38 



NATURAL HISTORY 



have been given to account for the long-distance migrations of ducks, 

 geese, the Arctic tern, golden plover, and other remarkable feathered 

 travelers. Food cannot be the deciding factor, jjecause many birds 

 leave for the south while food is still abundant. Neither can tempera- 

 ture be the only cause, because a 

 majority of migrating birds go 

 south when the weather is still 

 warm, while robins and other 

 lairds often stay behind and win- 

 ter successfully in cold climates. 

 Humidity, atmospheric pressure, 

 winds, have all been considered 

 as playing a part in migration, 

 but it is more likely that some- 

 thing within the bird rather than 

 any external environmental factor 

 is the impelling cause for this 

 impressive phenomenon. For in- 

 stance, among the hormones pro- 

 duced by the ductless glands, are 

 sex hormones which may stimu- 

 late the bird to the extraordinary 

 activity that results in long mi- 

 gratory flights. How to account 

 for the direction and exactness of 

 these migratory flights is another 

 matter, even more difficult to 

 explain. 



Changing climatic conditions 

 probably influence plants more 

 directly than animals, because the 

 latter are more capable of move- 

 ment, and, consequently, better able to escape from unfavorable 

 surroundings. Nevertheless, living things make up a world of 

 shifting organisms, always on the move. 



Ways of Locomotion 



Much of the delight that the naturalist experiences comes from 

 observing and interpreting the ways and devices by which the move- 

 ments of organisms are brought about. 



WINTER HOME 



The annual migration routes of the 

 Arctic tern. It covers about 22,000 

 miles in its yearly round trip from its 

 winter range in the Antarctic to the 

 summer breeding range in the Arctic. 

 Note the different routes taken going 

 and coming. 



