11 
THEORETICAL CONSIDERATIONS. 
Before proceeding to treat in detail of the phenomena presented by the 
reccrds of the migration observers, it may be well to glance for a moment 
at the general causes of migration, and at some of its more striking 
and interesting features. 
Without entering into a discussion of the causes which long ago 
started birds on their periodical change of habitation, we shall not be 
far out of the way in considering their present migrations the result of 
inherited experience. To be more explicit, the first migrations were 
doubtless very limited in extent and probably were intelligent move- 
ments which through repetition became habitual, and the habit was 
transmitted from parent to offspring until it has become, as we see it 
now, the governing impulse of the bird’s life. It is undoubtedly true 
that love of the nesting ground, which is to them their home, is the 
foundation of the desire for migration;* and year after year they find 
their way thousands of miles back to the same box or tree by the exer- 
cise of memory—not always the memory of the individual, but the 
memory inherited from numberless preceding generations which have 
passed and repassed over the same route. 
In the study of the yearly cycle of migrations there are two movements 
for which we must seek the cause—the restless pushing northward in 
the spring, in spite of cold, rain, sleet, and snow; and the southward 
journey in the fall. We have already stated that the northward move- 
ment is caused by a strong home love—an overpowering desire to be 
once more among the familiar scenes of the previous summer. The re- 
turn movement is obviously the result of two causes—the approach of 
winter and the failure of the food supply. Of these two, the latter is 
probably by far the more powerful, since it is well known that single 
individuals of species which retire far to the south often remain behind, 
and, favored by an abundance of food, withstand the most severe 
weather. Thus, many Red-headed Woodpeckers remain through the 
winter in the cold climate of northeastern New York, frequenting 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 in the thick pine forests; while Ducks and 
even Wilson’s Snipe have been known to remain throughout the whole 
{*I cannot concur with Professor Cooke in the belief that ‘love of the nesting 
ground * * * is the foundation of the desire formigration.” Ina lecture on Bird 
Migration which it was my privilege to deliver in the U. 8. National Museum, April 
3, 1886, I said: ‘* Some ornithologists of note have laid special stress upon the ‘ strong 
home affection’ which prompts birds to leave the South and return to their breeding 
grounds. To me this explanation is forced and unnecessary. Birds desert their 
winter homes because the food supply fails; because the climatic conditions be- 
come unsuited to their needs; because the approach of the breeding season gives rise 
to physiological restlessness; and because they inherit an irresistible impulse to move 
at this particular time of the year.”—C. H. M.] 
t Merriam, Bull. Nutt. Ornith. Club, Vol. III, No.3, July, 1878, pp. 123-124, 
