EXTERNAL PARTS OF BIRDS, — THE TAIL. 123 



have apparently ten; but really twelve, of which the outer one on each side is spurious, very 

 small, and hidden between the bases of the secoml and third feathers. Birds of prey (Baptores) 

 have about twelve. In pigeons the rule is twelve or fourteen, as in all our genera; but sixteen 

 are found in some and twenty in one case. In birds below these, the number increases directly; 

 there are often or usually more than twelve in the grouse, and there may be sixteen, eighteen, 

 or twenty, as among our own genera of Tetraonidcc. Wading birds, often having but twelve, 

 furnish instances of as many as twenty. Those swimming birds with large well-formed tails, 

 as the Longipennes, and some Anatidce, have the fewest, as twelve, sometimes fourteen, rarely 

 sixteen ; those with short soft tails have the most, as sixteen to twenty-four (forty in some 

 domestic pigeons). Among the penguins there are thirty-two or more. Tlie Archceopteryx 

 appears to have had forty, — a pair to each free caudal vertebra; and this may be considered 

 the prototypic relation between the bones and feathers of the tail. The 



Typical Shape of the Tail, as a whole, is the fan. The modifications of form, how- 

 ever, which are greater and more varied than those of the wing, are susceptible of better 

 definition, and many of them have received special names. Taking the simplest case, where 

 the rectrices are all of the same length, we have what is called the even, square, or truncate 

 tail. The other forms depart from this mainly by shortening or lengthening of certain 

 feathers. A tail nearly or quite even may have the two central feathei-s locg-exserted, as seen 

 in the jaegers (Stercorarius), and tropic-birds (Phaethon). The most frequent departure from 

 the even shape results from gradual shortening of successive rectrices from the middle to the 

 outer ones. This is called, in general, gradation or graduation (Lat. gradus, a step) ; such 

 shortening may be to any degree. More precisely, graduation means shortening of each 

 successive feather to the same extent, — say, each half an inch shorter than the next; but 

 such exactitude is not often expressed. When the feathers shorten by more and more, we 

 have the true rounded tail, probably the commonest form among birds ; thus, the gradation 

 between the middle and next pair may be just appreciable, and then increase regularly to an inch 

 between the next and the lateral feather. The opposite gradation, by less and less shortening, 

 gives the wedge-shaped or cuneate (Lat. cuneus, a wedge) tail ; it is well shown by the 

 magpie {Pica) in which, as in many other birds, the middle feathers would be called long- 

 exserted were the rest all as short as the outer one is. A cuneate tail, especially if the feathers 

 be narrow and lanceolate, is also called acute, or pointed, as in the sprig-tailed duck (Dafila) 

 or sharp-tailed grouse (Pedioecetes) . The generic opposite of the gradated is the forked tail ; 

 in which the lateral feathers successively increase in length from the middle to the outermost 

 pair. The least appreciable forking is called emargination, and a tail thus shaped is said to be 

 emarginate ; when it is better marked, as, for instance, an inch of forking in a tail six inches 

 long, the tail is imly forked or furcate (Lat. furca, a fork). But the degrees of furcation, like 

 those of gradation, are so insensibly varied, that quaUfied expressions are usual; as, ''shghtly 

 forked," ''deeply forked." Deep furcation is usually accompanied by more or less narrowing 

 or filamentous elongation of the lateral pair of rectrices, as in the barn swallows (Hirundo) 

 and most of the sea-swallows (Sterna). An advisable term to express such an extreme furca- 

 tion is forficate (Lat. forfex, scissors), when the depth of the fork is at least equal to the 

 length of the shortest feathers ; it occurs among our birds in those last named, in the species 

 of the flycatcher genus Milvulus, and elsewhere. Douhle-forked and double-rounded tails 

 are not uncommon ; they result from combination of both opposite gradations, in this way : 

 The middle feathers being of a certain length, the next two or three pairs progressively 

 increasing in length, and the rest successively decreasing, the tail is evidently forked centrally, 

 rounded externally, which is the double-rounded form, each half of tlie tail being rounded ; 

 it is shown in the genera Myiadestes and Anous. Now if with middle feathers as before, 

 the next pair or two decrease in length, and then the rest increase to the outermost, we have 



