THE CONDOR 
VOL. VI 
1 16 
from these various sources form the largest amount of material on bird migration 
ever collected in this country, and permit broader and safer generalizations than 
have heretofore been possible. 
CAUSES OF MIGRATION. 
For more than two thousand years the phenomena of bird migration have 
been noted; but while the extent and course of the routes traversed have of late 
become better known, no conclusive answer has been found to the question, wdiy 
do birds migrate? Some dismiss the subject with the statement that fall migration 
is caused by failure of the food supply, spring migration by love of home. All are 
familiar with the rush of w'aterfowl northward so early that they are often forced 
by storms to retrace their flight; and all know that robins, bluebirds, and swallows, 
following closely in the rear, sometimes lose hundreds out of their flocks by cold 
and starvation. If strong home love causes these birds thus to hazard their lives, 
why do they desert their home at the earliest possible moment; and if fall migra- 
tion is caused by lack of food, why does it commence when food is most abundant? 
Data recently collected at the Florida light-houses by the Biological Survey show 
that southward migration begins at least by the loth, and probably by the ist of 
July, insect-eating birds departing when their food supplies are most plentiful, and 
seed eaters just before the heyday of harvest. 
The broad statement can be made that the beginnings of migration ages ago 
were intimately connected with periodic changes in the food supply, but this 
motive is at present so intermingled with others unknown, or but imperfectly 
known, that migration movements seem now to bear little relation to the abund- 
ance or absence of food. 
HOW DO BIRDvS FIND THEIR WAY? 
How do birds find their way over the hundreds or thousands of miles between 
the winter and summer homes? Among day migrants sight is probably the prin- 
cipal guide, and it is noticeable that these seldom make the long single flight so 
common with night migrants. Sight undoubtedly plays a part in guiding the 
night journeys also; on clear nights, especially when the moon shines brightly, 
migrating birds fly high, and the ear can scarcely distinguish their faint twitter- 
ings; if clouds overspread the heavens, the passing flocks sink their course nearer 
to the earth, and their notes are much more distinctly heard; and on very dark 
nights one may even hear the flutter of vibrant wings but a few feet overhead. 
So far as known, birds never intentionally migrate above the clouds, and when 
suddenly formed vapor cuts them off from sight of the earth, they lower their flight 
until the friendly landscape is again visible. Nevertheless, something besides sight 
guides these travelers in the upper air. In Alaska a few years ago members of the 
Biological Survey on the Harriman expedition went by steamer from the island of 
Unalaska to Bogoslof Island, a distance of about sixty miles. A dense fog had 
shut out every object beyond a hundred yards. When the steamer was half way 
across, flocks of murres, returning to Bogoslof after long quests for food, began to 
break through the fog wall astern, fly parallel with the vessel, and disappear in 
the mists ahead. By chart and compass the ship was heading straight for the 
island; but its course was no more exact than that taken by the birds. The power 
which carried them unerringly home over the ocean wastes, whatever its nature, 
may be called a sense of direction. It is probable that this faculty is exercised 
during migration. 
Reports from light-houses in southern Florida show that birds leave Cuba on 
