446 



NATURAL HISTORY. 



and one never tires of watching them as they skim along just over the 

 surface of the sea, now rising to the crest of the wave, and then descending 



into the trough. 



With the tropic-birds we take up another series of swimming birds, 

 with very different features from those already mentioned, and which have 

 a structure which points in the direction of herons. The tropic-birds thein- 





Fig. 377. — Red-billed tropic-bird (Phaethon sethereus). 



selves are well named, for they are chiefly found in the seas of the tropical 

 regions, sometimes hundreds of miles from land. They have large wings; 

 but their flight is labored, the strokes being rapid and incessant, and the 

 long middle tail-feathers — red in some species, white in the others — 

 trailing behind. 



Allied to these are the frigate-birds of the same regions, of which only 

 two species are known. They are also called man-of-war hawks and 



