Bird experts have warned of a dramatic fall in the number of what is arguably Surrey's rarest bird, the Dartford warbler.

The Rare Breeding Birds Panel (RBBP) has said in its latest report that the heathland bird “has suffered a dramatic population decline at some of its most important breeding sites in South-east England.”

The bird has been spotted before by excited birdwatchers on Reigate Heath.

However, in recent years, experts have warned that its numbers have plummeted because of severe winters, and in the summer of 2010, a spate of heathland fires on Reigate Heath raised more concerns that it could be lost from the area.

The new report from the RBBP paints a bleak picture for the Dartford warbler.

The Royal Society for the Protection of Birds (RSPB) has been working to help the Dartford warbler since the harsh winter of 1962 to 1963, and extensive habitat management to regenerate and improve heathland had enabled a vital recovery of the Dartford warbler in England.

In 2004, there were almost 1,000 pairs in the Thames Basin and Wealden Heaths.

However the impacts of recent harsh winters have shown signs of knocking the population back at some key sites.

Sadly, in 2010, just 50 pairs of Dartford warblers were reported across areas of Berkshire, Hampshire, Surrey and Sussex.

Dave Burges, conservation officer for the RSPB in the South-east, said: “It is believed that warmer summers and, until recently, milder winters had allowed Dartford warblers to not only increase their numbers in the South-east, but to also spread further north in England.

“But the recent run of harsh winters appear to be taking their toll on this bird.”

Heathland, a scarce but hugely important wildlife habitat, once covered vast areas of Southern England.

But today, lost 75% has been lost what is left is highly fragmented.

Together, the Thames Basin Heaths, the Wealden Heaths and Ashdown Forest, form a vital complex of heathlands in Southern England that support a whole range of wildlife, including threatened UK breeding bird populations.

The RSPB has said heathland continues to face significant threats, despite intensive conservation action over the last 20 or more years.

But the charity has said there is significant potential to restore heathland where it has been lost relatively recently, making sites bigger and reconnecting them, which makes dependent wildlife more resilient to the pressures of a changing environment.

The RBBP report also found that, in contrast to the declines of the Dartford warbler, other birds are faring better.

The firecrest – one of the UK’s smallest birds – is increasingly nesting in Britain, from further South in Europe.

It is commonly found in Surrey, Sussex, parts of Hampshire and there are growing reports from across Kent.

Firecrests first colonised the UK 50 years ago, and since then the population has risen, with the latest report from the RBBP suggesting there may be more than 1,000 pairs nesting in Britain, primarily in Southern England.

Mark Holling, secretary of the RBBP, said: “In the last 50 years there have been a number of species which have nested for the first time in Britain.

“Some, like the purple heron, have only nested once. But others, like the little egret and firecrest, have gone on to become established and relatively widespread nesting birds.

“These shifts shown by some nesting birds fit the pattern of climate change with species moving from further South in Europe to colonise the UK.”

Mr Burges added: “Perhaps the firecrest’s success in Britain shouldn’t come as a surprise.

“The warmer summers we’ve seen in recent decades favour this bird, and during the winter it leaves its nesting sites to winter along the coast of South-west England or on Continental Europe, so avoiding the worst of our winter weather.”

But he added: “Sadly, the Dartford warbler is a casualty of the combined double whammy of weather and climate - a changing climate in the South of its range is affecting it, with rapid declines in its Spanish and Portuguese heartlands, while in the North of its range, where the summer climate is improving, it is being badly affected by harsh wintry weather.”

Reigate Heath is designated a Site of Special Scientific Interest (SSSI) - one of the most important wildlife designations nationally.

The heath is also a Local Nature Reserve (LNR) because it is an important example of the lowland heath habitat, including its heather and acid grassland.

As well as these features, the heath has a number of Bronze Age burial mounds between 3,000 to 5000 years old, which have been designated Scheduled Ancient Monuments.

Reigate Heath is important because over the last 200 years, some 80% of lowland heath habitat worldwide has disappeared.

As a result, a great many species that can only exist on heathland have either become extinct or are becoming increasingly rare.