The organisation says it is unclear exactly what is to blame for the dramatic fall in the bird?s numbers, but says that one reason could be the explosion in the population of muntjac deer, which has drastically reduced the availability of the bird?s favoured habitat.
A BTO spokesman said: ?Deer browsing in woodland is certainly a component we have looked at, and is one of several factors that is contributing to loss of habitat and therefore the bird?s decline.?
?The population of muntjac has increased hugely in the south-east of the UK, so it has probably had more impact than any other deer species. There is no doubt there is a causal link there, but to what extent is very difficult to measure.?
As well as pressures on the nightingale?s breeding territories in this country, the BTO believes more must be learned about the bird?s migration to and from Africa, and about its wintering area south of the Sahara Desert.
Since 1998, nightingale numbers have declined by over 50% and many of the bird?s former breeding haunts have fallen silent, despite huge local conservation efforts in traditional coppice and scrub habitats.
In an effort to halt the decline, the BTO says it is planning a new survey next year.
The spokesman explained: ?The decline shown by the nightingale is so great that the species would qualify for the Red List as a Species of Conservation Concern.?
?We plan to fund further research to investigate why these declines continue while there are still sufficient birds to study. Maintaining high-quality habitat in core areas will be crucial if the species is to survive as a breeding bird in Britain.?