Name: How birds travel thousands of miles every year - CrowdScience, BBC World Service Podcast
Uploaded: 2022-05-22T14:12:12.000Z
Duration: 36 min 9 s
Description: VoiceTubeの動画で発音を聞きながら英語表現を覚えよう！学べる英語：

What's the longest journey you've ever  been on and how did you manage to reach  

your destination without getting lost? - I'm  going this way! - Oh, we're going this way!  

My recent trip to Belize took me 5,000 miles,  11 hours on a plane, six hours on a bus and  

a lot of time zoning out while watching pretty  terrible films. But long before we humans devised  

our ingenious technology and transportation for  getting around, animals of all kinds have been  

making their own spectacular journeys across  the globe. And that's what we're exploring on  

CrowdScience from the BBC World Service. I'm Anand  Jagatia, and I'm taking you on a two-part journey  

to the Americas to try and understand some of the  most impressive feats of animal migration on Earth.  

- This is a greycap bird. This individual probably  made about three or four trips already to Belize.  

Birds like this greycap arrive in Belize  from North America for the winter, year in,  

year out, but how do they, or in fact any migrating  animal, know where they're heading? It's something  

that's been bothering one of our listeners  in Kenya, called Moses. Here's his question:  

Moses: Hi CrowdScience. My question is, how are  long-distance migrating birds like the Arctic tern  

able to navigate vast oceans and distances in  flight? How come they never get lost? Similarly,  

how do turtles not get lost traversing  thousands of miles under ocean water?  

Anand: Thanks Moses. It's such a great question and it's  also a huge one. So much so, that we've decided to  

make it into a two-part CrowdScience special:  the science of how animals actually find their  

way around is a fast-moving and really exciting  area of research, and we're dedicating the next  

episode to that. But before we can answer Moses's  question about how animals navigate over such huge  

distances, you need to understand where they  go and why they go there in the first place,  

and it's these amazing voyages that we're covering  in this episode. So let's get to it.   

that animals are migrating all over  the planet, large animals, small animals, birds,  

whales, insects, you name it, fish...  Anand: This is David Barrie. He's a sailor with plenty of  

his own experience navigating out at sea, but he's  also written a book on animal migration called  

DB: At one extreme you've got a  bird like the Arctic tern which, with the help of 

tracking devices, we now know may travel a hundred  thousand kilometres in the space of a year between,  

as it were, the Arctic and the Antarctic and back  again, and then you've got a huge animal like the  

humpback whale, which will spend a long period of  time feeding down in the southern ocean around  

Antarctica, building up its reserves of fat before  cruising steadily north into warm waters around  

the equator where it will breed and have its young.  Then you've got the monarch butterfly, which is a  

famous example of a migrant, and less famous, but I  think almost as fascinating, is the bogong moth of  

Australia, which starts its life in Queensland and  then it flies about a thousand kilometres south,  

travelling by night, and spends the summer months  hidden away in little caves, little cracks in the  

rocks on the top of the snowy mountains in new  South Wales, and then amazingly at the end of  

the summer months it emerges and then flies  all the way back to Queensland, and we're now  

beginning to get to grips with how these  navigational feats are performed.  

Anand: You might have seen some of the astonishingly long journeys  that animals can make on television in wildlife  

documentaries, and the fact that many animals,  from whales to wildebeest, migrate over their lives  

probably isn't news to you. But that wasn't always  the case. People had some very different ideas for  

what happened to the animals around them when they  vanished for large parts of the year.   

DB: Migration was kind of a controversial idea until the 19th  Century. I mean, nobody was really quite sure where  

birds went, for example, during the winter. And  there were these bizarre theories that people  

cooked up like, you know, swallows maybe went to  sleep at the bottom of ponds during the winter  

months when they disappeared. But it was really  in the 19th Century that people finally confirmed  

that migration was a real thing and it was first  really established in relation to birds and  

one of the more bizarre episodes was the discovery  in north Germany of a stork which had been  

shot with an arrow that was plainly of African  origin. This poor wretched beast arrived in  

northern Germany with this arrow stuck in it and  and it was obvious that it must have been to  

to Africa because of this arrow. So that  was the first sort of hard evidence, if you like...  

Anand: The poor bird was still alive, sort of  struggling? 

it got shot because it ended up in a museum,  poor thing. But after that, people of course  

gathered more and more evidence of of migration  in a whole host of different animal species.  

Anand: It might sound slightly ridiculous that people  back then seriously thought that swallows went to  

sleep in the bottom of ponds, but when you think  of the alternative: that swallows fly thousands  

of miles from the uk all the way to South Africa,  you can see why people were a bit sceptical that  

migration was a real thing. Take the case of birds  like the Arctic tern, which listener Moses asked  

about, and David mentioned might travel a hundred  thousand kilometres in a year. What could possibly  

be worth the immense effort that a trip like that  would involve? Why don't birds just stick to the  

Tim Guilford: Not all birds migrate by  any means. In the tropics, long-distance migration  

is relatively rare; in the seasonally constant rich  tropical regions of Earth then it makes sense to  

to breed and live all year round in approximately  the same place.  

Anand: This is Tim Guilford, professor of 

animal behaviour at the University of Oxford, where  he studies bird migration. 

feature of our planet is that it is seasonal, so  as soon as you start getting into higher latitudes  

then you have lots of sunlight in summer and  not very much in winter, and this drives massive  

seasonal differences in resource availability.  In the Arctic, for example, is very unproductive  

on land in the winter, a very harsh place to live, but  in the short summer season it's almost constant  

energy, and that feeds through into resources,  plant growth, insects, and therefore food.  

And so in order to exploit them, you've got to  make these very long journeys. It's got to be a  

trade-off between the risks and energetic costs  of these long journeys, and the rewards of  

essentially being able to exploit an endless  summer. There are many different kinds of migration,  

even long-distance migration, and different  species and different groups of animals and  

different groups of birds handle this problem in  in very different ways. For example, some species  

are only partial migrants, which means that only  a proportion of the individuals will migrate,  

and presumably there are differences between  these individuals in their energetic balances or  

whatever which makes the trade-off change. But in  terms of how migration is sort of executed, if we  

can use that word, this also varies between small  songbirds, which have to make very long journeys  

over harsh terrain, for them, like crossing the  Mediterranean or the Sahara, places they can't stop,  

and seabirds which can effectively stop  wherever they want on their nonetheless long  

journeys. So sometimes the dangers and the trials  that have to be overcome lead to very different  

Anand: Different species  of birds will face different balances between 

these trade-offs, and some species take their  dedication to migration to the next level. 

TG: Some really sustained migratory birds, like the bar-tailed  godwit, they will start to digest, internally, the  

organs that they don't need whilst they're on  passage, whilst they're making these journeys,  

they actually eat their  own gizzards and stomachs, in large measure,  

in order to make themselves lighter so they can carry  more fuel, and then when they reach a stopover site,  

or reach their destination, the first thing they  do is to regrow those internal organs. I mean it's  

an extraordinary level of preparation. You're only  allowed to carry a small hand baggage, essentially,  

on these long flights, and so you've got to  adapt to that.   

must be unbelievably good for a bar-tailed  godwit to eat part of itself just to get there.  

Our understanding of bird migration has come a  long way from thinking that birds hibernated at  

the bottom of ponds. But how exactly have  scientists mapped out avian flight paths  

across the globe? Sounds like an obvious question,  but birds have wings and we don't, so how have they followed them?  