EXTERNAL PARTS OF BIRDS. — THE TAIL. 117 



ttvelve, of which the outer one on each side is spurious, very small, and hidden hetween the 

 hascs of the second and tliird feathers. Birds of prey {Rnptoren) have about twelve. In 

 pigeons the rule is twelve or fmirteei], as in all onr genera; hut si.Kteeii are found in some and 

 twenty in one case. In hirds 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 Tetraonidce. Wading birds, often having but twelve, furnish in- 

 stances of as many as twenty. Those swimming birds with large well-fijrined tails, as the 

 Longipennea, and some Anatidm, have the fewest, as tvi'clve, sometimes fourteen, rarely 

 si.xteen ; those with short soft tails have the UKJSt, as sixteen to twenty-four. Among the 

 penguins there are thirty-two or more. The Archmopteryx 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 fjf the wing, are susceptihle 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 fcathei's long-exserted, as se(ni 

 in the jaegers (Stercorarius), and tropic-birds (Phaethon). The most fi'cquent 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, probal)ly the commouest form among birds ; thus, the gradation 

 between the middle and next pair may be just aj)preciable, and then increase regularly to an hich 

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

 gives the wedge-shaped or cuneate (Lat. cnneiis, ;i 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 (Pedicecetes'). 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 truly /offefZ or furcate (Lat. furca, a fork). ]5ut the degrees of furcation, like 

 tiiose of gradation, are so insensibly varied, that qualified expressions are usual; as, "slightly 

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

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

 and most of the sea-swallows (Sterna). An advisable terin 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 MilvuluH, and elsewhere. Double-forked and doiMe-rounded tails 

 are not uncommon; they result from combination of Ijoth opjiosite gradations, in this M'ay : 

 I'lie middle feathers being of a certain length, the next two or three jiairs 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 the tail being rounded ; 

 it is shown in the genera Myiadeste.'t and Anoufs. 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 



