434 APPLIED BIOLOGY 



and affect human interests. It is important to note that 

 such a grouping of birds with similar habits does^not always 

 correspond to a scientific classification based on similarity 

 of both external and internal structure. For example, there 

 are swimming birds in groups 2, 3, 4, 5 named above; but 

 loons, gulls, pelicans, and ducks show little structural evi- 

 dence of relationship aside from their swimming adaptations, 

 and hence, in a scientific classification, must be placed in 

 separate groups. However, those who do not specialize in 

 bird study cannot do better than remember the most familiar 

 birds either according to their habits, or as associated with 

 their relatives mentioned in groups 1 to 16 above. 



358. Instincts of birds are highly developed. They have 

 sharp eyes and a good memory. Parrots and ravens show 

 an extraordinary power of imitation and ability to take some 

 instruction. 



The most highly developed instincts of birds are connected 

 with migration and reproduction. It is well known that 

 most North American birds migrate northward in spring, 

 and after the breeding return southward in autumn. What 

 causes them to migrate, and especially what guides them on 

 the journey, has long puzzled naturalists. A vast amount of 

 information concerning the times and paths of migration of 

 many species in the Northern Hemisphere has been recorded 

 in the special books of ornithology. 



The distance covered in migrations varies with species. 

 Some move from the arctic to temperate regions in 

 autumn, and return in spring (e.g., certain " snow-birds ") ; 

 others migrate between tropical and temperate regions; 

 and still others go from tropical to arctic regions. Some 

 species migrate at night, others by day. Some migrate at 

 great heights. In some remarkable cases, the young birds 

 go southward before their parents ; and in other cases, the 

 parents go first. In these cases, we cannot understand how 

 the young are guided, unless by a few old birds which have 



