466 SANDPIPERS AND RELATED SPECIES 



position of its prey, it searches by touch alone. This it does by gently 

 and rapidly tapping the sand with the tip of its bill, or, if on sloppy 

 ooze, by ploughing the mud steadily with its bill, or drawing it about 

 over the surface. By examining the ground on which dunlin had 

 been working, Mr. Dewar found that " the imprints made by the bill 

 are of three kinds, distinguished not so much by the sharpness of their 

 differences as by the frequency with which the average forms occur. 

 They are a slight double dent in the sand made by a gentle pressure 

 with the point of the bill ; a shallow probing, an eighth to a quarter of 

 an inch in depth, usually, but not invariably, divided into two com- 

 partments by a transverse septum of sand ; a deep probing, a quarter 

 to half an inch or more in depth, and complete in the sense of having 

 no septum." l Watching dunlin in the act of tapping and probing, when 

 the conditions were favourable for seeing whether the mandibles were 

 closed or open, Mr. Dewar observed, on one occasion, that during 

 rapid probing they were separated all the time, and, on another 

 occasion, "the bill was sometimes opened and sometimes closed 

 during the downstrokes," but he believed that the apparent closure 

 was due to his " inability to see a trifling separation of the mandibles 

 towards the tip of the bill." This view that the mandibles are slightly 

 separated during tapping and probing is supported by the pattern of 

 the impression and the frequent occurrence of septa. The absence 

 of septa in the deep or complete probings is accounted for by the 

 probability that in these a capture has been made, and the material 

 forming the septum has been swallowed with the animal extracted. 

 Mr. Dewar's observations of the birds at work, and of the impressions 

 left on the sand or mud, indicate that they press or tap the surface 

 with their slightly opened bills to locate their prey, and that the 

 separation of the mandibles increases as the bill is thrust deeper in, 

 thus forming the septate probings. The suggested reason for the 

 separation of the mandibles is that it " makes introduction of the bill 

 more easy, it increases the tactile area, and may, by comparison 



1 See also Macgillivray, British Birds, iv. p. 212. 



