Avifauna
The history of mammals suggests three broad overlapping phases: natural colonisation after the ice age, human-caused extinctions, and introduction by humans of non-native species. The greater mobility of birds makes such generalisations hard to substantiate in their case. Modern humans have done great damage to bird species, especially the raptors, but natural variations in populations are complex. For example, Northern Fulmars were present at Skara Brae during the Neolithic period, but in medieval times their breeding range was restricted to St Kilda. Since then they have spread throughout the British Isles.
Most of about 250 species of bird regularly recorded in Britain venture into Scotland, and perhaps up to 300 more occur with varying degrees of rarity. A total of 247 species have been assessed and each placed onto one of three lists, red, amber or green, indicating the level of concern for their future. Forty species are red-listed, 121 are amber-listed and 86 are green-listed.
The Scottish Crossbill, Loxia scotica, which inhabits the coniferous forests of the Highlands, is Britain's only endemic bird and, with only 300 breeding pairs, one of Europe's most threatened species. Its shape, red/green hue and habit of hanging upside down has led to comparisons with parrots. St Kilda has a unique subspecies of wren, the St Kilda Wren Troglodytes troglodytes hirtensis, which has adapted to perching on the rocks and cliffs of this treeless Atlantic island, and consequently has developed larger and stronger feet than the mainland variant. It is also slightly larger, has a longer beak, a drabber though more varied colouring, and a "peculiarly sweet and soft" song. The subspecies was recognised in 1884 and was protected by a special Act of Parliament in 1904 to prevent its destruction "at the hands of ornithologists, egg-collectors, taxidermists and tourists".
Read more about this topic: Fauna Of Scotland