THE TAIL 33 



wise, it is described as curved outward or inward, according to 

 the side which shows the bulging outline. 



Shape of the tail as a whole. — The usual shape of the tail is 

 like that of a fan, but there are many and very important modifi- 

 cations of this form. Some of these are characteristic of certain 

 groups and are much used in classification ; thus most terns 

 can be separated from the gulls by this feature alone. If 

 the tail feathers are even in length, the tail is said to be even, 

 square, or truncate/ If the central pair is the longest, and 

 each successive outer pair is shorter, the tail is graduated.^ If 

 each pair is shorter by a constant amount, the tail forms a 

 regular angle, and might, if at all common, be called an angu- 

 lated^ tail. A much commoner variety is said to have each 

 successive pair shorter by an increasing amount ; this forms the 

 rounded tail ; * sometimes each successive pair is shorter by a 

 decreasing amount, and this forms a icedge-shaped or cuneate 

 tail. If the central pair is excessively long, the tail is said to be 

 exserted;^ Avhen not so excessively elongated, it is jwinted. 



The opposite of graduation is very common among birds ; that 

 is, each successive outer pair is longer than the preceding pair. 

 If this is true merely to an inappreciable extent, as in the English 

 sparrow, the tail is emarginate ; ^- but when the difference is great 

 enough to make a very distinct angle, as in the chipping sparrow, 

 the tail is forked ; " and the prefixing of the word slightly or deeply 

 tells how great the forking is. In deeply forked tails, like those 

 of the barn swallow, the outer rectrices are narrowed so as to be 

 Jilamentous. Such tails are said to be forjicate.^ 



Sometimes there is a combination of the two plans above 

 given. If the middle pair is short, and about three pairs out- 

 ward are successively longer, and the last two successively 

 shorter, the outer and middle pairs having about equal lengths, 

 a doubly rounded^ tail is formed. If the middle pair is long 



8 9 



apgar's birds. — 3 



