12 THE kf.Latiun ok siwkkoWs tQ \(.UK ri/rrRK. 
of identifying in the parent's beak the insects usually fed to nestling 
birds is exceedingly difficult, ye1 ii yields more satisfactory results 
than examination of the stomachs of the nestlings, not only because 
stomach examination shows nothing as to frequency of meals, but also 
because 1 1 1 « * soft insects given by most of the smaller birds to their 
young are generally unidentifiable in the stomach. 
EXPERIMENTS WITH (Al'TlVE RIRDS. 
Very often birds that are too shy to be watched in the field may be 
kept in captivity and experimented with. If the experiments are 
carefully conducted much can be Learned as to the amount of food 
eaten, preferences in food, and questions relating to the dissemina- 
tion of the seeds of fruit and weeds by birds. Birds selected for 
experiment should be recently trapped, because those thai have 
been iong in confinement usually develop unnatural tastes. In test- 
ing preferences in insect food it is convenient to place the insects on 
a piece of cork anchored in the center of a bowl of water. This pre- 
vents ihem from escaping and makes the conditions almost identical 
in the case of each kind. The insects should be equal in volume. 
Thus, a fair experiment would be made if a ladybird [Cocdnetta 
9-notata) and a 12-spotted cucumber beetle (Diabrotica 12-punctala) 
were both pu1 on lhe cork island at once for the bird to select from, 
because both insects are of about the same size. The same principles 
apply to fruits and seeds, though, of course, the use of the cork is not 
important in their case. The food of nestling birds may be sometimes 
Studied to advantage by removing the young from the nest and plac- 
ing them in a cage almost out of reach of the parent bird, so that in 
feeding them the latter will drop a large proportion of the food .just 
inside tlie cage. 
LABORATORY EXAMINATION OF STOMACHS. 
The contents of the crops (or gullets) and stomachs of wild birds 
are examined to find just what (dements of food the bird has chosen 
and the proportion each bears to the total amount of food; and if the 
number of stomachs examined be large enough, the difference due to 
individual variation is eliminated. The identification of the food 
found in a bird's stomach is difficult, for two reasons: (1) Because of 
the great variety of substances that maybe found in the stomach, 
and (2) because of t he fact thai t he semi-digested pieces and fragments 
of insects, fruits, and seeds are often so comminuted that exact iden- 
titicat ion is well-nigh impossible. The method of procedure employed 
in the laboratory of the Biological Survey in making stomach exami- 
nations is as follows: The stomach is slit open with a scalpel, and its 
contents are first washed into a pan by a jet of water from a wash bottle 
and then, with the exception of the sand ami gravel, poured into a 
