新托福听写训练 第3期（在线收听）

You know, it's kind of fashionable among students of birds to study well and exhausted species, especially in danger ones like golden eagles or spotted owls. But I often think that everyday-birds, birds that really are part of our lives are simply overlooked, so I'd like to spend some time talking about a very common bird “black crows.” It might surprise you to know that crows are among the most challenging birds to observe and study. First of all, they look alike. Picking out one or several individual crows in a flock in finding them again later is almost impossible, people study in larger animals can put some kind of mark on them, so they can tell them apart, well, you can trap large animal like a bear in a mobilized or a tranquilized gun, then it is easy to put a tag on it. But try doing that to a crow you probably kill it. Secondly, crows are highly intelligent survivors, they adapt easily to wildly varying situations. This adds to the difficulty of studying in them, because they pick up so many individual allies habits, so you can never be sure about any conclusion you reach about crows from observing them applies to the whole species or just those particular crows you being watching. One general observation about crows that can't be made the reasonable degree of certainty is that in the last forty years, more and more crows have been found living in large cities. They are attracted by people who produce a normal surmount of garbage and leave them places that crows can easily get to, it make for distances they must travel to hunt a lot shorter.