CYPSELIN^. 169 



The Swifts form a remarkable group of birds, organized speci- 

 ally both for speedy and continued flight, many of them being 

 capable of sustaining themselves in the air for the whole day 

 without once resting. They are distinguished from the Swallows by 

 having only ten tail feathers, by the wings being longer, narrower, 

 and more or less falcate, the first and second quills generally 

 about equal, and the secondaries short and hidden by the coverts ; 

 by the smaller and differently formed bill, and by the structure 

 of the feet. The gape is very wide ; there are no rictal bristles ; 

 the tarsus is short, and the toes are short, with sharp, strongly- 

 curved claws. 



The sternum is entire posteriorly, with the keel very large, 

 for the attachment of the powerful pectoral muscles. The humeri 

 are very short, and the furcula also short and much arched. The 

 stomach is muscular, the intestines short ; there are no coeca ; and 

 they have large salivary glands, which secrete a viscid mucus or 

 gelatine, of which they either compose their nest entirely, or they 

 use it much in consolidating other materials. The feet of most 

 have the toes composed of only two phalanges. They are usually 

 described as having all four toes directed forwards ; but the hind 

 toe is usually directed inwards, though capable of being placed 

 forwards, and in a few also opposable. 



These differences of structure have caused some naturalists, as 

 mentioned above, to totally separate the Swifts from the Swallows, 

 but a distinct, though it may not be a very close, link occurs in 

 Dendrochelidon, and the chief differences are only the much greater 

 development of the volar power, i. e. the pectoral muscles and 

 sternum, and other concomitant but subordinate points of organi- 

 zation. As to the number of tail feathers, we will see in the 

 Magpies that a difference of this nature barely makes even a 

 generic distinction. 



The whole organization of the Swifts is calculated to 

 produce the maximum power of flight, and they are the fastest 

 flying birds in the world, out early and late home, and even, it is 

 said, copulating on the wing. They rarely or ever descend to the 

 ground, and few of them perch even on trees, but roost in caverns 



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