TROPIC BIRDS. 59 



developed extensible pouch beneath the bill 

 (gular sac), fig. 40, with other minor charac- 

 ters. The young are hatched naked, fig. 41; 

 but are soon covered with down. They are 

 helpless, being at first fed by regurgitation 

 and do not fly until nearly or quite fully 

 grown; they subsist wholly upon fish. 



The order may be divided into well es- 

 tablished natural groups as follows:- 



TROPIC BIRDS. 



These are perhaps the smallest of the 

 Totiplamate Swimmers. In them the gula 

 sac is reduced to the minimum size ; the beak 

 is comparatively small, not hooked at the 

 end but pointed like those of the terns, fig.. 

 40. In fact, the birds have a very tern- 

 like appearence, but differ from them in 

 having the central tail feathers greatly elon- 

 gated and projecting from beyond the oth- 

 ers, fig. 42. The feet are small, fig. 43. 



