254 POULTRY AND WILD BIRDS 
factors in classification, but in field work not so much atten- 
tion can be paid to it, as the details of structure cannot gen- 
erally be distinctly seen. A few points about the bill will 
therefore suffice for the present purpose. 
(1) The two parts are called the upper and lower mandi- 
bles. They consist of projections of the skull bones covered 
by a horny material. 
(2) The bill is said to be of medium length if it is about as 
long as the head ; if longer than-the head, it is considered long, 
and if shorter it is called short. 
(3) A weak, slender bill shows that the owner must live 
on insects. A short, conical bill, fitted for crushing seeds, 
indicates the seed eater, and a stout, hooked bill indicates a 
bird of prey. Other interesting forms are the long, curved 
bill of the Curlew and the crossed mandibles seen in the 
Crossbills. 
THE WINGS 
Structure. —In the skeleton of the wing there are three 
joints, corresponding to the upper arm, forearm, and hand in 
human anatomy. Each of these is provided with a row of 
large, stiff feathers or quills. The largest and most impor- 
tant of these, the primaries, are on the last joint (the hand). 
The secondaries are on the forearm, and the tertiaries are on 
the upper arm. These quills form the spread of the wing and 
are the feathers on which flight depends. The others are 
smaller and serve to cover the bases of the quills and to dress 
the wing. They are called wing coverts. 
The Form of Wing varies greatly. It is long and pointed 
in birds of easy flight, like Swallows, Hawks, and Terns, and 
those whose flight is more labored have short, rounded 
wings. 
