456 University of California Publications in Zoology [Vol. ii 



would pass them by. This theory has not been supported by 

 the results of this investigation, nor, on the other hand, has it 

 been broken down. Although young birds were distinguished 

 from adults, yet the unusual insects found were not constantly 

 taken from the stomachs of young birds. 



Combination of Field and Laboratory Work 



Judd (1901) has suggested, as the best means of determining 

 the food of birds, an examination of the available food supply 

 combined with stomach examinations of birds taken in the same 

 locality. There are many points which commend this method. 

 Yet there is great difficulty in determining the available food 

 supply. Even an experienced observer cannot estimate even 

 with moderate accuracy the comparative numbers of insects and 

 weed seeds in any given locality. A bird is able to see many 

 articles of diet which such an observer doubtless overlooks. Let 

 me cite a case in point : A meadowlark was seen to fly to a grain 

 field and collect cutworms to feed its young. Careful investi- 

 gation by me of the place where the cutworms were collected 

 failed to reveal any. The same was noted with a pair of Brewer 

 blackbirds who persistently collected cutworms in a pasture. 

 Continued investigation in the same pasture did not allow of 

 the collection of a single cutworm. 



An attempt was made to follow this particular line of investi- 

 gation, but was finally given up on account of the multitude of 

 personal errors which are easily introduced. It seemed best to 

 concentrate on the usual method of stomach examination, thereby 

 making comparison with previous work possible. 



Relation op Birds to Insect Outbreaks 



The value of birds as insect destroyers is more noticeable at 

 the time of an insect outbreak. Their importance in maintaining 

 an equilibrium depends largely upon their effect when insects 

 occur in abnormal numbers and thus become noticeably injurious. 

 In two insect outbreaks investigated the western meadowlark was 

 found to take a very active part in insect destruction. 



