An ambitious scheme to reintroduce Britain's biggest bird of prey to the skies has seen numbers of the threatened species soar, the Royal Society for the Protection of Birds said yesterday.

The Red Kite has experienced a dramatic increase in its population this year as the spectacular birds begin to return to areas of the UK for the first time in 150 years.

The birds, which were on the brink of extinction in the UK 20 years ago, now number well over 2,000, after the RSPB successfully established a number of secret breeding colonies.

Yesterday the charity revealed that 44 chicks had already hatched at their North Yorkshire hideaway - which included the birth of quads for the first time in the region.

The RSPB reintroduced the birds, which have a wingspan of nearly two metres, to England and Scotland 15 years ago.

They established colonies in the Chilterns and Scotland using birds imported from Spain and Sweden.

After the Red Kites started breeding successfully, the RSPB created a new colony in 1999, using birds born in the wild in the Chilterns.

The programme has ''surpassed all expectations'', according to the charity as numbers have quickly soared to around 100 in the region.

Doug Simpson, project manager for the North Yorkshire colony, said the birds were now starting to spread their wings and establish new breeding sites away from their old home.

''We are delighted that the kites are now spreading out.

''The idea behind the release schemes throughout England and Scotland is to form a chain of kite colonies which will eventually link up and see the birds distributed throughout the country,'' he said.

The RSPB, along with English Nature, who jointly administer the scheme, will now track the movement of the birds through radio transmitters and build up a picture of their behaviour patterns.