THE BXTEBNAL STETJCTUEB OF BIEDS. I4I 



plumage of their species and sex. Some Birds, e. g., the Black 

 Eedstart (Rutidlla tithys) and the Rosy Bullfinch {Pyrrhula 

 rosacea), however, rear a progeny before attaining it *. 



The young of most Birds do not shed the quill-feathers in 

 their first year, and in many even an otherwise complete moult 

 does not seem to take place during that period. 



When the plumage of the sexes differs, the young resembles 

 the mother, save when the latter is more conspicuously coloured 

 than her mate, in which case they resemble the male parent. 



When the adults of both sexes are alike, the young is different 

 from either. The young of both the black and the white Swan 

 are of a dusky colour, while the black-necked Swan has white 

 young. ' 



It has been supposed that when adult birds assume at the 

 breeding-season a plumage which differs from their winter 

 dress, the young are intermediate in colour. The Linnet alone, 

 however, suffices to disprove this dictum. 



Feathers do not by any means grow — save in rare cases — 

 all over the body of Birds, but only along certain definite tracts, 

 the forms and arrangements of which are very characteristic of 

 different kinds. Such an arrangement in a Bird is called its 

 pterylosis, and the special description of these conditions is 

 called " pterylography " f. This does not apply to down. 



While considering the form and structure of different parts 

 and appendages of the external skeleton, it will be well also to 

 note the conditions presented by the body as a whole and its 

 various parts and members. In other words, we must study 

 the external topography of Birds — their head, neck, body, tail, 

 wings, and legs. 



The Head and Neck. 



The Head. — This is always more or less rounded and pyra- 

 midal, and almost always covered with feathers. It terminates 

 in front in the beak or hill, consisting of an upper jaw, or 

 maxilla, and of a lower jaw, or mandible %, each of which is pro- 

 vided with a more or less horny investment, and is naked or bears 

 but a few feathers. The crown of the head is the vertex, behind 

 this is the occiput. 



* I am indebted to Mr. Seebohm, RL.S., for a knowledge of these two 

 examples. 



t See below, p. 164. 



t The maxilla and mandible are very often called the upper and lower 

 mandibles. 



