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ĐỀ SỐ 22During the last 25 years, Britain's urban sparrow population has declined by as (0)_MUCH _ as two-thirds, and the bird has almost disappeared from many of (1) _________former haunts. The decline has been blamed on everything from cats to garden pesticides. Moreover, modern buildings have far (2) _________ few nooks and crannies (3) _________the birds can nest. Factors (4) _________ these may well be involved, but alone they fail to explain the severity of the decline, or the fact that other urban birds have been less affected.Denis Summers-Smith is the world's leading expert on sparrows, so when he (5)_________ up with a theory to explain their decline, it has to be (6) _________ listening to. He suggests that the culprit is a chemical added to unleaded petrol. It would be deeply ironic if a policy that was intended to improve the nation's health was to prove responsible for the decline of (7) _________ of its favorite species. According to Summers-Smith, social species such as the sparrow require a minimum population in a specific area to breed successfully. If, (8) _________ whatever reason, numbers drop (9) _________ this threshold, the stimulus to breed disappears. The most dramatic example is the passenger pigeon, which in the late nineteenth century went from(10) _________ the world's most common bird to total extinction within 50 years.

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