SEC. in EXTERIOR PARTS OF BIRDS 131 



the part of the body they represent : Thus, 5, the head tract {j)t. 

 capitaliSj Lat. caput, capitis, head; Fig. 24, 4) clothes the head, and 

 generally runs into the beginning of both dorsal and ventral tracts. 

 6. The wing tract {pf. alaris ; Lat. ala, wing ; Fig. 24, 5) repre- 

 sents all the feathers that grow upon the wing, excepting those of 

 the humeral tract. 7. The tail tract {^pt. caiidalis ; Lat. cauda, tail ; 

 Fig. 24, 6) includes the tail-feathers proper and their coverts, and 

 those about the elseodochon, and usually receives the termination of 

 the dorsal, ventral, and femoral tracts. 8. The leg tract {pt. 

 cruralis ; Lat. cms, cruris, leg ; Fig. 24, 7) clothes the legs as far as 

 these are feathered, which is generally to the heel, always below 

 the knee, and sometimes to the toes or even the claws. — I need not 

 enumerate the apteria, as these are merely the complements of the 

 pterylse. The highly important special " flight-feathers " of the 

 wings and " rudder-feathers " of the tail are to be examined beyond, 

 in describing those members. 



Endysis and Eedysis. — Putting on and off Plumage. — Newly 

 hatched birds are covered for some time with a kind of down, 

 entirely different from such feathers as they ultimately acquire. 

 It is scanty, leaving much or all of the body naked, in most altricial 

 birds, such as are reared by the parents in the nest (Lat. altrix, 

 female nourisher) ; but thick and puffy in some AUrices, and in all 

 Prcecoces (Lat. prcecox, precocious) which run about at- birth. Since 

 many birds which require to be reared in the nest are also hatched 

 clothed, or very speedily become downy, a more exact distinction 

 may be drawn by using the terms ptilopcedic and psilopcedic (Gr. 

 tttIXov, ptilon, a feather ; \//iX6s, psilos, bare ; and irats, pais, a child) 

 respectively for those birds which are hatched feathered or naked 

 — a chicken and a canary-bird are familiar examples. It is the 

 rule that the higher birds are born helpless and naked, requiring to 

 be reared in the nest till their feathers grow ; the reverse with 

 lower birds, as the walking, wading, and swimming kinds. It 

 offers, however, many exceptions ; thus, no birds are more naked 

 and helpless at birth than young cormorants. Probably all pree- 

 cocial birds are also ptilopsedic, and all psilopjedic birds altricial ; 

 but the converse is far from holding good, many AUrices, as hawks 

 and owls, being also ptilopsedic. In other words, psilopsedic birds 

 are always altricial, but ptilopsedic birds may be either altricial or 

 praecocial. In any case, true feathers are soon gained, in some 

 days or weeks — those of the wings and tail being usually the first 

 to sprout. The acquisition of plumage is called endysis (Gr. 'ivSvcn's, 

 endusis, putting on). The renewal of plumage is a process familiar to 

 all, in its generalities, under the term " moult," or eedysis (Gr. c/cSuo-ts, 

 ekdusis, putting off). Feathers are of such rapid growth, and make 

 such a drain upon the vital energies, that we easily understand how 



