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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.
Great spotted cuckoo chicks in carrion crows’ nests will______.
选项 A、rob young crows of their food
B、evict the eggs of crows or kill them
C、drive young crows out of their nests
D、compete with young crows for room
答案A
解析 根据题干中的关键词great spotted cuckoos和carrion crows’定位到第三段，而答案来自该段第二、三两句：Unlike some nest parasites，great spotted cuckoos do not evict the eggs oryoung of their hosts．They do，however，compete with them for food．其中，however之后的compete with them for food“与它们(指代carrion crows)竞相争食”为答案，与该句表达一致的是选项[A]rob young crows of their food“抢夺乌鸦幼鸟的食物”。选项[D]与原文最接近，但是原文的food被换成了room“空间”一词，属于偷换概念，故确定本题答案为[A]。
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