Bird Migrations. 
Poughkeepsie, N. Y., March 6. —Editor Forest 
and Stream: The following quotation appeared 
at the close of a charming article by Elmer Rus¬ 
sell Gregor in Forest and Stream of Feb. 6 
on the annual migrations of birds: 
"In the feathered world, migration plays a 
most important part in the life of each little 
songster of wood and field. We know the why 
and the wherefore of the autumn or Southern 
flight; it is entirely a matter of food supply. 
As for the northern or spring flight we can 
assume no logical reason. Why the birds should 
desert a land of plenty and attempt a journey 
often thousands of miles in extent, often reach¬ 
ing their summer homes before the snow and 
ice of winter have released their hold on earth 
and tree, is at present a mystery and perhaps 
ever will be.” 
The query as to the causes of the northern 
or spring exodus has prompted me to make an 
effort to explain, at least in some measure, those 
laws which govern migrations, not only those 
of birds, but of certain of our mammals as well. 
It is a subject of which, so far as I am in¬ 
formed, but very little of an explanatory char¬ 
acter has been written. Anything which may 
here be advanced shall be of a tentative nature. 
The basis of my explanations will be the re¬ 
cent theory of Dr. iMarsden Manson, as stated 
in his “Evolution of Climates.”* 
This theory has been accepted by many of 
the advanced scientists of to-day throughout the 
world. Prof. Schaberle, formerly of the Uni¬ 
versity of Michigan, has recently announced 
that Dr. Manson’s theory may be considered as 
proven.! Furthermore, the work of the United 
States Astro-Physical Observatory, as may be 
seen in the recently issued report, is favorable 
to the same conclusion as that reached by Prof. 
Schaberle. 
This is not the place to state even briefly the 
principles of Dr. Manson’s theory. I may say, 
however, that it makes clear and logical the 
various climates which have ruled upon the sur¬ 
face of this globe since life began, and also that 
it has made adherents in the ranks of science 
from its announcement. Physicists, geologists 
and astronomers; these have accepted the 
theory because of that unity and simplicity 
which distinguishes it from the many compli¬ 
cated and labored dissertations on the subject 
which crowd our book shelves. I shall not say 
that its acceptance has been universal. That 
could not be. There are too many who “do not 
wish to be disturbed” even in the ranks of 
science to permit such a result. The majority, 
however, of those who approach the subject in 
a fair, unprejudiced manner are accepting Dr. 
Manson’s conclusions. 
Almost without exception, however, scientists 
are agreed that previous to the period termed 
the glacial or ice age, climates were non-zonal; 
♦Evolution of Climates, by Marsden Manson. Revi 
and enlarged and reprinted from the American Geoloe 
p.^86, with 9 plates. August, 1903. ^ 
ij. M. Schaberle, Science, March 6. 1908. 
that is, that they were of the same general tem¬ 
perature everywhere from pole to pole. 
First, that there was an epoch of torrid heat 
followed by one of tropical heat and succeeded 
by one of temperate heat, which gradually 
passed into one of excessive cold during which 
period the higher lands were snow covered. 
From these regions descended those immense 
glaciers, many reaching the sea level. Since 
this ice age climates have become zonal, a con¬ 
dition which seems to us most natural because 
man remembers naught to the contrary. 
The geological record shows us, however, that 
everywhere from pole to pole the same life 
TWO OF THE CURRIER PARTY AT THE SUMMIT OF 
MT. SHASTA. 
existed during all the periods before the latter 
part of the temperate tertiary epoch. This im¬ 
plies that the ranges of all the various species 
were then vastly wider. Fossil sequoias and 
poplars" found in the Arctic complete families 
mostly found on the Paciflc coast. 
Aside from those differences of temperature 
resulting from elevation (“land masses were 
thrust up above the then existing snow line, 
such snow line being independent of latitude”), 
there were, in the nature of things, few rea¬ 
sons for migrations of either fauna or flora. 
The first wanderings would have been due to 
these causes and were undoubtedly as limited 
as these causes. 
'When finally the gradual transition from 
earth-heat control to sun-heat control had 
taken place and the ice age began, these wan¬ 
derings to and fro became systematic and 
periodical. The stronger and more active in¬ 
dividuals pushed further on than their fellows 
or they climbed ' up further on the mountain 
sides, thereby forming a class apart. They 
mated and founded new varieties. This pro¬ 
cess of natural selection continued for many 
thousands of years. The fur-bearing animals 
were in time produced, and as their protective 
coverings increased in warmth, these extended 
their search for such conditions of temperature 
of of food supply as their well-being demanded. 
So here we have in its earliest and simplest 
form the origin of the migratory movements of 
animals which have developed to such an extent 
in this day under the present zonal distribution, 
of climates. Birds and beasts travel thousands 
or hundreds of miles at the oncoming of spring 
or fall. 
During all the later part of the long tertiary 
period the process of mountain building or sub¬ 
sidence were changing the face of nature. The 
fauna which had previously enjoyed so wide 
a range of equable temperature and a general 
food supply discovered that those ranges had 
been restricted in certain directions, while per¬ 
haps they had become more extended in others. 
The land bridges which had previously given 
them passage had sunk beneath the encroach¬ 
ing waters. Mountain ranges had reared their 
heads into the clouds and were snow covered, 
forming impassable barriers in the paths of cer¬ 
tain species. The true migration began to take 
form, keeping pace with the constantly chang¬ 
ing climatic conditions, and in general terms 
they may be said to have begun with the ice 
age. 
The direct burning rays of the sun near the 
equator were the whips which drove onward 
to a more temperate region both birds and 
beasts. The increase of cold and the curtail¬ 
ment of their food supply drove them back 
again. The sheep and goat families accom¬ 
plished their migrations in altitude and became 
specialized, remaining there to-day. Certain of 
the largest birds followed their example. Others 
of the feathered tribe, needing no land bridges, 
scorning even the mountain barriers, “took to 
themselves the wings of the morning and fled 
to the uttermost parts of the earth.” 
Food supply was not the only cause for these 
flights, but comfort and well being, and after 
the passing of ages, the newly acquired habit 
of life. They were unconsciously striving for 
the continuance of the former level tempera¬ 
ture condition which had been, during millions 
on millions of generations, the whole life his¬ 
tory of their species. Many varieties, however, 
failed to pass this extreme test of their powers. 
■When the ice age reached its maximum of 
cold and the glacier front had crept far down 
the Mississippi valley, when the tops of the 
Rocky Mountains and the Mexican and Central 
American Cordilleras were capped with snow 
and ice, glaciers streaming far down their sides 
toward the sea, several species found their re¬ 
treat cut off either by sea or ice. It is my be- 
