90 BIRDS' NESTS 



appliances (natural tools, such as bill and feet), or 

 even mental development. 



We will deal first vi^ith those nests that are made in 

 tunnels in the earth. It is somewhat remarkable how 

 comparatively few species resort to this method of 

 providing a procreant cradle ; but although few in 

 number, the representatives of this particular kind of 

 nest-building are drawn from widely divergent groups. 

 These tunnels or burrows may either be driven into 

 the face of sand or earth cliffs, or formed in the 

 ground itself. Curiously enough we find amongst 

 these feathered excavators some of the least likely 

 birds — species one would think to examine them very 

 ill equipped for such kind of work. One of the most 

 familiar of these tunnel nests is made by our well- 

 known Sand Martin, all the more interesting because 

 these birds rank amongst the few known Passeres 

 that excavate out of the five thousand five hundred 

 known forms. As a rule these Martins prefer a cliff 

 of fairly firm clay, loam, or earth, sand pits and the 

 layer of soil at the top of a quarry being favourite 

 sites, but less frequently their excavations are made 

 in soft sandstone, sufficiently hard nevertheless to 

 take all the cutting edge off a hardened steel knife 

 blade ! When once the locality has been selected the 

 birds seek out a suitable spot in which to commence 

 their shaft, often making several attempts before 

 finally deciding. A small circular hole is first formed 

 with the bill (one of the weakest looking, by the way, 



