TOEFL Listening: ETS-TOEFL听力机经 - U8R4K733RB1YKLMBQ$

Lecture: Migratory birds: Narrator: Listen to part of a lecture in a biology class. Professor: Ok, so we've spent a lot of time this past week talking about migratory birds and how they survive their long journeys. Well, there's some interesting research that's being done to determine what certain migratory birds eat during their journeys and more importantly what they eat at different times during migration and this research involves using breath collectors; tiny breath collectors. Here's how it works: it consists of a small mask connected to a balloon filled with oxygen. The mask is fitted over the birds beak and the bird inhales the oxygen from the balloon and then when it exhales it replaces the oxygen with carbon dioxide. So then, researchers analyzed the carbon dioxide for the carbon signature of the food the bird ate recently. Now the carbon signature the researchers measured is the ratio of Carbon-12 to Carbon-13. To different forms of carbon in the bird's breath and the ratio will vary depending on what the bird ate. Male Student: You said they could determine what a bird ate at different times during migration. How do they do that? Professor: What a good question! What happens is over a period of time, as the bird digests its food and the body absorbs the nutrients, the carbon signature moves from the birds breath to its blood and then to its feathers. The carbon signature in the breath tells us what the bird ate earlier that day. The signature in the blood plasma tells us what it ate two or three days before and the signature in the red blood cells tells us what it ate two or three weeks before and finally, the carbon signature in the bird's feathers tells us what it ate a month or so ago. So, scientists can basically create a dietary record by analyzing a bird's breath along with the carbon signatures in different tissues from the same bird. Ok, so what is the carbon signature in a bird's feathers is different from the signature in its breath? Male Student: That would tell us that the bird changed its diet over the course of its migrating, wouldn't it? Professor: Yes it would. In fact, that's what tests revealed about white-throated sparrows on Block Island. We learned that during the course of their migration, white-throated sparrows switched their diets from berries to corn when at a stopover site on Block Island. Now this indicates that they're eating out of bird feeders because that's an ingredient in birdfeed and there's no corn grown on this particular island. So, researchers know that birds are using birdfeeders, but they don't know why. Is it because there isn't enough of the food they usually eat available to them or is it because they prefer corn? So, we need to know. Do the sparrows using switch to diet of corn when they migrate? Is this a more nutritious diet for the migration period? Or are they just getting by on corn when what they really need is berries? This raises the question about the importance of feeders to the birds as they migrate. If we can understand the sparrows diet we can provide them with proper nutrition while they're migrating. On top of that, Block Island used to have a lot of farming, but now residents rely on tourism. Tourists often come to see the flocks of migrating birds. Female Student: So when you talk about proper nutrition for the sparrows, it could be corn. It could be berries. Professor: Right. That's what needs further research. Male Student: So, what about other animals? Could breath collectors be used to find out about their diet? Professor: Yes. Of course, you need a different size mask. This research is not only significant to biologists studying birds. For instance, biologists are studying the eating habits of bears to find out if they eat different foods when they're nursing than when they're not nursing because they want to know whether nursing bears are more carnivorous than non-nursing bears. How would you like to be the biologist who puts the mask on the bear? Of course, the bears are sedated before that's done, so it's not as risky as it might seem. Anyway, similar research is being conducted all over the world since the information it provides could be very useful to conservation efforts.