86 PROCEEDINGS OF THE 



tliey are often found there. Mr Selby says they breed at the 

 great Smoo cave near Durness, and "in other caverns of the 

 limestone rocks " on the north coast. Mr J. Crawford informs 

 me tliat only on two occasions has he seen the Swift at Tongue, 

 and that they did not remain to breed, but only stayed for two 

 or three days. It is not easy to account for their numbers at 

 Durness, which I am informed are considerable, when we ask the 

 questions, " How did they come there ? " and " Why should they 

 not occupy the caves on the west coast, as well as those on the 

 north]" 



Perhaps the following hypothesis may account for this some- 

 what curious local distribution. First, I would be inclined to 

 suppose that the line of migration lies between Bonar Bridge in 

 the south-east, and Durness in the north-west, and midway between 

 the north and west coasts; or, in other words, the said line 

 of migration, after reaching Bonar, runs in a north-westerly 

 direction; and this would seem to receive probability from the 

 fact that occasional specimens are observed along the shores of 

 Loch Shin, and again about Loch More. That more are not 

 seen at these intermediate stations may possibly be owing to the 

 migration taking place either at a great height, or during the late 

 evening. Second, the reason that Swifts do not visit the west 

 coast may be owing to either of two causes, or partly to both, 

 viz.: — that, as their migration follows the course of the great 

 dividing valley formed by the rising grounds on either side of the 

 chain of lochs — including Loch Shin, Loch Gream, and Loch 

 More — it would lead them out of this course to surmount the 

 h'vAi ranges of mountains which lie between these lochs and the 

 Atlantic; again, another reason may be found in the fact that the 

 caves of the west coast being principally in granite rocks, fewer 

 crevices suitable for nesting purposes are to be found than 

 in the limestone rocks of the north coast. In the range of cliffs 

 above Inchnadamph they might find perfectly suitable nesting 

 places, but this being only an isolated cropping-out of the lime- 

 stone, it may not offer sufficient inducement to them to diverge 

 from their above mentioned north-westerly course. 



