CLASS AVES. BIRDS 



447 



bird. Colors and color patterns of many 

 birds, as well as of other animals, conceal 

 them in their surroundings. A striking ex- 

 ample of protective coloration is the ptarmi- 

 gan, which is white in winter when snow is 

 on the ground, but a mottled brown in other 

 seasons. 



Bird songs 



The songs of birds, as previously ex- 

 plained, are produced by the air passing 

 through the syrinx. For one who wishes to 

 study birds, a knowledge of bird songs is 

 very helpful since one hears a great many 

 more birds than he is able to see. Songs 

 should be distinguished from call notes. The 

 former are usually heard during the breed- 

 ing season and are generally limited to the 

 males. Call notes, on the other hand, are 

 uttered throughout the year and correspond 

 in meaning and effect to our conversation. 

 The birds' songs and call notes serve ( 1 ) to 

 warn of danger; (2) to bring together birds 

 in a gregarious species; (3) for communica- 

 tion between parents and young; (4) to at- 

 tract mates; and (5) to announce nesting 

 territorv. 



Some birds, such as the pelican, are voice- 

 less. The great power of mimicry among 

 birds is interesting; among those possessing it 

 are the catbird, mocking bird, and parrots. 

 Do young birds sing instinctively the song of 

 their species? It appears that calls and simple 

 songs are instinctive, but the more elab- 

 orate songs require considerable practice by 

 imitation before perfection is attained. 



Bird flight 



One of the most important functions of 

 birds is that of flight. The bodies of flying 

 birds are boat-shaped so that they offer little 

 resistance to the air. The attachment of the 

 wings high up on the trunk, the high posi- 

 tion of such light organs as the lungs, the 

 low position of the hea\7 muscles and diges- 

 tive organs, the resulting low center of 



gravity, all tend to prevent the body from 

 turning over. 



Birds are able to glide or skim by spread- 

 ing their wings and then moving forward 

 by means of their acquired velocity. In soar- 

 ing, birds do not depend entirely upon ac- 

 quired velocity but rely upon favorable air 

 currents. 



The rates of speed at which birds fly. 

 vary considerably. The carrier pigeon in this 

 country attains a maximum racing speed of 

 about 60 miles per hour. Ninety miles per 

 hour has been recorded for ducks, but the 

 usual speed is 40 to 70. The duck hawk has 

 been clocked flying 165 to 180 miles per 

 hour in a nose dive. A speed of nearly 200 

 miles per hour has been reported for an 

 Old World swift. Most passerine (perching) 

 birds fly at a much slower speed. 



Formerly, birds were supposed to hiber- 

 nate during the winter in caves, hollow trees, 

 or, in the case of swallows, in the mud at 

 the bottom of lakes and ponds. We now 

 know that most birds spend the winter in a 

 more congenial southern climate, though a 

 few, such as the poor-wills, possess the un- 

 usual ability of becoming torpid, and ac- 

 tually hibernate. 



Bird migration 



Birds are the most famous of all animals 

 that migrate (Fig. 317). The world cham- 

 pion long-distance migrant is the arctic tern; 

 the extremes of its nesting and wintering 

 ranges are 11,000 miles apart. Since the 

 routes taken are circuitous, these terns may 

 fly 25,000 miles each year. In the autumn 

 some birds gather in flocks and move south- 

 ward, returning the following spring. Cer- 

 tain species migrate east and west. Birds that 

 breed in the far north may spend the winter 

 in parts of the temperate zone. Some birds 

 do not migrate; for example, the great 

 horned owl and bob-white. Certain other 

 birds move southward only when the 

 weather becomes very severe. 



Most birds migrate on clear nights at an 



