WHY DO BIRDS MIGRATE? 55 



short, we cannot but feel that this experiment constitutes the strongest 

 argument for the existence of a sense of direction as yet derived from 

 the study of wild birds; with this established, the so-called 'mystery 

 of migration' becomes no more a mystery than any other instinctive, 

 functional activity. 



Why Do Birds Migrate? Any attempt to reply to this question 

 should be prefaced by the statement that birds have been migrating 

 for an incalculable period. The existing phenomena are not therefore 

 to be explained solely by observable causes, but they may often have 

 their origin in influences which have long ceased to be potent. In 

 other words, the migration of birds, as well as the birds themselves, 

 is an outcome of those gradual adjustments between an organism and 

 its environment which has led, on the one hand, to activities which 

 existing causes only in part explain, and on the other to the evolu- 

 tion of certain types of form and color the reason for which we cannot 

 now wholly determine. We observe that bird migration is most highly 

 developed in those parts of the world which are subjected to marked 

 seasonal changes. In endeavoring, therefore, to ascertain the factors 

 governing migration either north or south of subtropical regions, we 

 find our problem greatly complicated by questions of temperature and 

 food which seem to exert a powerful influence on the movement of 

 birds. 



Fortunately we are not obliged to be*gin our examination of the 

 subject in this, its most complex form, but in the tropics may find 

 perfectly well-defined instances of bird migration in which the matters 

 of food and temperature seem to play no part. 



With tropical land-birds there is, as a rule, no well-marked migra- 

 tion; while their numbers may fluctuate in response to an increasing 

 or diminishing supply of food, they make no journeys to a nesting- 

 ground. 



Tropical sea-birds, however, are often great wanderers and, dur- 

 ing the year, many cover vast distances, within the tropic zone, in 

 their search for food. They cannot, however, nest on the water, and 

 when the season of reproduction approaches, they are, of necessity, 

 forced to go to the land. Now it is of the highest importance for us 

 to know that their visits to their breeding resorts are made with the 

 same regularity which attends the journey of Oriole, Bobolink or 

 Warbler. They return each year to the same place and they all reach 

 it, almost to the day, at the same time. The Brown Pelicans of eastern 

 Florida come in thousands to Pelican Island the first week in November; 

 Boobies and Man-o'-war-birds return to certain Bahama keys in 

 January; the Noddy and Sooty Terns appear on Bird Key in the Tor- 

 tugas, the last week in April. 



Temperature, obviously, has nothing to do with these journeys, 

 since with the Pelicans the average daily temperature is decreasing, 

 with the Terns increasing, while the Boobies and Man-o'-war Birds 

 have probably experienced no change of temperature. Nor are the 



