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 no} obliged to begin 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 N ovember; 
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 
