lowed, may find an adequate explanation in the temporary leader¬ 
ship of some individual within sight or hearing of the others, who 
knows at least a fraction of the way by experience or who strikes 
out a safe path by means of landmarks. 
Finally, it must be remembered that all who start upon this 
winged crusade do not reach the holy land. The annual loss of 
bird life during migration is unquestionably enormous. Birds are 
not driven by an unfailing instinct that carries them all automat¬ 
ically to their destination. The blunderers and the stupid ones are 
relentlessly eliminated in countless numbers. The more resource¬ 
ful ones, the quicker witted, the more vigilant, accomplish the 
grand tour amid perils innumerable with many a hair-breadth es¬ 
cape and the survivors are those choice spirits who, having thus 
won their spurs by noble effort, or because they possess the birth¬ 
right of a superior endowment over their fellows, become the 
ancestors of other birds. So it is that winning qualities are en¬ 
grafted upon the race by hereditary transmission. It is to be 
greatly wonderel at that, after ages of such rigid selection, we 
' should at last have birds to-day whose performance is so remark¬ 
able that we are tempted to attribute it to powers uncanny and 
unknown ? 
II. Why Do Birds Migrate? 
Theories to Ac- Having discussed some of the theories ad- 
count for the Fall , . . . ^ . , . , ~ . 
Migration vanced in explanation of how birds find tneir 
way during migration let us consider some of the reasons which 
have been given to solve the origin of the migration habit. Why 
do birds migrate at all ? At once it is seen that the fall migration 
seems to present fewer difficulties than the spring migration. 
The Temperature It has been maintained by some investigators 
Theory the approach of cold weather causes birds 
to go south in the fall and it is quite true that if all birds attempted 
to remain in northern latitudes during the winter many would 
doubtless succumb to the cold. The main factor in such a disas¬ 
ter, however, would not in all probability be low temperature in 
itself but rather scarcity of food dependent upon low temperature 
during the winter months. The fact that there are repeated in¬ 
stances of birds, such as robins, song sparrows, etc., which ord¬ 
inarily migrate south, remaining occasionally in their summer 
habitat throughout the entire winter, demonstrates that these birds 
are able to endure low temperature when they have a plentiful food 
supply. In this connection two well known facts are significant. 
First, the ordinary bodily temperature of a bird is always several 
degrees warmer than in the case of man, and secondly, the fall 
migration begins and is largely completed before the weather be¬ 
comes cold. 
