THE ANIMAL CLIMAX-THE CHORDATES 



333 



In order to secure its food and avoid its 

 enemies the bird is compelled to rely on its 

 keen eyesight. Its large eyes are protected 

 by bony capsules which apparently prevent 

 the pressure of passing air from forcing the 

 eyes back into the head. The brain also is 

 considerably larger than in reptiles. How- 

 ever, the difference in size is owing prima- 

 rily to those parts having to do with sharp 

 vision, as well as balance and muscular 

 coordination, functions essential to flight. 



The birds in general exhibit some rather 

 fascinating habits which are not always 

 well understood, as, for example, the return 

 of a pair of purple martins each year to a 

 particular bird house on precisely the same 

 day, the building of a specific type of nest 

 by the young bird without ever having 

 done it before, or, and still more perplex- 

 ing, the migrations of many species cover- 

 ing vast distances, often over great bodies 

 of water. Many observations have been 

 made, demonstrating that birds do these 

 feats with remarkable regularity, and all 

 efforts up to the present to discover the 

 sense involved have been unsuccessful. 



It has been thought that birds return to 

 their nesting grounds, and in many species 

 to the same nesting site, year after year at 

 particular times because they are able to 

 measure the exact angle of the sun. This is 

 logical when one recollects what the bees, 

 using similar devices, are able to do. Crows 

 taken several hundred miles from their 

 nesting groimds and released will return 

 ( almost to a bird ) within the period of time 

 predicted, which has been based on the 

 "cruising" speed of the bird. One might 

 suspect that the bird would recognize land- 

 marks along the route and use them to 

 guide it back again. The birds have been 

 kept in covered cages or actually placed on 

 turntables and swung continuously through- 

 out the trip, but even then they will return 

 in the expected time. Some have guessed it 

 to be simply a matter of chance — the birds 

 merely fly at random and finally stumble 

 upon their way home. That this is not true 



F!g. 13-53. Skeleton of the domestic fowl. The skeletal 

 design is adapted to accommodate flight and bipedal 

 locomotion. The large keel provides a secure point of 

 attachment for the powerful wing muscles. 



has been shown by many experiments. For 

 instance, if some birds are taken twice as 

 far from home as others and released simul- 

 taneously, the birds farthest away should 

 require four times as long to get back by 

 the wandering technique. This is not the 

 case, however, because they arrive in about 

 one half the time that it would take them if 

 they flew on in a random manner. This 

 homing instinct, present to some extent in 

 other animals, is remarkably developed 

 among the birds, but its explanation is still 

 a mystery. 



Why or how birds are able to follow such 

 varying paths in their migrations is as 

 mysterious as their homing instincts. Every- 

 one is conscious of the fall migration of 

 great flocks of birds all heading south. Of 

 course, a possible explanation is immedi- 

 ately obvious: once the breeding season is 

 over and the young are able to care for 

 themselves, the approaching winter with its 

 accompanying food shortages and severe 

 cold might be reason enough for heading 



