may use, particularly the long-distance birds, such as the albatross, that navigate across the ocean. The surface of the water may offer more clues than we appreciate. It may also be that the cues need only be approximate when the bird is far from the destination, but that a different set of cues comes in as the bird senses that it is getting closer until eventually the bird can come back to the same tree where it landed the year before. It may be that a sense of smell comes into play, although on that, as with so much besides, more research is needed.
TIM GUILFORD: The challenge for us as scientists really is to understand how you can use relatively simple machinery to solve such a staggeringly complex task. And I think that this sort of hierarchical approach to the solution is part of that resolution. Nature has this way of finding relatively simple solutions to problems that look unbelievably complex until you dig down into them.
Returning to what she had said at the close of the programme, Barbara Helm contended that it would take a formidable super computer to solve all the problems that migrating birds have to solve simultaneously. Picking up on that, Tim Guilford tried to imagine a passenger aircraft that turned its wheels into fuel while 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 to make the journey more efficient while in the air. That is what migratory birds are capable of doing. They digest their internal organs so they do not have to carry as much weight, he said. ‘It’s a staggering business.’
THE PALAEOCENE–EOCENE THERMAL MAXIMUM
About 50 million years ago, the earth’s climate changed faster than at any time in our geological record, reaching temperatures much higher than they are today. That event is known as the Palaeocene–Eocene Thermal Maximum, the result, it’s thought, of vast volumes of carbon dioxide being released into the atmosphere over a few hundred years, along with methane, another greenhouse gas. The Arctic and Antarctic became subtropical, with crocodiles where there’s now ice. Some life forms went extinct, others adjusted in the warmer acidic oceans before the earth cooled 100,000 years later.
With Melvyn to discuss the Palaeocene–Eocene Thermal Maximum were: Dame Jane Francis, director of the British Antarctic Survey and professor of palaeoclimatology; Mark Maslin, professor of palaeoclimatology at University College London; and Tracy Aze, lecturer in marine micropalaeontology at the University of Leeds.
In geological terms, Jane Francis told us, where ages are measured in millions of years, this episode was a spike of warmer temperatures in a period when the normal temperatures were already warmer than today. This spike can be narrowed down to about 20,000 years, when there was more carbon in the atmosphere and, overall, the world’s temperature increased by about 5°C, although the significance of that, as we were to hear, was greater than that number might suggest. Most of the dinosaurs had died out on the land surface around 10 million years before and, after that, life on earth was starting to recover. There is a lot of information available in the rock records about this period, as the relevant rocks are relatively young, while older rocks from this period are more compressed and it becomes harder in those to date a short period.
JANE FRANCIS: We have a whole list of causes of the warming. Where did all the carbon come from? Did it come from volcanoes? Did it come from burning peat that was around, melting permafrost or methane that was trapped on the sea floor that was released? There’s been a lot of work trying to understand why it happened and then what happened afterwards.
The interest in this period started in the late 1980s with drilling near Antarctica, where scientists were looking at the sediments and the rock record. They noticed something very exciting, which was a sudden change in the sediment cores they extracted.
JANE FRANCIS: They saw that some of these small shells, marine organisms that lived in the oceans, particularly on the bottom of the sea floor, suddenly disappeared from their cores, as they were working through. These things called forams, foraminifera, went extinct. Geologists are really excited when things go extinct because they want to know why.