110 
BIRDS 
completely change the air with each breathing movement and 
thus secure a greater supply of oxygen. The rapid oxidation 
requires that a large 
supply of food be di¬ 
gested and assimilated 
rapidly and it makes the 
normal body temperature 
of birds higher than that 
of other vertebrates. 
Plumage. — Birds are 
the only vertebrates hav¬ 
ing feathers and their 
plumage shows great 
variety in form and color. 
In some species there are 
certain colors which al¬ 
ways predominate on the 
males, while the females 
have little color; in other 
species it is hard to dis¬ 
tinguish between the sexes. The brilliantly colored males 
are supposed to attract the females at the mating season, 
while the dull colored females are inconspicuous and less 
likely to be attacked by enemies while 
hatching their eggs, or caring for their 
young. We may say, therefore, that they 
are protectively colored. The color of 
birds varies during the first two or three 
years of life. 
91. Classification. — Birds are usually 
divided into groups according to their 
structure. The shape and size of the beak 
and of the feet and wings are the charac¬ 
teristics most used in the general classifi¬ 
cation. This is illustrated by a single 
Figure 98. —Skele¬ 
ton of a Mallard 
Duck. 
A , Ostrich’s foot — adapted for running; 
B, duck’s foot — adapted for swimming; 
C, hen’s foot — adapted for scratching; 
D, plover’s foot — adapted for wading; 
E , hawk’s foot — adapted for tearing ; 
F, crow’s foot — adapted for perching ; G, 
woodpecker’s foot — adapted for climbing. 
