THE SWALLOWS 263 



of them, are to us artificial. But it adds wonder- 

 fully to one's interest in the birds to find their nests 

 built as they must have been during the long ages 

 before human dwellings dotted the land, and I 

 happen to be very familiar with a piece of coast- 

 line on which the sight may be witnessed summer 

 after summer. It is a cliff on the northern shore 

 of a sea loch, blessed therefore with a southern 

 exposure, and penetrated by half a dozen con- 

 siderable caves. These were hollowed out by the 

 sea in a past age, but an uprise of the land has 

 lifted them about forty feet above sea-level, and 

 interposed between the cliff-foot and the beach a 

 broad strip of dry land cumbered with fallen 

 rocks. The swallows build in the caves, attaching 

 their clay structures to the roofs, and in the case 

 of at least one large and vaulted cavern it is 

 possible to believe that they were tenants of these 

 roof-sites when the floor was used as the dwelling 

 of primitive man. On the face of the cliff outside 

 a much more numerous colony of house-martins 

 have their abodes, their globe-like nest being 

 attached to the under angles of the ledges. 



But the chief interest of these neighbouring 

 colonies resides in the illustration they afford of 

 the swallow mind and its power of recognizing 

 essentials. In the course of its long evolution 

 the swallow formed the habit of nesting in caves, 

 but when it found the first open barn it was able 

 to recognize that here were all the essentials of a 

 cave. So now we have swallows "using every man- 

 made cave to which they can find access. Barns 

 with open doors, grain lofts with a broken win- 



