THE BIRDS 211 



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 fiock 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 true, 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. 



