o 



s 



38 SHAPE AND NUMBER OF RECTRIC'ES. 



were they thick, nntfliit. This disposition is perceived at oiico in the accom- 

 pauying diajrrani, whore it will also he seen that uprendinrj of the tail is 

 simply the greater divergence of a from b, _^ 



while dodnij the tail is bringing a and b to- ^_ 



gether directly under c. The act is accom- 



plished hy certain muscles that pull on either _^ — 



ide at the bases of the quills collectivel}' : a '^b^ 



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. Shape of a rectrix, is shown in PI. i, fig. 5. The feather is some- 

 Avhat clubl)ed, or olilong, widening gradnally and nearly regularly towards 

 the tip, whore it is gently rounded. But the oljvious departures from this 

 are various. A rectrix broad to the very tip, and tliere 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 lanceolate one is broader at the base, and 

 tapers regularly and gi'adually 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, tlie middle one of a tropic bird (gen. 278), 

 etc. Wlien such protrude suddenly and far beyond all the rest, I call them 

 lonrj-exserted, after an analogous term in botany. An unusually stiff' feather 

 is called riijid, as in woodpeckers and other birds that use the tail as a prop 

 or support. AVhen the rhachis [)rojects bejMnid the vexilla, the feather is 

 sjnnose, or better, mucronale (L. spina, a [iriokle, or mucro, a point; e. g., 

 chimney-swift, tig. l^;]). The bob-o'-link (gen. 87) and sharp-tailed finch 

 (fig. 84) both approximate towards this condition. When the vexilla are 

 wavy-odged, the feather is crenulaie (fine example in Plotus, gen. 276). 

 "While the great majority of rectrices are sirair/ht, 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 concavo-convex in transverse section. The typical 



§ 69. Number of rectrices is twelve. This holds in the vast majority 

 of birds. It is so unifin'm throughout the great group Oscine», that the 

 rare exceptions are, perfectly anomalous; in the other group of Passeres 

 (Clamatores) it is usually twelve, but sometimes ten. Among tStrisores 

 there are never more than ten rectrices. In Scansores, the nuudier varies 

 from eight to twelve; eight is rare, as in the genus Crotophaga (no. 126) ; 

 other cuckoos have ten; the woodpeckers have apparently ten, but, there 

 are really twelve, of which the outer pair on each side arc 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, avc may lind up to twenty, as in the great 



