XATURAL HISTORY OF BIRDS. 



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■X. 



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

 figure), the so-called ' iiiidillc coverts,' which in many birds, jiMrlicularly anmng the 



Passercs, are arranged in a manner dif- 

 ferent from the other feathers, as they 

 overlap each other with tlii'ir inner 

 edges, while all the other fi^athcrs have 

 the outer margin free, and the inner one 

 .covered by the overlying feathei-s. The 

 caudal tract embraces the tail feathers 

 (rectrices) and their u]>iier and under 

 coverts. They are in paii-s, and are 

 counted from the outside towards tlie 

 centre. Their number varies from eight 

 to thirty-twi), but twelve is the rule, less 



FlO. I. — Feathers of a passnrine wing, seen from above; the rare exception. 

 a, aliilu; p, primaries; Ic, leaser euverti*; tfr, greater t-» • i i 



coverts; ;«•, priniar.v coverts; nir, miiiiUo coverts; », Besides these iiorni:)! feathers there 



secondaries; «c, scapulars; /, lerliaries. , ... . . . 



are several modincations for special pur- 

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

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

 eating birds, :uul the eyelids of m:iny birds of i>rey, 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 hemmergeier), or dorsal, or on each side of the sjiinal 

 tract (some kites) ; or post-femoral and inguinal (herons, Leptosouta, tinamous)." AVe 

 may also mention the so-called semii>lumes, feathers intermediate between contour 

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

 ajiteria are neai-ly filled with them, and Garrod asks why they may not be regarded as 

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

 8emi]dumes are found in some forms, as, for instance, the ornamental feathers in the 

 Marabou stork {Leptoptilos duhms). 



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

 they have attained tiieir full size, the vascular j)apill:i enclosed in the quill dries uj), 

 fonning the ' ])ith,' and from that moment no further growth, nor any renewing of 

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

 are thrown off, shed, and replaced Ijy an outgrowth of new ones. This j)roce.ss, which 

 we call molting, presents some variations and modifications in the different groui)S 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, ami eiiually, 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 otiier 

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

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

 nmlt 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 a])pearance. 

 This renewal of a ]i;iit of the plumage is generally very rapid, and the time between 

 the autinnnal total molt and the ]>artial one in s]>ring, as a rule, iierhajis, .shorter than 

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



