A、Mankind’s economic development. B、Hunters and other animals. C、Climate change and air pollution. D、The length of migration pat

Scientists say a huge percentage of bird species are in danger because their habitats, or homelands, are disappearing. Traditional migration paths take birds through countries that are not protecting locations to stop, rest and feed. The scientists studied the migration, or flight paths, of almost 1,500 species. They decided that 91 percent of them passed through dangerous areas. The major danger for migratory birds is development. Building and paving has covered over nature where birds stop and feed as they move from one part of the world to another. For example, a bird migrates from its breeding grounds in the Arctic. It flies all the way to the southern hemisphere in Australia and New Zealand. Along the way, the small birds stop at Yellow Sea coast in China, North Korea and South Korea. One of the scientists who worked on the study says "many of these critical sites have been lost owing to urban, industrial and agricultural expansion."
    The problem, according to investigators, is that many of these small birds die along their migration because they don’t have a safe place to eat and rest. There is no place to restore their energy for the next leg of their journey. "They simply perish along the way," says one of the scientists. Countries in North Africa, Central Asia and those along the coasts of East Asia are having the most difficult time conserving land. The scientists say these countries do not have enough areas that are safe for birds. In addition, many of the reserves do not overlap enough with the paths of migrating birds. One species that is now extinct is the Eskimo bird. "Our world gets poorer every time we lose a species," one of the scientists says. The researchers say countries need to work together and come up with safe stopover areas for birds that pass through their boundaries. For example, one country might have preserved safe zones for migrating birds. But a neighbor country might not. A bird might perish. One scientist who is not involved in the report tells the Los Angeles Times that while some habitats are changing, more work can be done to make urban areas safe for birds. He says small changes—like planting more native plants or keeping cats out of areas birds would be likely to use—could make a big difference.
19. What threatens migratory birds most?
20. What does the scientist say about the critical sites?
21. What do countries have to do to protect birds?
22. What can be done to make urban areas safe for birds?