OCTOBER, 1902. 
THE MIGRATION OF BIRDS. 
191 
It will be needless to go further into detail regarding this mat- 
ter. A little reflection shows readily that all birds are dependent 
upon certain kinds of food, that they are especially fitted by na- 
ture to obtain this particular food, and in nearly all cases are 
physically incapable of obtaining any other, and hence that in 
order to insure their existence they must migrate when their par- 
ticular kind of food fails. In winter the food of our summer resi- 
dents fails. Their animal food is not found where they are wont 
to capture it and their vegetable food is scarcely to be found at 
all. The food of the robin lies below the snow, that of snipe and 
sandpiper is covered by ice, and that of swallow and nighthawk 
is not now on the wing. The cherry tree is bare to the rose- 
breasted grosbeak, and for the warbler or vireo there is not even 
a leaf where he may look for an insect. 
It has been maintained for years that the question of food 
will never explain the vernal migrations, however well it serves 
that purpose in autumn. Our study of the phenomena pertain- 
ing to bird life, until recently, has been very limited in the south. 
In the north, however^ these phenomena have been worked out 
in detail. It has been shown that the most important factor in 
the fall migration is the food supply. The assumption that the 
question of food will not explain the vernal migration has prob- 
ably been due to our ignorance of the physical conditions of the 
south. According to Dr. Aug. Weismann, ponds, rivers and 
creeks become dry, insects disappear and even vegetation fails 
in many regions in the south in summer. Dr. C. H. Merriam 
has more recently discussed this question from the same stand- 
point. Mr. C. R. Ricker says : ''During the dry season many 
forest trees shed their leaves and the grass and small shrubs 
wither, while the palms and the majority of other trees bear their 
fruit; * * * *" (35). Hence we are at present quite safe 
in saying that birds must migrate both in spring and fall because 
the food supply fails. In fact, the investigations carried on in 
the south during recent years seem to corroborate these views. 
The investigations of Mr. Ricker along this line are especially 
valuable and it is hoped that future investigation in the south 
will help establish the facts that are still in a state of doubt. Mr. 
Ricker says further : ''The abundance or scarcity of a large num- 
ber of species I found subject to daily fluctuation, dependent upon 
the presence of the army ant or some fruit suddenlv ripening, 
there being scarcely two consecutive days when birds could be 
35. Ricker. The Auk. 1890, p. 131. 
