140 GRAY LADY AND THE BIRDS 
long distances as that of the biggest birds that grow. 
Ideas like this have been believed until a comparatively 
short time ago, and it is only within the last fifty years 
that there has been much real progress toward the truth 
of it all. And this is the way it has been brought about. 
In our country the band of Wise Men at Washington, 
forming the United States Biological Survey, have for 
twenty years been gathering facts about the migration 
of birds. This body has sent out naturalists to travel 
through the North American continent from Guatemala 
to the Arctic Circle, to meet with other scientific men on 
their way,and keep careful notes of what they see, so that 
reports are had in the spring and fall each year from 
hundreds of observers. 
‘““ These reports give the date upon which each particular 
kind (or species, as they call it) of bird is seen, when 
it becomes plenty, and when it moves on again. The 
lighthouse keepers also give much information by noting 
the times at which they find the birds that are dashed to 
death against the lanterns in the tower. In short, the 
Wise Men have more material at hand than ever before 
from which to shape the story that day by day increases 
in wonder. 
Causes of the Migrations 
‘“‘Tt is more than two thousand years since the wonders 
of bird travel have been noted; and while the distances 
and routes of travel are better known, we cannot yet give a 
positive answer to the question, ‘ Why do birds migrate ?’”’ 
“Please, Gray Lady,” said Sarah Barnes, “I thought 
you said it was because in fall the insect food begins to 
