172 NEW ZEALAND NATURE-STUDY BOOK 
Certain parts of the body do not bear any feathers, but 
are covered by those on the ‘feather tracts.” 
The feathers may be grouped as (i) quill-feathers or 
those carried by the wings and tail, and hence used in 
flight ; (2) clothing-feathers, sometimes called contour- 
feathers, or those provided for the purpose of keeping the 
bird warm, and giving rise to those beautiful curves, 
which add so much to its grace and symmetry ; (3) down 
or small-feathers below the clothing-feathers, which aid 
still further in keeping in the warmth. These are well 
seen in water-birds such as the Duck, Goose, and Swan. 
An examination of a quill-feather will bring out the 
following details. The lower parts consist of a hollow 
horny quill, which is prolonged into a solid flattened 
shaft, from either side of which springs the web or vane. 
The outer surface of the vane is smooth, shiny, and slightly 
convex, the inner surface being relatively rough and concave. 
The shaft at its junction with the quill bears a small tuft of 
down. The vane is made up of a number of flat rod-like 
projections, the barbs, which spring from either side of 
the shaft. These in turn consist of numerous delicate 
threads, the barbules, placed on the barb in much the 
same way as the barbs are arranged on the shaft, Each 
barbule is furnished with a hook and flange, and the whole 
are arranged in such a way that the hooks of one row of 
_ barbules interlock with the flanges of the next row. By 
this simple expedient the vane maintains an unbroken 
surface, the wind being unable to force the barbs apart. 
The Air Supply. The work performed by birds in 
flight is perhaps greater in proportion to their size than 
that of any of the other vertebrates. A man, when taking 
violent exercise, requires to breathe quickly, ie. he has 
need of an increased supply of oxygen ; it is not surprising 
then that birds should be provided with breathing facilities 
