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During the last 25 years,Britain's urban sparrow population has declined by as much as two-thirds ,and the bird has almost disappeared from many of its former haunts.The decline has been ...1... on anything from cats to garden pesticides

Blamed

Moreover,modern buildings have far ...2... few nooks and crannies where the birds can nes

Too

Factors ...3... these may well be involved,but alone they ...4... to explain the severity of the decline,or the fact that other urban birds have been less affected.

Like-fail

Dennis Summers Smith is the world's leading expert on sparrows,so when he ...5... up with a theory to explain the decline ,it has to be worth listening to.

Comes

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 ...6... to prove responsible for the decline of ...7... of its favorite species.

Were-one

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

For-below

The most dramatic example is the passenger pigeon,which in the late 19th century went from ...10... the world's most common bird to total extiction within 50 years.

Being