For decades, the arrival of the first V-shaped flights of Bewick's swans in Britain's wetlands after a 2,000-mile journey from Siberia heralded the arrival of winter.
This year, a dramatic decline in numbers of the distinctive yellow-billed swans skidding into their winter feeding grounds could be the harbinger of a more dramatic shift in weather patterns: global warming. Ornithologists at the main reserves that host the birds, the smallest of Britain's swans, said only a handful had appeared on lakes and water courses. Normally, there would be several hundred.
The latest arrival in a decade of Britain's seasonal influx of 8,000 Bewick's swans throws into sharp relief the debate on the effects of climate change as it enters a crucial week. As the Government's forthcoming Climate Bill is finalised, Sir Nicholas Stern, a former World Bank economist, is expected to warn in a report on Monday that failure to tackle global warming will provoke a recession deeper than the Great Depression.
But far from Westminster, the potential ecological impact of the same phenomenon was being noted in the absence of the high-pitched honking call of Bewick's swans on reservoirs and wetlands from the Ouse to the Severn estuary. The Wildfowl and Wetlands Trust (WWT) said its first three birds had arrived at its Slimbridge reserve in Glouc-estershire, only on Thursday, the latest arrival since 1995.
In Welney, Cambridgeshire, where there are normally 100 Bewick's by the end of October as the vanguard for a winter population of 1,000; a solitary male was this week the sole representative. The Royal Society for the Protection of Birds said that two of its reserves in East Anglia which host the bulk of the British population - the Ouse Washes and Nene Washes - were also devoid of Bewick's. Experts said that the slow arrival was due to warmer than usual conditions on the continent, in particular the birds' other main wintering grounds in the Netherlands, and an absence of the north-east winds that aid their migration from the Arctic tundra of northern Russia.
The disruption to the swans' migration pattern fits into an emerging pattern of fluctuating numbers of bird species and population movements blamed on climate change. Redwings, another winter visitor to the British Isles, started arriving from Scandinavia only this week. Normally, they come in early September.
Other species which normally leave Europe for the winter, such as the blackcap, are now staying through the year. The WWT and other bird conservation groups said that it would take weeks to assess whether the late arrival of the Bewick's, named after the 18th-century English engraver and ornithologist Thomas Bewick, would affect the overall numbers wintering in Britain.
Since reaching a peak of about 9,000 in 1992, numbers of the swans have fallen by about 5 per cent. In 2004, numbers of wintering ducks, geese, swans and wading birds fell to the lowest level for a decade.
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