188 CHAPTERS ON THE NATURAL HISTORY 



Delicate and slender in general contour, the sexes are barely to 

 be distinguished by external characters, while the plumages for 

 season and age markedly vary. 



Confining ourselves to the North American forms, it is to be 

 observed that the pattern of their coloration is quite constant for 

 the majority of them. White predominates, and this, in some 

 species, may be rose-tinted upon the under parts. Most fre- 

 quently the head is black-capped, and the back, or mantle, as it 

 is here called, is of a delicate pearl-blue or clear gray. The pri- 

 maries of the wings are black, or dark-colored. As exceptions 

 we mav note that Trudeau's tern has a white head with a black 



FIG. 50. TRUDEAU'S TERN. 



Natural size. Drawn by the Author. 



stripe upon either side (Fig. 50); and in others the black cap is in- 

 terrupted by a white frontal crescent. In the genus Hydroclicli- 

 don the species are nearly entirely black, while the Noddy terns 

 have the whole body of a fine fuliginous brown. The bill is vari- 

 ously colored, the chief shades being yellow, red, black, and 

 orange, often two of the colors being combined. Structurally, 

 they depart but slightly from the gulls, and this departure is 

 \<T\ gradual as we pass from the more laro-sternine types to the 

 distinctly tern-end of the series. In flight they are buoyant, 

 while at other times they far exceed the gulls in dash and vigor. 

 p]specially is this last the case when they are feeding, for they 

 have the habit of balancing themselves on wing over the water 



