﻿EXTERNAL ANATOMY. FORKED TAIL. 103 



(9&) 9- Forked tails are of two kinds ; one which 

 presents a single, the other a double, fork. There can be 

 no doubt that this structure has great influence in faci- 

 litating the evolutions of such birds as possess it ; for 

 we invariably find that fork-tailed birds are excellent 

 flyers. Nevertheless, it must not be considered as 

 necessary to extreme swiftness, for although it is found 

 in many swallows, one half of the family have the tail 

 more or less even. The swifts, whose rapid flight is 

 perhaps superior even to that of the swallows, have the 

 tail perfectly even ; and there are comparatively few of 

 the oceanic birds belonging to the Petrels and neigh- 

 bouring genera — which rarely approach land — pos- 

 sessing a forked tail. It is hardly necessary to say, that, 

 by this structure, the two outer feathers are by far the 

 longest, the others gradually shortening until the gra- 

 dation terminates in the central or middle pair. The 

 house swallow is a notable instance of this form, but, 

 perhaps, no two genera possess it in a higher degree, 

 considering their size, than that of Psalurus, among 

 the night-jars (Caprimulgidce) , and Milvulus among 

 the tyrant fly-catchers. The scissar-tail (Gubernetes) , 

 which is almost equally remarkable in this respect, as 

 stated by Azara, is in the habit of opening and con- 

 tracting its tail during flight, and has hence received a 

 name from the Indians, likening the tail to a pair of 

 scissars. It is unnecessary to cite further instances of a 

 simply forked tail, particularly as they occur in nearly 

 all natural groups, without having the peculiar charac- 

 teristic of any one. It is nevertheless remarkable that 

 not one instance of this structure occurs in the Scansorial, 

 or climbing tribe, and very few even among the most aber- 

 rant of the Rasorcs. Doubly-forked tails, on the other 

 hand, are very rare (fig, 51.). The most remarkable 

 is that of a species of night-jar (Psalarus bifurcatus) 

 we discovered in Brazil ; in this, not only the two 

 outer, but the two middle tail feathers are greatly 

 elongated, the intervening lateral ones being gradu- 

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