OBSEBVATIONS ON THE OYSTERCATCHEB. 51 



(78 per cent.) being accounted failures. Of the sixty-one which 

 reached the second stage, four (6"6 per cent.) were completely 

 opened up in one stage, shell being extruded, in each case, equal 

 in area to that more usually extruded in two portions. The 

 average size of fifty-six shells which reached the second stage was 

 fifteen-sixteenths of an inch (^| in.), the largest being one and 

 one-eighth inches (LJ in.), and the smallest thirteen-sixteenths 

 of an inch (^f in.). The four shells reaching the first stage 

 averaged one and a sixteenth inches (lj 1 ^ in.), the largest being 

 one and one-eighth inches (1^ in.). A longer series, however 

 would probably lower the average. I did not systematically 

 measure the failures, as they did not show visually any variance 

 from the others. The largest measured was one and one-eighth 

 inches (1^ in.) in length. 



By direct observation I have seen all the methods in use at 

 each stage of opening the shells — thrusting, hammering, or 

 longitudinal rolling at the first stage; lateral leverage, thrust- 

 ing, hammering, or longitudinal rolling at the second. Often 

 the several mode3 are put in force successively when strong 

 resistance is encountered. The birds appear to vary in the 

 degree of pertinacity with which they attack the more difficult 

 shells, some birds giving in after a single bout, others returning 

 repeatedly and with renewed energy. In the first paper I 

 recorded the introduction of the upper mandible alone into the 

 aperture of the shell. This fashion occurs but rarely, and in 

 some cases, at least, is apparently dependent on the mode of 

 carrying the shell by the outer lip. The more usual plan is to 

 carry the shell crosswise between the tips of the mandibles, and 

 to insert the bill into the interior of the shell for the purpose of 

 opening it. 



An examination of the failures yielded some interesting 

 results when the shells had been dealt with on rock. As a rule, 

 each presented on the lower surface of the last whorl a single 

 bulb of percussion, at a point on the external surface correspond- 

 ing roughly to the edge of the operculum inside, and in the 

 second stage, when not completed, at a point in line with the 

 margin of the first opening and the edge of the pillar. Experi- 

 mentally, similar bulbs can be produced by making five or six 

 thrusts inside the shell, but it is difficult to adhere to a single 



