AVES 



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leave in August or September, when there is still an abundance of 

 food. Another theory is that of securing better and more pro- 

 tected breeding grounds. Nest concealing is possibly a factor. 

 Chapman believes that the origin of this great pilgrimage " is 

 found in the existence of an annual nesting season," and that it is 

 exactly paralleled by the annual migration of certain fishes to their 

 spawning grounds, and the regular return of seals to their breed- 

 ing-rookeries. But what seems to us most strange is that the 



Fig. 225. Bower bird (Chlamydera maculala) with bower. (From Brehm.) 



same species of birds takes the same definite route of migration 

 for generations, except that its range is gradually lessened or 

 extended. Chapman gives as an illustration of the stability of 

 routes of migration the bobolinks, which are Eastern birds, now 

 spread westward to Utah, yet, instead of migrating directly south 

 through Texas and Mexico, they, "true to their inherited habit, 

 retrace their steps and leave the United States by the round- 

 a-bout way of Florida, crossing thence to Cuba, Jamaica, and 

 Yucatan, and wintering south of the Amazon." 1 The extent 

 1 Chapman's " Bird Life," p. 60. 



