【英语时差8,16】鸟类的饮食与羽毛（上）（在线收听）

 Sometimes it’s easy to understand how an evolutionary change took place, but it’s more difficult to understand why. For example, consider the beautiful birds of paradise which live in the forests of New Guinea. The males of many bird-of-paradise species have evolved colorful plumage, used for dramatic courtship displays. How this plumage evolved is no mystery: The healthiest males usually have the most spectacular plumage, so females choose whichever male has the gaudiest feathers. With this criterion, it’s not surprising that male plumage would grow more and more elaborate over the generations. Understanding WHY the plumage evolved is a little trickier. After all, there are some bird-of-paradise species that are really quite drab. These birds devote most of their energy to finding food and raising their young, instead of wasting it on courtship. What makes these near neighbors so different?