Flocks of golden plovers and lapwings roosting in fields in winter are a traditional British countryside scene - but possibly not for much longer.

Some of these farmland birds have developed a new habit guaranteeing sleep, free from rude awakening and the possibility of becoming a fox's dinner - they have switched to sleeping on factory rooftops.

A paper outlining the "relatively new phenomenon" of daytime roosting on large roofs of commercial and industrial premises by these normally strictly down-to-earth species is published in the latest edition of the journal British Birds.

Ornithologists say in their report that rooftop flocks of up to 600 have been found. The habit has been observed in the North-East with roosting on factory roofs at Birtley, Newton Aycliffe and Washington.

The rooftop gatherings begin in July, as flocks begin forming after the nesting season, and continue through autumn and winter until spring, with highest numbers from September to February.

"They appear to be quite tolerant of proximity to human activity, traffic or noise from industrial processes within buildings," says the report.

"Several assemblies are in busy town centres, where a number of roofs may be used for sleeping, idling and preening by the same flock over a long period."

Researchers noted grey asbestos corrugated roofs were a favourite spot, probably because the fields where lapwings traditionally roost have a grey or brown appearance.

The paper describes how lapwings and golden plovers fly on to the rooftops after sunrise and leave 40 minutes after sunset to head for fields over two miles away to feed under cover of darkness.

As for the reason for the moving on to roofs, suggestions include safety from ground predators and more protection from bad weather. The birds gather on the sheltered side of sloped roofs and benefit from heat transmitted from below