82 The Bird 



name to^ two great divisions of birds : Rati'tce (those with 

 flat breast-bones, raft-hke), including the ostrich, rhea, 

 emeu, cassowary, and apteryx; and Carina' Ice (birds 

 with keeled breast-bones, boat-like), including all other 

 living birds, whether flyers, as the thrushes, storks, and 

 gulls, or swimmers like the penguins. But this differ- 

 ence in breast-bones is far from being as profound as 

 other differences existing between certain birds which 

 are alike in having keels to their sternums. The pres- 

 ence or absence of a keel is not of great taxonomic im- 

 portance. 



The size of the keel is a pretty sure criterion of the 

 flying powers of a bird, that is, judging not the actual 

 duration of flight, but the actual muscular power and 

 amount of energy used in flying (Fig. 59). The alba-, 

 tross, and other birds which, trusting to the air-currents 

 to bear them upward, flap seldom and soar much, have 

 comparatively smaller keels than do those birds which 

 flap their wings more frequently. Thus the pigeon has 

 a very good-sized keel; while in the humming-bird this 

 bone is enormous, compared to its spread of wings. Dr. 

 Frederick A. Lucas has expressed this very graphically 

 in a diagram, where it is supposed that the albatross, 

 pigeon, and humming-bird have an equal spread of wings. 

 On comparison, the keel of the first is seen to occupy but 

 a small fraction of the surface of the same bone in a 

 humming-bird. To account for this we must realize 

 that the wings of the humming-bird execute from six 

 hundred to a thousand strokes a minute; while the alba- 

 tross may soar for miles with wings held outstretched 



