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 nestling period. 



BEAKS OF NESTLING BIRDS COM- 

 PARED WITH THOSE OF THE 

 ADULTS. 



1. Adult sdssor-biU. 



2. Nestling scissor-bill. 



3. Nestling cocatiel, 



4. Adult cocatiel. 



5. Nestling tinamou, showing a tooth- 

 like armature. 



