Part of a lecture about bird migration — the seasonal journeys that

Section 4. You will hear part of a lecture about bird migration — the seasonal journeys that birds make each year. First, you have some time to look at questions 31 to 35. Now listen carefully and answer questions 31 to 35. MAN: Good morning. Today's lecture is about one of the true wonders of the animal kingdom, bird migration, the immense seasonal journeys that millions of birds undertake every single year. Let's begin with the obvious question: why do they bother, given how dangerous these journeys are? The principal driver is food. As winter closes in and the insects, seeds or fish a bird depends on become scarce in one region, the bird moves to a warmer area where food is still plentiful. A second, closely related reason is reproduction: many birds travel to reach places safe and rich enough for breeding, then return in spring to raise their young. So it is not warmth in itself they chase, but food and a safe nursery. MAN: Migration is exhausting, so birds prepare with great care in the weeks beforehand. Above all, they eat, far more than usual, laying down a thick layer of fat beneath the skin. This fat is not spare weight; it is carefully stored fuel, and it will be burned steadily over thousands of miles. Many larger birds also save energy in the air by flying in a distinctive V formation, in which each bird rides in the wake of the one ahead, so that the whole flock can travel much further than any single bird could alone. Before you hear the rest of the lecture, you have some time to look at questions 36 to 40. MAN: But how on earth do they find their way, often to the very same field they left the previous year? Remarkably, birds seem to use several methods at once, cross-checking one against another. By day, many take their bearings from the position of the sun in the sky. By night, they switch to the pattern of the stars. And underlying both, birds can actually sense the Earth's magnetic field, using it like a built-in compass, while older, experienced birds also memorise landmarks such as coastlines and great rivers. MAN: The journey itself is perilous, and huge numbers of birds die on the way. Some are picked off by predators; falcons in particular gather along the well-worn routes, knowing a meal will pass. Others are swept far out to sea and exhausted by sudden storms. And there is a newer, man-made threat: the loss of the wetlands and marshes that migrating birds rely on as resting sites, the service stations of their journey. As these sites are drained and built over, birds arrive at each stage weaker, with fewer places to feed and recover. MAN: Those that survive finally reach their destination, warm feeding grounds and a safe place for breeding, before the whole cycle turns and they begin the return trip. But the pattern itself is now shifting. As the climate warms, careful records show that many species are setting out earlier in the year than their parents and grandparents did. That might sound harmless, but it can be dangerous: a bird that arrives too early may find that the insects or plants it depends on have not yet appeared. We'll look at those consequences next week. That is the end of Section 4. You now have some time to check your answers.