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  3. To the people of Europe, cuckoos are harbingers of spring. To European birds, though, the cuckoo’s call is less welcome—for cuck

To the people of Europe, cuckoos are harbingers of spring. To European birds, though, the cuckoo’s call is less welcome—for cuck

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问题     To the people of Europe, cuckoos are harbingers of spring. To European birds, though, the cuckoo’s call is less welcome—for cuckoos, notoriously, are parasites of others’ nests. They lay their eggs in them and leave the hapless foster-parents to do the hard work of bringing up baby.
    But are those parents always so hapless? How the cuckoo gets away with it has been the subject of much speculation, and some biologists think that, at least some of the time, cuckoos may bring subtle benefits which help offset the undoubted costs they impose. And, as a paper just published in Science outlines, for one sort of cuckoo and one sort of surrogate parent, that does appear to be the case.
    Daniela Canestrari of Oviedo University, in Spain, and her colleagues looked at relations between great spotted cuckoos and carrion crows. Unlike some nest parasites, great spotted cuckoos do not evict the eggs or young of their hosts. They do, however, compete with them for food. The result is that crows which raise both their own and young cuckoos to the point when they can fly fledge fewer of their own chicks than do crows which fledge a clutch without cuckoos. This suggests, as might be expected, that cuckoos are a bad thing.
    Dr. Canestrari, however, decided to look a little deeper. She suspected that though their fledged clutches were smaller, crows with cuckoos in the nest were more likely to fledge at least some young than were unparasitised crows. That might be enough to compensate for the lower number of fledgling per nest.
    Her idea was that cuckoo nestlings engage in a form of chemical warfare with predators. They certainly secrete a noxious mixture of chemicals. This suggests these chemicals have a specific purpose, rather than being mere waste products. And when she offered meat with cuckoo nestling secretion to wild cats and birds of prey, which are predators of crow nests in the study area, they would not touch it. That indicates having a cuckoo or two in the nest really is a defence.
    To prove the point, she did some experiments—swapping cuckoo nestlings from the nests where they had hatched into other nests without cuckoos. The receiving nests turned out to be more likely to yield at least some fledgling crows than did other nests that had never had cuckoos in them. Moreover, 16 years’ worth of observations of nests suggested that, though the difference was small, having cuckoo chicks in the nest was actually a better strategy on average than not having them.
The best title for the text is______.
选项 A、Cuckoos—Bringer of Spring
B、Cuckoos—Pest among Birds
C、Cuckoos—Protector of Birds
D、Cuckoos—Invader or Defender?
答案D
解析 首先，全文讨论话题为cuckoos，只有第一句提到它们是春天的预告者，下文再无对此进行讨论，故[A]项可以排除。而[B]项的Pest among Birds“鸟类中的害群之马”与前几段信息基本吻合，但与后三段提到的“杜鹃可以用来抵御掠食者"这些信息又不符，故[B]项过于片面；而[C]项的Protector of Birds“鸟类的保护者”只能对应后几段信息，与前几段不符，同样不够全面，故[D]Cuckoos—Invader or Defender“杜鹃——入侵者还是保卫者”为本题答案。
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