38o A HISTORY OF BIRDS 



In many birds, such as Pelicans, Gannets and Cormorants, 

 the furcula is firmly fixed, articulating, by means of special joints, 

 with the end of the keel of the sternum below, and with the 

 coracoids above. In the Pelicans the union with the sternum 

 becomes still more complete, the joint between the furcula and 

 sternal keel giving place to an actual fusion of the juxtaposed 

 bones ; and in the Frigate-birds yet further rigidity is attained 

 by the fusion of the base of the coracoids with the end of the 

 sternum. The advantage of this rigidity seems apparent enough 

 in the case of the Gannets which invariably, and the Pelicans 

 frequently, capture the fish upon which they feed by violently 

 plunging upon their victims from a height ; but the same 

 mechanism obtains also in the Cormorants and Darters which 

 dive only from the surface of the water, and with closed wings. 

 Is this method of diving a recently acquired habit? In the 

 Frigate-birds, as we have remarked, fusion has rendered all 

 movement between the shoulder girdle and breast-bone impos- 

 sible, yet these birds rarely dive ; since, however, much, if not 

 most, of their food is obtained by rapid plunges through space, 

 as they endeavour to catch the fish disgorged by Gannets and 

 other birds — which they harass for this purpose — 'before it reaches 

 the water, it would seem that this peculiar mechanism has been 

 developed to facilitate the rapid downward thrusting of the 

 body through space rather than to meet the impact with the 

 water. This view is borne out by the fact that the Eagles and 

 Falcons also present similar modifications of this kind, the great 

 width and breadth of the furcula compensating for the fact that 

 it does not articulate with the keel of the sternum. 



The intimate connection of the furcula with the coracoids 

 and sternum just described is, however, by no means essential 

 for strong and long-sustained flight, inasmuch as in the Pigeons 

 and Sand-grouse which fly extremely well, the furcula is a very 

 delicate bone and only slightly attached to the coracoids. In 

 some Parrots this bone is wanting altogether, though it is 

 admitted, of course, that Parrots are not great fliers. 



As a rule a deep keel to the breast-bone (sternum) is corre- 

 lated with vigorous and long-sustained powers of flight, as in 

 the case of the Swift, Humming-bird, Sand-grouse and Pigeon, 

 for example ; conversely, a low keel indicates a feeble flight. In 

 the Eagles and Steganopodous birds, e.g:, Gannet and Pelican, 



