【答案解析】The lecture talks about some in-born abilities of migratory birds to achieve long-distance migration. One important factor is that they can eat sufficient food to store enough energy for a long-distance travel. Some birds can store fuels by doubling their weights in case of a 24-hour flight. Another proficient method is thermal soaring, which means that birds can fly upward more easily with the help of thermals. For example, the bird Argentavis can depend on the energy acquired from the pampas to glide even though it has a large body. Also, the birds will reduce their flapping frequency to save more energy. [听力原文] W: Migration is the regular seasonal journey undertaken by many species of birds. Long-distance migration depends on a multitude of factors: physical, geographical, historical and ecological factors. We will discuss how they are likely to affect migration. One of the main reasons why birds can fly so long a distance is that they have reserved sufficient energy for a long-distance flight. Any physical movement would involve an energy cost, which is typically taken from stored fuel during long-distance migration. It has been suggested that, in order to prepare for their long journey, most birds would deposit fuel stores before their flights. Some migrants would almost double their body weights by storing fat before migration. The hummingbird weighs only 4.8 grams and can use stored fat to fuel a non-stop, 24-hour flight. Another effective means of migration is thermal soaring, which means that birds can gain altitude by riding up thermals and convert height advantages into distance by gliding. Argentavis is probably too large a bird to be capable of continuous flapping flight, so they would have extracted energy from the atmosphere for flight, relying on the thermals present on the Argentinean pampas to provide power for soaring. Argentavis is an excellent glider, which could take off by running downhill. What's more, when birds are on their migration journey, they will reduce the flapping frequency to save their energy.