YOUNG BIRDS AND RECORDS OF THE PAST 143 



rate until after very careful thought. But there are 



other facts of a similar nature which are more easily 



grasped. These are illustrated by the beaks of young 



birds : which, as everybody knows, present a marvellous 



range in size and shape, and 



some present features which 



are of exceptional interest, 



since they represent degrees 



of specialisation often 



unique. 



Take the scissor-bill tern, 

 for example. In this bird 

 the lower jaw is half as 

 long again as the upper 

 jaw, and both are flattened 

 from side to side to the 

 thinness of a paper-knife 

 from tip to base. Now 

 this bird feeds after a quite 

 peculiar fashion, skimming 

 along over the surface of 

 streams with the lower jaw 

 plunged into the water : as 

 it ploughs its way through 

 shoals of small fish one after another is caught, as in a 

 cleft stick, between the upper and lower jaw, and passed 

 back to the mouth and swallowed. But in the nestling 

 bird no sign of this most extraordinary beak is present, 

 the two jaws being of equal length. Similarly the long 

 beak of the curlew, and the strange upwardly turned 

 beak of the avocet, and the crossed mandibles of the 

 crossbill, and the pouch of the pelican are not developed 

 till the end of the nesthng period. 



BEAKS OF NESTLING BIRDS COM- 

 PARED WITH THOSE OF THE 

 ADULTS. 



1. Adult sdssor-bill. 



2. NesUing scissor-bill. 



3. NesUiug cocatiel. 



4. Adult cocatiel. 



5. Nestling tinamou, showing a tooth- 

 like armature. 



