BIRDS, CATERPILLARS, AND PLANT LICE. 113 



feeding a few European birtls in captivity, or upon the mere 

 casual observations of any one, to establish facts. 



No one, however, is warranted in atteniptino- to dis})ute 

 assertions made hy eminent naturalists, unless he is prei)ared 

 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 puldished, and some field 

 notes from a few observers, that the read(!r may judge of 

 the ('haracter 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 caterjjillars, and 

 during nine years of this time I had an op})ortunity to com- 

 pare notes on this subject with many other field workers. 

 During the prosecution of tlu^ work against the gipsy moth 

 by the State Board of Agriculture more than a thousand 

 men Avere 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 recjuested 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 em})loyed 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 ^e\Y 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 l)y ex- 

 perience in the earlier studies was turned to good account 

 in conducting those made later. The value of such observa- 



