What do we mean by the “Geographical Distribution” of birds? Are not
birds to be found everywhere, over both land and sea? Are they not,
then, universally distributed? As a class they certainly are, but not as
species nor even orders. Parrots are not found in frigid regions, nor
are snowflakes and snowy owls found in the tropical regions. Our Wood
Warblers and Vireos are not found outside of America, while there are no
birds of Paradise anywhere in America. We shall see that most of the
birds found in the eastern hemisphere differ from those found in the
western, speaking broadly, but that many of the island birds are
different from birds of continents.
Since most birds migrate shorter or longer distances in search of a
place to rear their young, and return again to warmer regions to pass
the winter months, the question at once arises, What is the geographical
distribution of such migratory birds? That is not so difficult as it may
seem at first glance. We have only to inquire what governs the movements
of the species in question in such a way that its appearance at certain
places at certain known times may be confidently expected. The study of
migration and breeding has shown that the impulse to move northward in
the spring to the old nesting-places where the young are reared is more
reliable than the impulse to move southward on the approach of cold. The
birds are more certain to appear at their old summer homes in spring
than they are to be found at any particular place during the winter. But
if there be any objection to this view it will yet remain true that
where a bird rears its young should more properly be called its home
than the place to which it is forced by the approach of cold or the lack
of food. In either case, therefore, we may regard the home of the bird,
and therefore treat its distribution geographically as the place where
it habitually rears its young. Having settled the question as to what
shall determine the distribution of the separate species, it remains to
study the physical conditions of the earth for the sake of finding what
it is that determines the limits to which the different species may go.
We know that the distribution of land and water over the earth has not
always been the same as it is now, but that many places that are now
covered with water were once dry land, and that in many places where
there is now land there used to be water. Now, America is wholly
separated from Uro-Asia-Africa, but once they were connected together by
a broad neck of land where Bering Sea now lies, and there may have been
another neck of land connecting Europe with Iceland and Greenland and so
with North America. Now Australia and New Zealand are wholly separated
from all other lands, but they were not so long ago. So of the larger
islands in general, they have not always been isolated as now, but
connected with great land masses, sharing with them the animals which
roamed over the whole vast regions. For in the earlier times before Man
had appeared upon the earth, before the great Glacial Period, the whole
earth was tropical in climate, making it possible for plants as well as
animals to live anywhere upon the earth, as they cannot now. Then
extensive migrations north and south were not necessary, but instead
there were roamings about in all directions, or great invasions of new
regions by hosts of animals of one kind.
As the land sank away here and there, and the sea covered it, barriers
were thus formed to further roamings, except by the birds of strong
flight or animals that could swim long distances, and there could no
longer be an intermingling of the animals of the whole land surface of
the world. Since all animals are inclined to change somewhat to meet or
keep pace with the changes that are going on in vegetation and the
general physical conditions of the earth, those that have been separated
in this way will grow more and more unlike. In some such isolated
regions there may not be much change in their environment and so they
will change but little, if at all, and so will not keep pace with those
in other regions where life is a constant struggle with others for
supremacy. It is just as true in the natural world as in the commercial,
that competition is necessary for the highest development. It is
probably true that the disturbances which caused the land to sink in
places and so disconnect what had been connected lands, possibly a
splitting up of one great flat land mass, also brought about the changes
which made out of one great tropical world the one that we know with its
frigid, temperate and tropical zones. So that just at the time when the
animals of the different regions were separated from each other forever
there came these changes in physical conditions which would make them
change to meet the new conditions. But that is a long story for the
geologist to tell. Of course the sinking of the land in different
regions occurred at different times, probably thousands of years apart
in many cases. And the changes from tropical to temperate and frigid
must have been very gradual also, or there would have been no animals
left alive in the northern and southern regions. Only those near the
equator could have lived.
Probably New Zealand was the first considerable land mass to be
separated absolutely and for all time from all other land, because here
we find the lowest type of birds and lower animals. There are no
terrestrial indigenous mammals even. Such birds as were not able to fly
across the now wide stretches of ocean did not continue to develop
rapidly because there was little change in their environment and because
there was little or no competition with other similar forms. So to-day
we find them either very similar to what they were when their island
home was made an island home, or else even degenerated into flightless
creatures. Australia seems to have been the next tract of land cut off,
for here, too, we meet with the lower forms which show the lack of the
keen competition which their relatives further north had to sustain.
When North America was cut off from Siberia, marking the close of more
or less extensive interchange of communication of the animals of both
regions, there was little difference in their animal life; but following
this separation there came about a more rapid change in the Orient than
in the Occident. It may not be quite clear why this was so, but that it
was cannot be doubted, for some of the lower forms of animals which
still inhabit America have been completely destroyed in the Orient. At
the time of their separation these forms were found in both places. What
seems a probable explanation of this more rapid change in the Orient may
be briefly stated. The configuration of the Orient is such that animals
would have a far greater range east and west than north and south. A
great mountain range and a great desert are thrown as barriers across
the way of the northward and southward movement. In America there is a
continuous gateway to the north and south, but barriers to an eastward
or westward movement. With such creatures as the birds freedom to move
north and south would always lessen competition, while the crowding of
one group or race upon another eastward or westward would increase the
competition. But Geology tells us that in the Orient such westward
invasions have actually occurred, causing the death of the less hardy
forms and the modification of all forms of animal life.
It must not be understood, from what has been said, that all the
animals, especially the birds, found in any one country or island, are
different from the birds found in all others, for that is not true.
There are many species of birds that are found practically all over the
earth. But what is true is that each country or region of any
considerable extent, or group of oceanic islands has some species which
are not found anywhere else in the world.
From what has already been said it will be clear that the world may be
divided into several different regions, according to the animals which
are peculiar to the different ones. Following Newton’s system, because
it seems the most logical, at least so far as the birds are concerned,
we have first