An endemic species is one that is found only in a specific geographic region, whether it be a country, island, state, or county. Species that are native to an area but can also be found elsewhere are not considered endemic.
Many islands host a variety of endemic species due to their isolated evolution. However, the British Isles are an exception, with very few endemic species. This is because, until relatively recently, the islands were connected to continental Europe via a land bridge. It was only around 8,000 to 10,000 years ago that rising sea levels flooded the land bridge, isolating the islands from the mainland.
Frequent glaciations also contributed to the loss of much of the islands’ biodiversity, as they were buried beneath thick ice. When the ice retreated, Europe was repopulated from the south. However, not all species made it to Britain and Ireland before the sea levels rose enough to cut them off from the mainland, particularly species less tolerant of cold. This explains why there are no snakes in Ireland and why, for example, France has several more species of amphibians and reptiles despite being just a few miles away.
The UK has just one true endemic bird species: the Scottish crossbill (Loxia scotica), which is found exclusively in the Caledonian Forests of Scotland. It is also the only endemic terrestrial vertebrate in the country.
The Scottish crossbill was first described as a subspecies of the common crossbill in 1904 by German taxonomist Ernest Hartert, who noted that the bills of crossbills in the Scottish Highlands were larger than those of the common crossbill. Later, it became clear that this distinctive feature persisted despite frequent invasions by common crossbills from the continent, some of which remained in Scotland to breed and could potentially hybridise with the Scottish crossbill.
In 1980, the British Ornithologists’ Union recognised the Scottish crossbill as a distinct species. However, some scientists continued to debate whether it should be classified as a subspecies or race of the common or parrot crossbill, both of which inhabit the same forests.
A long-term study by the RSPB, published in 2006, concluded that the Scottish crossbill has a distinct flight pattern and unique calls, described as a ‘Scottish accent’. It is believed that these unique calls help the birds make sure they pair exclusively with mates of their own species.
All three species of crossbill have evolved crossed mandibles, which enable them to extract seeds from cones. The common crossbill has the smallest bill, best suited for extracting seeds from spruce cones. The parrot crossbill, with its larger bill, is adapted for prising seeds from pinecones. Meanwhile, the Scottish crossbill, with an intermediate-sized bill, is capable of extracting seeds from a variety of conifer cones. Like all crossbills, Scottish crossbills can be either right- or left-handed, with their bill tips crossing in either direction.
The subtle differences in bill size are not easily used by ornithologists in the field to distinguish crossbill species. However, their calls can be identified through sonograms, or sound spectrograms, created from recordings.
The RSPB study also found that young Scottish crossbills inherit both bill size and call traits from their parents, providing further evidence that they are a distinct species.
It is estimated that there are between 8,000 and 20,000 individual Scottish crossbills. Although no specific data on population trends is available, the species has been placed on the Amber list due to its status as endemic, making it of international importance and at risk of extinction. Threats to the species include habitat loss from commercial forestry, the expansion of wood and pulp plantations, the underplanting of new conifers, and overgrazing by red deer, which prevents forest regeneration.
