NATURAL HISTORY OF BIRDS. 



alula; p, primaries; Ic, lesser coverts; gc, greater 

 coverts; pc, primary coverts; me, middle coverts; s, 

 secondaries; sc, scapulars; t, tertiaries. 



a long description ; but we would like to call attention to the middle row (me in the 

 figure), the so-called ' middle coverts,' which in many birds, particularly among the 



Passeres, are arranged in a manner dif- 

 ferent from the other feathers, as they 

 overlap each other with their inner 

 edges, while all the other feathers have 

 the outer margin free, and the inner one 

 covered by the overlying feathers. The 

 caudal tract embraces the tail feathers 

 (rectrices) and their upper and under 

 coverts. They are in pairs, and are 

 counted from the outside towards the 

 centre. Their number varies from eight 

 to thirty-two, but twelve is the rule, less 



FIG. 1. Feathers of a passerine wing, seen from above; the rare exception. 



Besides these normal feathers there 

 are several modifications for special pur- 

 poses ; filoplumes, with slender axis and rudimental barbs, are often merely for ornament, 

 while the hair-like vibrissae, which have no barbs at all, line the mouths of many insect- 

 eating birds, and the eyelids of many birds of prey, toucans, and ostriches. " Some 

 plumes have the barb-tips breaking off as dust (powder-down), and these may be scat- 

 tered (and transitory, as in the laemmergeier), or dorsal, or on each side of the spinal 

 tract (some kites) ; or post-femoral and inguinal (herons, Z/eptosoma, tinamous)." We 

 may also mention the so-called semiplumes, feathers intermediate between contour 

 feathers and down, and occupying the edges of the feather-tracts; in the hoatzin the 

 apteria are nearly filled with them, and Garrod asks why they may not be regarded as 

 degenerated feathers ; they are usually concealed by the contour feathers, but long 

 semiplumes are found in some forms, as, for instance, the ornamental feathers in the 

 Marabou stork (Leptoptilos dubius). 



Feathers, generally, do not, like hairs, continue to grow indefinitely. Where 

 they have attained their full size, the vascular papilla enclosed in the quill dries up, 

 forming the ' pith,' and from that moment no further growth, nor any renewing of 

 tissue, takes place in the feather. Therefore, as soon as the feathers are worn out, they 

 are thrown off, shed, and replaced by an outgrowth of new ones. This process, which 

 we call molting, presents some variations and modifications in the different groups of 

 birds, but may, as a rule, be said to take place annually after the breeding season, with 

 its wear and tear to the feathers, is over. During this general molt, all the feathers, 

 including wing and tail feathers, are shed gradually, and equally, on both sides of the 

 median line of the body ; the feather of one wing is thrown off simultaneously with 

 the corresponding one of the other, and the same relation takes place in the molt of 

 the feathers in each half of the tail. It is the exception, when ducks and some other 

 birds lose all the wing feathers at once, thus being deprived of the power of flight for 

 a short time. While wing and tail feathers are only molted once a year, a partial 

 molt of the smaller feathers often takes place early in spring, at which time also 

 most of the ornamental feathers, borne only a short time, make their appearance. 

 This renewal of a part of the plumage is generally very rapid, and the time between 

 the autumnal total molt and the partial one in spring, as a rule, perhaps, shorter than 

 between the spring and the autumn changes, sometimes being often a brief period of 



