A "Scottish accent" has confirmed a bird species can only be found in the Scottish Highlands and nowhere else in the world, according to the RSPB.
Debate has raged over the exact lineage of the Scottish crossbill
Debate has raged for years among experts about whether the Scottish crossbill was unique, or a sub-species of the common crossbill.
But the RSPB said its research had reinforced its status as endemic.
It said the bird's distinctive call and the fact young inherited bill size from their parents provided the proof.
The British Ornithologists Union has classed the Scottish crossbill as a separate and distinct species since 1980.
However, the RSPB admitted to having been sceptical in the past and had cited a lack of scientific evidence to back up its endemic status.
DNA tests had shown the Scottish crossbill, common crossbill and parrot crossbill - which visits from Europe - to be genetically similar.
The results of long-running research has now found, according to the RSPB, that the Scottish variety is a distinct species of its own.
The society said it had a "Scottish accent", or call, which it uses to attract a mate from among other Scottish crossbills.
The different calls of crossbills were distinguished by sonograms, or sound pictures, made up from recordings.
RSPB Scotland's senior researcher Dr Ron Summers, who led the study, said: "The question of whether the Scottish crossbill is a distinct species, and therefore endemic to the UK, has vexed the ornithological world for many years and split the bird watching community.
"This research proves that the UK is lucky enough to have a unique bird species that occurs here and nowhere else - and this is our only one."
The crossbill uses its unusual beak for extracting pine seeds
Dr Jeremy Wilson, head of research for RSPB Scotland, added: "Clarifying the status of the Scottish crossbill as a distinct species, and devising a survey method based on the bird's calls are exciting steps forward.
"We hope to carry out the first full survey of the numbers and distribution of Scottish crossbills in 2008, after which we will be better placed to understand how best to manage conifer woodlands in Scotland to secure the future of a bird found nowhere else in the world."
The Scottish crossbill uses its unusual shaped beak to prise pine seeds from tree cones.
The RSPB said the species could only be found in native pine forests in the Highlands.
Loch Garten at Abernethy, near Aviemore, is listed as one of the best places to see them.
Other birds - like the St Kilda wren, the red grouse, and some thrushes and starlings that are found in the Western and Northern Isles - are currently regarded as sub-species.
They may also in fact be separate species.
But so far not enough work has been done to convince scientists of their claim to that title.