Urban sparrows facing extinction says expert

The house sparrow, Britain's most familiar bird, may be on the road to extinction in large towns and cities, says the world's leading expert on the species.

It is being driven out by the same biological mechanism that a century ago wiped out the American passenger pigeon, which was also once a common bird worldwide, Dr Denis Summers-Smith believes.

The mechanism, known to scientists as the Allee effect, is the inability of highly social animals to breed successfully once their population drops below a certain level. A decline in numbers, once started, becomes self-reinforcing and impossible to stop.

Dr Summers-Smith, author of several books and many scientific papers on sparrows, thinks this may be happening to house sparrows in London and other British cities, where numbers began falling sharply in the early 1990s, for reasons which are not clear.

The British Trust for Ornithology says that between 1994 and 2001 house sparrow numbers in the capital fell 70 per cent, and there is no sign of a recovery.

"This is the most astonishing biological phenomenon," says Dr Summers-Smith, "and I do think that in London, Glasgow, Edinburgh and other urban centres the house sparrow may now be going extinct.

"The Allee effect is very probably implicated in its decline. The house sparrow may well be the passenger pigeon of the 21st century."

In the mid-19th century the passenger pigeon was numbered in many millions across North America, but it was hunted to extinction by 1914.

The Allee effect is named after American biologist Warder Allee, who more than 50 years ago realised that "undercrowding" can itself tip the balance towards extinction for species that need to gather in large groups to breed.

Like the passenger pigeon did, the house sparrow breeds in colonies, and Dr Summers-Smith believes that falling numbers are stopping breeding and - once below a crucial threshold - causing colonies to dissolve. House sparrows need to produce between two and three broods a year to maintain their numbers.

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