MIGRATION OF WARBLERS 19 



kill themselves by striking light-houses. When the dead birds at 

 the foot of any light-house are examined after a disastrous night, 

 more than half are always found to be Warblers. 



How the Warblers find their way in the long night journeys 

 is still a disputed point. Some believe that they are guided 

 entirely by sight and that mountain ranges and river courses form 

 prominent land marks to aid in finding the course. Others go to 

 the opposite extreme and attribute to a so-called 'sense of direction,' 

 the bird's wonderful success in retracing its way to the last year's 

 home. Still others think they have explained the case sufficiently 

 when they say the bird finds its way by instinct, while still others 

 deny the efficacy of instinct and affirm that the young birds are 

 led in their southward journey by the old birds, w^ho in turn 

 remember the route from their previous season's passage. All 

 observers are agreed that each Warbler intends to return each 

 year to the general vicinity of the last year's nest and that most of 

 them succeed. 



Almost as great a diversity of opinion exists as to the reasons 

 for bird migration, both as to its original cause and the factors 

 that at present work for its continuance. There are two general 

 theories in regard to the origin of migration. One, that the birds, 

 originally non-migratory, increased so in numbers that their home 

 became overcrowded and adventurous birds, passing beyond the 

 usual boundaries, found new and congenial nesting sites. From 

 these they were driven by the winter's shortage of food, to return 

 again the following summer. In this view of the case, the place 

 of residence in the winter is the bird's true home, which it deserts 

 in the summer for the purpose of reproduction. 



The second theory is the direct opposite of the one just given. 

 According to this second theory the nesting-site is the bird's real 

 home, from which it was driven originally by the advancing ice 

 of the Glacial Epoch, and the habit of migration thus induced has 

 been continued through the ages. Both theories base the origin 

 of migration on a failure of the food supply, the one a failure in the 

 winter home and the other in the summer. 



But whatever the cause, the migration of Warblers as now 

 conducted is at widely different periods. The Myrtle Warbler 

 presses north in the Spring when the trees are still bare of leaves, 

 while the Canadian Warbler forms one of the rear guard, after 

 vegetation has reached nearly full summer luxuriance. Instead 

 of waiting until the winter's cold and a shortage of food compel 



