Mystery as Britain's biggest inland bird colony vanishes into thin air

Britain's biggest inland bird colony has vanished almost without trace, leaving wildlife experts baffled.

Britain's biggest inland bird colony has vanished almost without trace, leaving wildlife experts baffled.

Up to 50,000 adult and young black-headed gulls would gather at Sunbiggin Tarn in Cumbria until two years ago, in a vast avian metropolis that spread over 10 acres - but now only a few score pairs are left.

For no obvious reason, nearly all the 25,000 adult birds seem to have taken a sudden dislike to this lake in a moorland basin 1,228ft above sea level in the Pennines west of Kirkby Stephen - and left.

"It's a mystery - we don't know why they went or where they've gone," said Geoff Longrigg, a local farmer and keen ornithologist who has studied the colony over many years.

"Such a massive number of nesting black-headed gulls is very rare and something that doesn't escape attention, so if another colony on that scale had become established elsewhere recently I'm sure we'd have heard about it.

"Perhaps they've just become dispersed among the many small colonies scattered about the Pennines and elsewhere. There's certainly no evidence to suggest they've suffered some major disaster."

What adds to the puzzle is the element of déjà vu. One of Europe's largest black-headed gull colonies used to be at the mouth of the river Esk, near Ravenglass, 40 miles to the west - then it disappeared in the 1980s, many of the birds possibly switching to Sunbiggin, which was already established.

David Baines, a local game conservancy researcher, said: "The end of the Ravenglass colony was also a mystery - it even led to checks for radioactive contamination with Sellafield being near by. But that was ruled out and no other reason was found.

"I don't think predation was a problem at Sunbiggin. Foxes regularly took eggs and young but the colony was so big it had no effect - in the same way there was no impact from gypsies filling a few buckets with eggs for food.

"The impression is that the gulls have suddenly, collectively, decided they needed a change of scene. What triggered that is something we don't know, despite this having occurred twice in Cumbria in 20 years. Perhaps we never will know what happened."

Mr Baines has vivid memories of the colony over the past 25 years. He would often take part in operations to fit rings to the legs of young birds to record their movements.

"There is nothing like the experience of walking into the centre of the colony with thousands of birds flying around, screeching loudly and dive-bombing. It's quite weird seeing it virtually deserted now."

The black-headed gull, Larus rudibundus, has not always been so common in Britain - its numbers declined so much during the 19th century that extinction was feared during the 1880s.

The population revived after legal curbs were introduced on shooting and egg harvesting. Studies during the 1980s put the number nesting on the coast at 82,000 pairs, with possibly 130,000 pairs inland.