196 BULLETIN OF WISCONSIN NATURAL HISTORY SOCIETY. VOL. 2. NO. 4. 
*of food, and that in the case of the migratory birds their food fails 
in the north in winter and in the south in summer. Hence we 
may be justified in saying that the principal cause for migration, 
both in spring and fall, is to be sought in the failure of food. 
The period of migration, however, in each season is of at least 
four months duration. In the spring it would seem that birds 
which arrive here find conditions for feeding less favorable than 
in the lands from which they have just departed. It has been 
shown, however, that it is advantageous for birds to return to their 
parental duties in spring as soon as convenient, and that the ap- 
proach of the breeding season causes a physiological restlessness 
w^hich sets them in migratory motion. In the fall, while we do 
find many of our birds departing at times when their food still 
seems to be abundant, we have life in nature continuously chang- 
ing. Just as the year has seasons with a difference in life, so, too, 
the life of a summer is different at different times. Certain in- 
sects appear and disappear at more or less definite periods ; plants, 
flowers, fruits and leaves all have their appointed times ; days be- 
come longer and shorter, and the very atmosphere carries an es- 
sence, an inherent something, that seems to belong to a certain 
time of the summer. All of these phenomena are operating over 
a vast territory at the same time, affecting different birds in differ- 
ent ways, and setting countless numbers in motion simultanecusly. 
While we may not be justified in saying that these changes in 
nature directly cause a dearth in the food supply of the migratory 
birds, they have been the forerunners, for ages past, of other 
changes the birds could not resist. Their influence upon birds 
has been operating since migration first took its origin, so that to- 
day thev set the birds in motion instinctively. 
Birds are set in migratory movement by a complex combina- 
' tion of changes in temperature, humiditv and living nature. The 
cause for migration, however, is the failure of food in two wide- 
spread areas — the north and the south — at opposite seasons of the 
year. 
CHAPTER HI. 
MIGRATORY ROUTES. 
The theory that birds follow certain definite courses in their 
migrations, and that these courses are determined by the physiog- 
raphy of the countries through which they extend, may now be 
considered an established fact. A migratory route, excepting 
\ oceanic routes, consists of one or more physiographic features 
which together form an almost unbroken path from the breeding 
