THE EXTERNAL STRUCTURE OF BIRDS. I4I 
plumage of their species and sex. Some Birds, e. 7., the Black 
Redstart (Ruticilla tithys) and the Rosy Bulltinch (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” ~. ‘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 bc//, consisting of an upper jaw, or 
maxilla, and of a lower jaw, or mandible t ¢, 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 i is the vertex, behind 
this is the occiput. 
* Tam indebted to Mr. Seebohm, F.L.S., for a knowledge of these. two 
examples. 
+ See below, p. 164. 
{ The maxilla and mandible are very often called the upper and lower 
mandibles. 
