CONCEALED OR COVERED NESTS 91 



throughout the class Aves), the bird clinging to the 

 bank face meantime and working round and round. 

 Then as the tunnel gets longer the bird is able to 

 stand in the excavation and cast out all the loose soil 

 with its feet. Bill and feet keep steadily at work, 

 chiefly in the early portions of each day, until the 

 gallery extends several feet into the cliff. Sometimes 

 the tunnel has to be deserted should a large stone, a 

 tree root, or other impediment block the way, or in 

 many cases when the cliff is too soft or too hard to 

 admit of successful boring. Although the holes vary 

 considerably in length and general direction, they 

 invariably slope upwards. Some tunnels are almost 

 straight, others turn to the right or left, and are from 

 three to five inches in diameter. As a rule these 

 tunnels are circular, but in some cases they are more 

 or less oval or rectangular. At the end of the gallery 

 the tunnel is enlarged into a sort of chamber about 

 six or eight inches in height, and here a loose nest is 

 formed of dry grass and straws, lined sparingly with 

 feathers. Sand Martins breed in colonies of varying 

 size, and in some places the cliffs are literally honey- 

 combed with their burrows, the birds returning year 

 by year to the same spot to rear their young. The 

 Rock Sparrow {Petronia stulta) also bores into banks 

 and makes its nest in a burrow three or four feet 

 in depth, a cutting or railway embankment being a 

 favourite situation. 



Our next burrowing Passere is a somewhat aberrant 



