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第一段
1 .Listen to part of a lecture in a biology class.
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第二段
1 .Now usually when we talk about birds flying long distances, we're discussing seasonal migration.
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2 .But there's some species that fly long distances not as part of a migration but as part of their regular foraging for food.
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3 .A great example is the albatross.
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第三段
1 .Albatross are seabirds that nest on islands and forage for food out in the open sea.
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2 .And you have one species that forages an average of a thousand miles from its nest.
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3 .And I read in another study where one albatross left a chick in its nest and went out in search of food.
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4 .And by the time it got back to the nest, it has flown nine thousand miles. Yes, Bob?
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第四段
1 .But why don't they just build their nest closer to their food supply?
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2 .I mean, for one thing, they must burn up a lot of energy flying back and forth and also the parents are gonna have to be away from the nest that much.
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3 .Aren't the chicks gonna be pretty hungry most of the time?
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第五段
1 .Ok, good question.
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2 .The chicks are capable of going for long periods of time without food,
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3 .which works out nicely since as you point out, they may not get to eat that often.
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第六段
1 .As far as the parents go, well, first, they typically can't get enough food in a single location.
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2 .So they have to visit several places on the same foraging trip.
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3 .And the locations of good foraging grounds tend to be very far apart.
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第七段
1 .And second, they can't always nest on an island that's closest to the best feeding ground because some of those islands have too many predators on them.
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2 .Predators that would just love some little chicks to snack on.
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3 .So I don't think they have much choice.
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第八段
1 .But it still works out because albatross fly using a technique called dynamic soaring,
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2 .which enables them to cover very long distances while expending very little energy.
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3 .If it weren't for that, you'd be right. They would probably burn up all their energy just flying back and forth.
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第九段
1 .Another factor is albatross lay only one egg at a time.
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2 .So when the parent returns with the food that one chick doesn't have to share it with a lot of other chicks.
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3 .Yes, Nancy?
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第十段
1 .So you're saying that they might easily fly a thousand miles over the open ocean when they're looking for food?
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第十一段
1 .That's right.
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第十二段
1 .Then how do they know how to get to the food?
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2 .I mean, which direction to take to get to the food and how do they find their way back home?
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第十三段
1 .Good point.
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2 .And the truth is we are not sure.
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3 .It's very difficult to keep seabirds in captivity where you can study them.
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4 .And it's very difficult to study them in the wild, you know.
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第十四段
1 .But we think that a lot of what we've learnt about songbirds probably applies to seabirds as well.
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2 .So we're thinking that albatross could make use of two different kinds of compasses if you will, a magnetic compass and a celestial compass.
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第十五段
1 .The magnetic compass somehow makes use of Earth's magnetic field,
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2 .much the way a standard compass does.
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3 .But to prove this, we would have to find some kind of magnetic sensory organ in birds.
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4 .And we are not sure that we have.
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5 .We have found in birds a mineral called magnetite,
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6 .which we think might be somehow related to this, because magnetite is a natural magnet.
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7 .But the problem is that we've also found magnetite in non-migratory birds,
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8 .which suggests that it may in fact serve a completely different function, not related to navigation at all.
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第十六段
1 .Um, and the other compass, the celestial compass makes use of the stars, more or less the same way humans have historically used the stars to navigate in the open sea.
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2 .So that's the way we think albatross navigate.
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第十七段
1 .So anyway, you know, think about it,
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2 .how about if you had to go a thousand miles every time you wanted to get a bite to eat?
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第十八段
1 .Yeah, and we complain about having to walk all the way across campus to get to the cafeteria.
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第一十九段
1 .Yeah.
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