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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.
Some biologists think that cuckoos______.
选项 A、are more evil than we think
B、will cause nothing but harm
C、may not be completely useless
D、can not compensate for their harm
答案C
解析 根据题干关键词some biologists定位到第二段第二、三行：some biologists think that，at least some of the time，cuckoos may bring subtle benefits which help offset the undoubtedcosts they impose．大意为“一些生物学家认为，至少有些时候，杜鹃能带来一点好处，从而弥补它们所造成的伤害”。显然，[A][B][D]三项都是否定，只有[C]稍微有点肯定，[C]项说may not be completely useless“并非一无是处”，这与原文may bring subtle benefits“可能会带来一点好处”一致，故[C]项为本题答案。
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