TOEFL Listening: ETS-TOEFL听力机经 - NJ3K13S84G1DL4Y4Q$

Lecture: The feeding behavior: Narrator: Listen to part of a lecture in an animal behavior class. Professor: I'd like to talk today specifically about shorebirds, birds that hunt and forage close to the shore in shallow coastal waters, mud flats, and other wetlands. There are over two hundred known species of shorebirds. In North America, for example, typically during the summer months, many shorebird species breed in the Arctic region in the far north and specific nesting areas and during that time they're highly territorial. They defend their areas energetically and generally don't gather in flocks. But during the winter months and during their migration to inform their warm weather habitats, shorebirds do congregate in mixed flocks along shoreline south of their nesting areas. For example, on both coasts of the United States, around seventy percent of all shorebirds feed in water that's four inches deep or less. And what's amazing about these feeding sites is that hundreds, thousands, even millions of birds will gather in a specific areas that offer rich food sources. And I'm talking about a pretty small area, maybe only thirty or so metres of shoreline. Male Student: A million birds? That's a lot of competition for food. Professor: It sure is. And of course, the food sources aren't unlimited, but look at it this way. Lots of people might be in a grocery store at the same time, but some are there buying oranges and others are buying cheese. Male Student: Right. So they're buying different stuff at the store? One bird might eat crabs while another eats insects? Professor: Right! There wouldn't be enough crabs for every bird. Each species feeds on something slightly different from the next. And in addition, each species has its own unique method of finding food. Let's look at a good example of two species that eat in the same area, the dunlin and the dowager. Dunlins and dowitchers are small shorebirds that feed in very shallow water, on basically the same food – small clams and worms. Dunlins have little receptors in their bills that allow them to detect movement. And using these, they are able to find their prey in the mud. Dowitchers forage in slightly deeper water, probing deeper to find food. They're able to stick their bills deeper into the mud, because they have longer bills than Dunlins. So dowitchers are looking for the same kinds of food as Dunlins. But because of their longer bills, dowitchers can eat in other places. And it's also true for other shorebirds. Some might walk along skimming the surface of the water. Others might be further up on the beach and so on. Female Student: So even though the birds are hunting within a few feet of each other, they're able to get the food they're looking for without actually competing? Professor: That's right. And this is happening along the entire shoreline, which brings us to a particularly interesting discovery that was made recently about the western sandpipers. Western sandpipers are another species of small shorebird that lives in great numbers in North America. For many years it was thought that western sandpipers ate mainly small crustaceans, mud snails and clams. But a recent study found that those small invertebrates might only be a supplementary food source for the sandpipers. Using high speed video recordings, and by studying the stomach contents of the sandpipers, researchers found that some sandpipers' main diet, or at least a sizable portion of it, actually consists of biofilm. Biofilm is a thin layer on top of the mud. It consists of micro organisms, including algae and bacteria. Biofilm is everywhere in stagnant ponds, hot springs, glaciers. It's full of carbohydrates and provides a rich energy source for the sandpipers, though previously we thought only snails liked it. Um, and scientists made this discovery by, well, at first as they watched the video of the sandpipers as they said, they were interested by how slowly the sandpipers were moving. The sandpipers weren't hunting as such. They were grazing like cows and sheep. And it's estimated that this grazing of biofilm might account for as much as half the sandpipers dayly nutritional intake. This is an important discovery, because they are not only predators after small prey, as we thought they were also eating food that's even lower on the food chain.