METHODS OF STUDYING THE FOOD OF BIRDS. 15 



It is set up early, so the birds may get accustomed to seeing 

 it, and not taken down till the observation is completed. 

 Sometimes several nests are brought one after another to the 

 same site. In the illustration opposite there is a chipping- 

 sparrow's nest in position and a robin's in waiting only ten 

 feet away. Except for the trouble and a very slight delay 

 in the work of the birds, there is no objection to striking the 

 tent every evening and pitching it again in the morning. 



At such short range there is generally no doubt as to the 

 identity of every object that is brought to the nest. Some 

 birds bring food in their gullets and feed by regurgitation. If 

 it is not possible to see what they are delivering, wait till the 

 old one has gone away, then go out and examine the young. 

 Four times out of five you can tell what they have swallowed 

 by looking through the transparent skin of their necks. In 

 case there is still a doubt, it is not difficult to make them dis- 

 gorge by placing a thumb and finger below the mass and 

 working it upward to the mouth. Simple honesty demands 

 that it be returned when you are done with it. 



See how many hours a day the old birds attend their young 

 and how many times they average to feed per hour. Estimate 

 the proportion of each kind of food from an examination of 

 your notes. Then by weighing samples of the different kinds 

 you can quickly compute the daily consumption. 



As a check on the above method weigh the young at the 

 same hour every day. Collect several excreta and find the 

 average weight, also observe the average number voided per 

 hour. The weight of excrement for the day plus the bird's 

 gain in weight for the day will give the weight of food con- 

 sumed, less the small amount lost by respiration. 



The excreta of young birds is so well wrapped in a coat of 

 albumen that it is not so objectionable to handle as might be 

 supposed. It may be obtained at any time by taking the 

 bird from the nest and keeping it out for a few minutes. 



In order to distinguish one nestling from another they may 



