EXTERNAL PABTS OF BIRDS. — THE TAIL. 
115 
Tail-Coverts are the numerous comparatively small and weak feathers which overlie and 
underlie the rectrices, covering their bases and extending a variable distance tOM^ard their 
ends, contributing to the firmness and symmetry of the tail. They pass smoothly out from 
the body, by gradual lengthening, there being seldom, if ever, any obvious outward distinction 
between them and feathers of the rump and belly ; but they belong to the pteryla caudalis 
(p. 87). The natural division of the coverts is into an upper and under set (tectrices super- 
iores, tectrices inferiores). The inferior coverts are the best distinguished from the general 
plumage, the anus generally dividing off these vent-feathers," as they are sometimes called. 
It is to the bundle of under tail-coverts, behind the vent, that the term crissum is most properly 
applied. Neither set is ever entirely wanting ; but one or the other, particularly the upper one, 
may be very short, as in a cormorant, or duck of the genus Erismatura, exposing the quills 
almost to their bases. While the upper coverts are usually shorter and fewer than the under 
ones, reaching less than half-way to the end of the tail, they sometimes take on extraordinary 
development and form the bird's chiefest ornament. The gorgeous, iridescent, argus-eyed 
train of the peacock consists of enormous tectrices, not rectrices ; the elegant plumes of the 
paradise trogon, Pharomacrus mocinno, several times longer than the bird itself, are like- 
wise coverts. Occasionally, a pair of coverts lengthens and stiffens, and then resembles true 
tail-feathers; as in the Ptarmigan (Lagopus). The crissal feathers are more uniform in 
development; they ordinarily form a compact, definite bundle, as well shown in a duck, 
where they reach about to the end of the tail. In some of the storks, they become plumes of 
considerable pretensions ; and in the wonderful humming-bird, Loddigesia mirabilis, the 
middle pair stiffens to resemble rectrices and projects far beyond the true taO. The 
Rectrices, Rudders, or true tail-feathers, like the remiges or rowers, are usually stiff, 
well-pronounced feathers, pennaceous to the very base of the vexilla, without after-shafts, as a 
rule, and with the outer web narrower than the other in most cases. They are always in 
pairs; that is, there is an equal number of feathers on the right and left half of the tail; and 
their number, consequently, is an even one. The exceptions to this rule are so few and 
irregular, and then only among birds with the higher numbers of rectrices, that such are 
probably to be regarded as mere anomalies, from accidental aiTest of a feather. They are im- 
bricated over each other in this wise : the central pair are high- ^ 
est, lying with both their webs over the next feather on either 
side, the inner web of one of these middle feathers indifferently 
underlying or overlying that of the other; all thus successively 
overlying the next outer one so that they would form a pyra- 
mid were they thick instead of being so flat. The arrange- 
ment is perceived at once in the accompanying diagram ; ~^ 
where it will be seen, also, that spreading the tail is the diver- * 
gence of a from^ft, while closing the tail is bringing a and h together under c. The motion 
is effected by certain muscles that draw on either side upon the bases of the quills collectively ; 
they are the same that pull the whole tail to one side or the other, acting like the tiller-ropes 
of a boat's rudder. The general 
Shape of a Rectrix is shown in fig. 23. Such a feather is ordinarily straight, some- 
what clubbed or oblong, widening a little, regularly and gradually toward the tip, where it is 
gently rounded off. But the departures from such shape, or any that could be assumed as a 
standard, are numberless, and in some cases extreme. In fact, none of a bird's feathers are 
more variable than those of the tail ; it is impossible to specify all the shapes they assume. 
While most are straight, some are curved — and the curvature may be to or from the middle 
line of the body, in the horizontal plane, or up and down, in the vertical plane. Some shapes 
