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During the last Ice Age, when all of Scotland and much of the rest of Britain was covered with snow and ice, birds were decidedly scarce. In fact, many of the birds that are present
today were not found in Britain at all. One such was the crow (Corvus corone), which discovered ice-free refuges at either end of the Mediterranean, in Spain and Greece. There, isolated one
from another, the two populations developed different plumage's; crows in the west became wholly glossy black, while those in the east became grey and black. Today the all-black
bird is called the carrion crow (Corvus corone corone) and the grey and black bird the hooded crow (C. corone cornix).When the ice retreated the landscape was quickly colonised first by plants and then by
animals, including birds - indeed, some birds actually played an important part in plant colonisation. As a result, the two crows moved northwards and eventually met up. The
carrion crow now occupies the western parts of Europe, the hooded crow the eastern and northern parts. Where the two sub-species of crow meet they interbreed quite freely,
producing hybrid young that are themselves quite capable of breeding. In Britain the line of hybridisation extends diagonally across Scotland. Over the past 70 years this hybrid zone
has moved northwards, perhaps as a result of climatic warming. In 1928 the line extended from Bute to Perth and thence northwards along the coastal zone to Fraserburgh. Today it remains stable in Bute, but now e
 xtends northwards through Speyside to Caithness. It is
interesting that the line has shifted in the north, but remained stationary in Bute and Cowal. The reasons for the differential change between the northeast and southwest are a matter for
speculation, but it is clear that the carrion crow has expanded its range largely through the Lowlands. In the northeast the hills are more gentle, the slopes less pronounced and the hybrid zone is
comparatively wide. In Cowal and the southwest the hills rise steeply from deeply cut valleys and as a consequence the hybrid zone is
considerably narrower. Yet even here the carrion crow occupies the valley floors, leaving the hills and mountains to the hooded. If the greenhouse effect is a reality and the climate
becomes warmer throughout Scotland, it will be interesting to keep a careful watch on the dynamics of these two well marked subspecies - particularly along the eastern shores of Loch Fyne in Cowal. |
