Four pairs of terns, which are breeding for the first time in London's Docklands, are receiving 24-hour protection from security guards to prevent vandalism to their nests.

The arrival of the terns is a major breakthrough for the London Docklands Development Corporation, which last year built specially constructed rafts in the harbour areas to attract nesting birds.

These floating islands, designed in consultation with the Royal Society for the Protection of Birds, are made to look like the birds' ideal breeding ground - sand and shingle in the case of terns.

According to the RSPB, the number of terns has been declining in many areas because there are few suitable places for them to breed.

'To attract them to areas where they would not have been previously is very good,' said a spokesman. The terns would normally come inland, nesting along rivers. Despite the Docklands apparently inhospitable facade, it suits them perfectly - all they need is somewhere to fish.

David Lowman, Development Corporation personnel officer and a keen ornithologist, noticed the terns flying around last year, looking for somewhere to nest.

He won the corporation's support in setting up the scheme to attract them to breed in the area. With backing from Texaco, the corporation built 12 rafts, each about 15 feet square at a cost of pounds 800, which were scattered around the harbours.

The rafts, specially designed to attract terns, include boxes for young birds to shelter from predators and a sloping ramp round the edge so, if they fall off, they can get back on quite easily. The terns have bred about 10 young between them.

'It is so successful,' said Mr Lowman. 'I am absolutely delighted. The Development Corporation is about urban development - buildings and roads. It is good to do things which introduce some variety in the environment.'

During the breeding season, Docklands security guards were asked to keep an eye on the rafts. Although they are in the middle of the harbour, they could still be reached by boat.

The Docklands area has also attracted a rare pair of black redstarts which are nesting in a crevice of an old building. There are only 20 pairs of the birds in the country, mainly in the south and east of England. The birds came to England after the Second World War, when they nested in bombed buildings and industrial areas.

Docklands also boasts ringed plovers and shelduck, according to a survey of the area by the London Ecology Unit.

Although these birds are not as rare as the black redstarts, they are none the less important, according to the RSPB.

'If you look at it as a whole it is a sign that the environment around Docklands can support all of these birds,' said a spokesman.

'There is food there. That has got to be a good sign. And the environment is healthy enough for them to breed successfully.'

The Corporation is so pleased with the results it plans to build a wildlife sanctuary around the disused East India Dock basin.

As well as a bird reserve it would show other features of shore life.

One of the rafts can be seen from the Docklands light railway, looking east as it crosses the bridge from South Quays to Canary Wharf.