The RSPB's Hannah Bartram and Safeway's Master of Wines Liz Robertson on the pros and cons of natural corks

Oak woodlands in Spain and Portugal, known as dehesas, have been used to produce cork and graze livestock for hundreds of years, making them a haven for wild birds.

The Royal Society for the Protection of Birds says the dehesas, which are home to many rare species are under threat as a result of the cork's decline in popularity. People should look for wine bottles with real corks in them, the society says.

Some 42 species of bird depend on the cork-producing woodlands, including the endangered Spanish Imperial Eagle, of which only 130 pairs remain, and other rare species including the black stork and black vulture.

Hannah Bartram, agriculture policy officer of the RSPB, said about 100 pairs of the eagles depend on the dehasas for survival.

Wine buyers should check their corks are natural

"They hunt over the dehesas and they also nest in the trees. So if that habitat goes ... that will be a really significant loss to the biodiversity of Spain and also to Europe," she told BBC Radio 4's Today programme.

Other smaller birds, including robins, chaffinches and song thrushes, migrate to the dehesas from northern Europe, along with blackcaps from the UK.

The dehesas are also home to a rich variety of butterflies and plants in spring and summer.

They provide more than 80% of the world's cork, of which two-thirds is used for wine and champagne stoppers.

The corks are produced from the bark of cork oaks, which can live for centuries. They are stripped every nine years, giving the tree time to regenerate its bark and so produce more cork.

One particular tree, known as the "Whistler Tree" because of the many singing birds attracted to it, is said to be 212 years old. This tree alone will have produced up to one million corks by 2000.

Production uneconomic

But the trees take between 25 and 30 years to become productive and are also at risk of a fatal disease known as Seca.

Production may become uneconomic because it takes so long for the trees to become productive, forcing farmers to replace them with crops such as sunflowers or faster-growing eucalyptus trees.

To add to the problem, plastic stoppers are gaining in popularity - partly because they are seen as a more hygienic alternative to the natural version which sometimes carries a chemical which causes a mouldy taste known as "taint".

Plastic stoppers have now taken a 1% share of the world market, and this percentage is growing.

The resulting disappearance of the cork trees may contribute towards impoverished soils and water shortages, causing huge problems for both wildlife and people.

'Look for the real thing'

So now the RSPB is to call on supermarkets to label the wine on their shelves to show what type of stopper is in the bottle.

Ms Bartram said: "It is vital that supermarket customers know which wines contain real cork stoppers. Until there is better labelling, consumers will just have to pop a cork this Christmas and hope it's a real one.

Spanish environmental expert Miguel Naveso urged British wine buyers to check whether their bottles had natural corks.