NATATORES. 



249 



species at first have the bill of a dark horn-colour, 

 orange-red at the base. As the bird gets older, the 

 dark colour recedes further from the base, while the 

 red becomes brighter, and finally only the tip of the 

 bill remains black in the Common Tern, while in the 

 Arctic species the black colour disappears entirely. 

 The only features which really appear to be constant, 

 and may consequently be considered indicative of 

 the species, are the comparative length of bill and 

 tarsus, and the length of closed wings in proportion 

 to the tail. In a few words, the differences may be 

 shortly stated as follows : 



COMMON TERN. 



Bill, longer and stouter, ave- 

 rage = 2 in., red, black at 

 tip. 



Tarsus, longer, average Si- 

 lines. 



Tail, equal to, or (generally) 

 shorter than closed wings. 



Under parts, whiter. 



Black-cap, extends perhaps a 

 little further, and is more 

 pointed in form. 



This last statement requires confirmation. In 

 most cases the shape and colour of the eggs may 

 be taken as a good indication of the species to which 

 they belong, but those of the Common and Arctic 

 Terns so nearly resemble each other, and at the 

 same time vary to such an extent in the colour and 



ARCTIC TERN. 



Bill, shorter and more slen- 

 der, average =1 in. 6 lines, 

 in old bird red throughout. 



Tarsus, shorter, average 6 

 lines. 



Tail, longer than closed 

 wings. 



Underparts, greyer. 



Black-cap, rather shorter and 

 more rounded in form. 



