188 BULLKTIN OF WISCONSIN NATTKAL HISTORY SOCII-TY. VOL. 2 NO. 4 
of time, if any, separates them. Thug we have a multitude of di- 
verse conditions : changes in temperature, humidity and Hving 
nature, any or all of which may have an influence upon migra- 
tory birds. 
In the yearly cycle of migration we have the northward move- 
ment in the spring and the southward movement in fall. With 
a very few exceptions, writers upon this subject seem to have 
been agreed that there are different influences operating in each 
instance. In the fall it was the approach of winter and the fail- 
ure of food, in the spring the love of home and nesting ground, 
and the difl^iculties of breeding in the south. There can be no 
doubt that of these various influences each may play some small 
part as a factor in the causes of migration. Let us analyze the 
various influences. 
From a merely superficial view of the physical conditions ex- 
isting during the migratory season it would seem that tempera- 
ture is a most important factor as a cause for the fall migration. 
Indeed, it was considered so for a long time, but at present it is 
beginning to be looked upon as only of .secondary importance. 
Almost every student of birds can cite cases where strictly mi- 
gratory species have wintered far to the north of their northern 
winter limits, when the food supply was sufficiently abundant to 
insure their existence. 
Prof. Cooke says : "Thus many Red-headed Woodpeckers 
remain through the winter in northeastern New York, frequent- 
ing the heavy timber where there is a great quantity of their 
favorite food ; and it is not unusual for a few Robins to spend 
the winter in north central Wisconsin, sheltered by the thick pine 
forests ; while ducks and even Wilson's snipe have been known to 
remain throughout the whole winter in Wyoming, near the hot 
springs, whose warmth kept the neighboring waters and ground 
from freezing" (33). 
Dr. O. G. Libby has noted a Wilson's Snipe remaining near 
an open spring in northern Wisconsin the whole winter. In- 
stances of this kind are met with almost yearly. During the win- 
ter of 1 899- 1 900 a remarkable case was brought before the writer. 
In November, 1899, the Red-headed Woodpecker was observed 
storing acorns in the crevices of the bark of our larger oaks (see 
frontispiece). Before winter had fairly set in a great many trees 
about Madison contained hundreds of the acorns. As the winter 
33. Cooke. Rep. on Migration in Miss. Valley. Bull. U. S. Dept. of 
Agr. Div. Econ. Ornith., No. 2, p. 11. 
