BIRDS, CATERPILLARS, AND PLANT LICE. 113 
feeding a few European birds in captivity, or upon the mere 
casual observations of any one, to establish facts. 
No one, however, is warranted in attempting to dispute 
assertions made by eminent naturalists, unless he is prepared 
to show that his own experience has been extended and varied 
enough to warrant him in assuming them to be in error. To 
justify my own position, I shall present here some revised 
portions of some papers previously published, and some field 
notes from a few observers, that the reader may judge of 
the character of the evidence offered to disprove the state- 
ment that birds do not eat hairy caterpillars. It will first 
be necessary, however, to explain how the evidence was 
secured. For more than thirty years I have observed, from 
time to time, the feeding of birds upon caterpillars, and 
during nine years of this time I had an opportunity to com- 
pare notes on this subject with many other field workers. 
During the prosecution of the work against the gipsy moth 
by the State Board of Agriculture more than a thousand 
men were employed, among whom were many who knew 
the more common birds. Some were keen field naturalists. 
In the early history of the work, when it was seen that 
birds were feeding on the hairy caterpillars, all those em- 
ployees who knew birds were requested to watch both birds 
and insects, and report results. There were eleven such ob- 
servers on the force at that time. Others joined the force 
from time to time, until the number of competent persons 
whose experiences were recorded was increased to thirty- 
eight. Some of these observers were employed only one 
season; others were in the employ of the Board for six, 
seven, or eight years, and made observations during each 
year. The conditions under which these studies were made 
were such that most of the birds could be observed within 
either a few feet or a few yards. Those which could not 
be so readily approached were watched with the aid of good 
field glasses or opera glasses, and, where there appeared to 
be doubt, birds were shot, and the contents of their stomachs 
were carefully examined. Much that was learned by ex- 
perience in the earlier studies was turned to good account 
in conducting those made later. The value of such observa- 
