6 CHANGES OF PLUMAGE. 
tant for purposes of classification. They have been specially studied, named 
and classified by the celebrated Nitzsch, who has laid down the following as 
the general plan obtaining in the vast majority of birds : — 
(6.) 1. The spinal or dorsal tract (pteryla spinalis, pl. 1, fig. 4, 1), 
running along the middle of the bird above from nape of the neck to the 
tail; subject to great variation in width, to dilation and contraction, to 
forking, to sending out branches, to interruption, ete. 2. The humeral tracts 
(pt. humerales, pl. 1, fig. 4,2), always present, one on each wing; narrow 
bands running from the shoulder obliquely backward upon the upper arm- 
bone, parallel with the shoulder-blade. 3. The femoral tracts (pt. femo- 
rales, pl. 1, fig. 4,3), a similar oblique band upon the outside of each 
thigh, but, unlike the last, subject to great variation. 4. The ventral tract 
( pt. gastrei, pl. 1, fig. 3,8), which forms most of the plumage on the 
under part of a bird; commencing at or near the throat, and continued to 
the anus; it is very variable like the dorsal tract, is usually bifurcate, or 
divided into right and left halves with a central apterium, is broad or nar- 
row, branched, etc. ; thus, Nitzsch enumerates seventeen distinct modifica- 
tions! The foregoing are mostly isolated tracts, that is, bands nearly 
surrounded by apteria that are complementary to them; the following are 
continuously, uniformly feathered, and therefore, in general, equivalent to 
the part of the body they represent. Thus, 5, the head tract (pt. capitis, pl. 
I, figs. 3,4; 4,4), clothes the head and generally runs into the beginning of 
both dorsal and ventral tracts. 6. The wing tract (pt. alaris, pl. 1, figs. 3, 5; 
4,5), represents all the feathers that grow upon the wing, except those of the 
humeral tract. 7. The tail tract (pé. caudalis, pl. 1, figs. 3,63; 4, 6), includes 
the tail feathers and their coverts, those surrounding the oil-gland, and usu- 
ally receives the termination of the dorsal, ventral, and femoral tracts. 8. 
The leg tract (pt. cruralis, pl. 1, figs. 3, 7; 4, 7), clothes the legs as far as these 
are feathered, which is sometimes to the toes, generally only to the heel. I 
need not give the spaces, as these are merely the complements of the tracts ; 
and the highly important special feathering of the wings and tail will be 
examined in describing those members for purposes of classification. 
§ 10. Progress anp Cuaner. Newly hatched birds are covered with a 
kind of down, entirely different from the feathers they ultimately acquire. 
It is scanty, leaving much of the body naked, in Altrices, or those birds that 
are reared by the parent in the nest ; but thick and puffy in a few of these, and 
in all Precoces, that run about at birth. But true feathers are soon gained, 
in some days or weeks, those of wings and tail being the first to sprout. The 
first plumage is usually only worn for a short time —then another is gained, 
and frequently several more changes ensue before the bird attains its mature 
covering. Feathers are of such rapid growth, that we can easily understand 
how exhaustive of vital energies the growth must be, and how critical a pe- 
riod the change is. The renewal of plumage is a process familiar to all under 
the term “ moult” (ecdysis). It commonly occurs at least once a year, and 
generally twice, in spring and fall; when old, faded and worn out feathers 
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