原文已被隐藏，你可用 快捷键 - 或点击 显示原文 按钮来查看原文
1 .<-NARRATOR:->Listen to part of a lecture in a biology class.
1 .<-FEMALE PROFESSOR:->Now usually when we talk about birds flying long distances, we're discussing seasonal migration.
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.
3 .A great example is the albatross.
1 .Albatross are seabirds that nest on islands, and forage for food out in the open sea.
2 .And you have one species that forages an average of a thousand miles from its nest.
3 .And I read in another study where one albatross left a chick in its nest and went out in search of food, and by the time it got back to the nest, it had flown nine thousand miles. [sees hand raised] Yes, Bob?
1 .<-MALE STUDENT:->But why don't they just build their nest closer to their food supply?
2 .I mean, for one thing, they must burn up a lot of energy flying back and forth, and also, if the parents’re gonna have to be away from the nest that much, aren’t the chicks going to be pretty hungry most of the time?
1 .<-FEMALE PROFESSOR:->Ok, good question.
2 .The chicks are capable of going for long periods of time without food, which works out nicely since, as you point out, they may not get to eat that often.
1 .As far as the parents go, well, first, they typically can't get enough food in a single location.
2 .So they have to visit several places on the same foraging trip, and the locations of good foraging grounds tend to be very far apart.
1 .Uh 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—predators that would just love some little chicks to snack on.
2 .So I don't think they have much choice.
1 .But it still works out, because albatross fly using a technique called dynamic soaring, which enables them to cover very long distances while expending very little energy.
2 .If it weren't for that, you'd be right—they would probably burn up all their energy just flying back and forth.
1 .Another factor is, albatross lay only one egg at a time, so when the parent returns with the food, that one chick doesn’t have to share it with a lot of other chicks.[seeing hand raised]Yes, Nancy.
1 .<-FEMALE STUDENT:->So you're saying that they might easily fly a thousand miles over the open ocean when they're looking for food?
1 .<-FEMALE PROFESSOR:->That's right.
1 .<-FEMALE STUDENT:->Then how do they know how to get to the food—I mean, which direction to take to get to the food—and how do they find their way back home?
1 .<-FEMALE PROFESSOR:->Good point. And the truth is, we are not sure.
2 .It's very difficult to keep seabirds in captivity, where you can study them, and it’s very difficult to study them in the wild, you know.
1 .But we think that a lot of what we've learnt about songbirds probably applies to seabirds as well.
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.
1 .The magnetic compass somehow makes use of Earth's magnetic field, much the way a standard compass does.
2 .But to prove this, we would have to find some kind of magnetic sensory organ in birds. And we are not sure that we have.
3 .We have found in birds a mineral called magnetite, which we think might be somehow related to this, because magnetite is a natural magnet.
4 .But the problem is that we've also found magnetite in non-migratory birds, which suggests that it may in fact serve a completely different function, not related to navigation at all.
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.
2 .So that's the way we think albatross navigate.
1 .So anyway, you know, think about it, how about if you had to go a thousand miles every time you wanted to get a bite to eat?
1 .<-FEMALE STUDENT:->Yeah, and we complain about having to walk all the way across campus to get to the cafeteria.
1 .<-FEMALE PROFESSOR:->Yeah.