OUR FEATHERED ENGINEERS. 75 



vellous perception the Sand Martins select the 

 portion of the cliff where the ground is neither 

 too hard nor too soft for boring. Regularly every 

 spring-time the little engineers return to the old 

 familiar colony ; fresh nests are made in many 

 cases by birds setting up housekeeping for the 

 first time, by others whose homes have fallen in 

 or been destroyed during the previous winter, 

 or by those who change their quarters for no 

 apparent reason to us. The tunnels formed by 

 the Sand Martins are about three inches in 

 diameter, sometimes more and sometimes less, the 

 little architects scratching away the sand with their 

 claws, their bodies working round like an ani- 

 mated drill, for two or three feet into the solid 

 bank. Sometimes these passages turn and twist 

 considerably. I have known them turn at right 

 angles, and now and then two points of ingress 

 are made to the same nest, whilst less frequently 

 one entrance gallery will branch out into two 

 passages, each leading to a different home. The 

 Sand Martins, like the thorough engineers that 

 they are, also provide for drainage by making the 

 tunnel gradually slope upwards. At the end 

 in a kind of chamber, the slight, slovenly nest of 

 dead grass and feathers is placed. Sometimes the 

 tunnels are deserted at a depth of a few inches 

 or even more, the soil apparently being unsuited 

 for boring ; and very frequently a boulder or a 

 large pebble stops the way, and the workings 

 are deserted. Both birds work at boring these 



