98 ON THE CLASSIFICATION OF BIRDS. 
graduated. Many of our own little warblers, whose 
flight is evidently feeble, and whose wings are almost 
rounded, leave us in autumn, and only return in the 
spring ; so that probably the very same journey is per- 
formed by birds of the most apposite qualities of flight. 
The ducks, which go to and from the northern regions 
at stated periods, afford us another instance of very short 
winged birds habitually migrating to great distances. It 
is obvious, nevertheless, that the degree of speed with 
which these several species proceed must entirely depend 
upon the formation of their wings. 
(88.) The TAIL of a bird is formed to act in unison 
with its wings, in performing the function of flight, 
during which it is expanded, and it is used as a rudder. 
There are very few birds without this member, although 
in many it is very short. It is longest in the llasorial 
types, and shortest in the Natatorial and GraUatorial: in 
the birds of this latter division, however, its office, in a 
great measure, would seem to he performed by the legs, 
which are extended backwards during fliglit, in the 
direction of the tail. W e can scarcely suppose, indeed, 
that the flight of the heron would be so steady and 
graceful, was the course of the bird simply directed by 
a tail which hardly exceeds three inches in length. It 
must not be supposed, however, that the degree of speed 
with which a bird flies is to be estimated by the de- 
velopment of this member ; for it generally happens that 
birds with very large tails have proportionahly short 
wings. The peacocks and turkeys are familiar instances 
of this relative disproportion, and the same may be 
traced through a large proportion of those birds by which 
they are represented in other groups. Hence it is evi- 
dent that the structure of the tail is in some birds more 
adapted for ornament than for flight. We shall now 
enumerate the different forms it assumes, and the espe- 
cial offices, where known, which these forms are intend- 
ed to perform. In speaking of the comparative length 
of this member we call that a .short tail which, at the 
most, does not exceed half the length of the wings ; a 
