THE BIRDS 208 
arise to take their place. This process of molting is 
usually accomplished gradually, without diminishing the 
powers of flight; but in the ducks and some other birds all 
the wing- and tail-feathers drop out simultaneously, leaving 
the bird to escape its enemies by swimming and diving. 
The molting-process usually takes place in the fall, after 
the nesting and care for the young is over, and often when 
the need for a heavy winter coat commences to be felt. 
Many birds also don what are called courting colors, ruffs, 
crests, and highly colored patches, in the spring, previous 
to the mating season, doubtless for the purpose of attract- 
ing or impressing their mates. In other cases the change 
appears to be related to the bird’s surroundings. A most 
beautiful example of this is the ptarmigans—grouse-like 
birds living far to the north. During winter they are per- 
fectly white and are almost invisible against the snow; but 
in the spring, as the snow disappears, the white feathers 
gradually fall out and new ones arise. The latter so har- 
monize “with the lichen-colored stones among which it 
delights to sit, that a person may walk through a flock of 
them without seeing a single bird.” 
There are also numerous birds, chiefly those that go in 
flocks, which possess what are known as color-calls or recog- 
nition-marks. These may consist of various conspicuous 
spots or blotches on different parts of the head or trunk, 
such as we see in the yellowhammer or meadow-lark; or 
one or more feathers of the wings or tail may be strikingly 
colored, as in many sparrows and warblers. During the 
time the bird remains at rest these usually are concealed 
under neighboring feathers, but during flight they are 
strikingly displayed. It may possibly be truc, as many 
have urged, that these color-signals are for the purpose 
of enabling various members of the flock to readily follow 
their leader; but this and many other interesting questions 
regarding the color of birds and other animals have not yet 
received final answers, 
