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A Truly Scottish Crossbill

Status of ‘UK’s only endemic bird species’ confirmed by the RSPB

New research by the RSPB has helped to settle one of the longest-running disputes in the ornithological world.

The status of the Scottish crossbill as endemic to Britain – occurring here and nowhere else in the world – is indeed justified, according to the results of a lengthy scientific study into the species.

Celtic’ crossbills differ in bill size from other crossbill species found in Britain, and just like native Scots, they have also been found to have a distinct Scottish accent or call, thought to be the method used by the birds to make sure they only attract and pair with potential mates of the same species.

Although the British Ornithologists Union has classed the Scottish crossbill as a separate and distinct species since 1980, many ornithologists, including those in the RSPB have always reserved judgement on this notion, believing there was insufficient scientific research for its formal acceptance.

Scotland ’s conifer woods are home to three types of crossbill – the common crossbill (with a small bill best suited to extracting seeds from the cones of spruces), the parrot crossbill (with a large bill suited to extracting seeds from pine cones) and the Scottish crossbill (with an intermediate bill size used to extract seeds from several different conifers).  All three are similar in both size and plumage, and DNA tests have showed that the birds are genetically similar, casting some doubt on the Scottish crossbill’s status as a distinct species.

In trying to discover exactly what features the birds used to identify each other, experts at RSPB investigated the calls of the three types of crossbill, and found that Scottish crossbills (as identified by bill size) also have quite distinct flight and excitement calls from other crossbills.  However, the most important evidence has come from RSPB’s long-term field study in the Highlands which focused on discovering if the birds mate with those with a similar bill size and call, and whether young Scottish crossbills inherit their bill sizes from their parents.

Results showed that, of 46 pairs of different types of crossbills caught, almost all matched closely for bill size and calls. In other words, the different types of crossbills were behaving as distinct species. The small number of ‘mismatched’ pairs was too few to suggest that the different types are not species, but enough to account for their genetic similarity. The fact too that young crossbills had bill sizes similar to their parents showed that they inherited their bill sizes, and also supports the species status of Scottish crossbills.

Although the three species differ in average bill size, the actual differences are small and cannot be used reliably in the field by ornithologists to identify crossbills. The calls, though, can be distinguished by sonograms, or sound pictures, made up from recordings.  Crucially, this provides the basis for a method to survey crossbills and, for the first time, gain a clear picture of their numbers and distribution in Scotland.

The next steps in the Scottish crossbill study are to find out its population size and habitat requirements. With the current estimate of 1,500 birds for its global population, being little better than a guess, a detailed survey is crucially important to put together the right conservation and management measures to protect and conserve it.

Interested in seeing Scottish crossbill? Then click here.

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