i 
38 SHAPE AND NUMBER OF RECTRICES. 
were they thick, not flat. This disposition is perceived at once in the accom- 
panying diagram, where it will also be seen that spreading of the tail is 
simply the greater divergence of a@ from 3, For 
while closing the tail is bringing @ and 6 to- — 
gether directly under c. The act is accom- Aes 
plished by certain muscles that pull on either = —— 
side at the bases of the quills collectively: a oe 
they are the same that pull the whole tail to one side or the other, just as 
tiller-ropes of a boat’s rudder work on that instrument. The general 
§ 68. Swarr of a rectrix, is shown in Pl. 1, fig. 5. The feather is some- 
what clubbed, or oblong, widening gradually and nearly regularly towards 
the tip, where it is gently rounded. But the obvious departures from this 
are various. A rectrix broad to the very tip, and there cut squarely off, is 
truncate; one such cut diagonally off is incised, especially when, as usually 
happens, the outline of the cut portion is concave. A linear rectrix is very 
narrow, with parallel sides; a danceolate one is broader at the base, and 
tapers regularly and gradually to a point. A noticeably pointed rectrix is 
acute; when the pointing is produced by abrupt contraction towards the tip 
it is called acuminate, as in woodpeckers generally. A very long, slender, 
more or less linear feather is said to be filamentous, as the lateral one of a 
barn-swallow or of most terns, the middle one of a tropic bird (gen. 278), 
ete. When such protrude suddenly and far beyond all the rest, I call them 
long-exserted, after an analogous term in botany. An unusually stiff feather 
is called rigid, as in woodpeckers and other birds that use the tail as a prop 
or support. When the rhachis projects beyond the vexilla, the feather is 
spinose, or better, mucronate (Li. spina, a prickle, or mucro, a point; e. g., 
chimney-swift, fig. 123). The bob-o’-link (gen. 87) and sharp-tailed finch 
(fig. 84) both approximate towards this condition. When the vexilla are 
wavy-edged, the feather is crenulate (fine example in Plotus, gen. 276). 
While the great majority of rectrices are straight, some are curved, either 
outwards or inwards, in the horizontal plane; those curved in a perpen- 
dicular plane are arched or vaulted—the latter particularly when the vanes 
are concayo-convex in transverse section. The typical 
§ 69. Numper of rectrices is Twetve. This holds in the vast majority 
of birds. It is so uniform throughout the great group Oseines, that the 
rare exceptions are perfectly anomalous; in the other group of Passeres 
(Clamatores) it is usually twelve, but sometimes fen. Among Sérisores 
there are never more than éen rectrices. In Scansores, the number varies 
from eight to twelve; eight is rare, as in the genus Crotophaga (no. 126) ; 
other cuckoos have ten; the woodpeckers have APPARENTLY fen, but there 
are really éwelve, of which the outer pair on each side are very small, almost 
rudimentary, hidden betwixt the bases of the second and third pair (see Key, 
III). Birds of prey have about twelve. Pigeons (all ours at least) have 
twelve or fourteen. In birds below these the number begins to increase ; 
thus directly, among the grouse, we may find up to twenty, as in the great 
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