FIELD WOKE!. 11 
is probably adequate. But these conditions are seldom realized, and 
under any other so many factors are involved that the question is too 
complex to be so simply determined. A discussion of what is neces- 
sary to be considered in order to determine the status of a bird will 
be found in the chapter on 'Classification of Bird Food' (pp. 16-18). 
METHODS OF INVESTIGATION. 
Knowledge of the food habits of birds is of great importance in 
rural economy. Ignorance of this subject is in part responsible for 
the grave mistake which was made in the introduction of the English 
sparrow. To the same cause has been due the passage of bounty 
laws for the encouragement of the indiscriminate slaughter of hawks 
and owls, notwithstanding the fact that some of these birds of prey 
are highly beneficial to agriculture. To obtain adequate knowledge 
of the food of birds in relation to agriculture a definite scheme of 
procedure must be followed. Simply observing the birds while they 
are feeding gives only fragmentary information and has often resulted 
in the protection of injurious or the persecution of beneficial species. 
The results thus obtained must be supplemented by other and cor- 
roboratory evidence. Recent investigations by the Biological Survey 
have been carried out by the following methods: (1) Observation of 
birds in the field; (2) experiments with captive birds; (3) examination 
of the contents of stomachs; and (4) a combination of field work 
and stomach examination. Economic ornithology is as yet so little 
advanced that a detailed account of these methods will not be amiss. 
FIELD WORK. 
Field work, as stated, yields results which must not be considered 
as a final solution of the problem, but only as a contribution to our 
knowledge. Nevertheless, it is indispensable as a part of the investi- 
gation, since the actual amount of damage done to ripening fruit or 
to grain or the good done by the destruction of weed seed or insect 
- by native birds can best be determined, in dollars and cents, by 
careful study of the scene of action. As an illustration of damage 
that could not have been ascertained by any other means an instance 
may be cited of a ripening oat field of 3 or 4 acres that was visited 
by a flock of about 100 goldfinches, where the quantity of grain actu- 
ally eaten was insignificant, but a loss of 5 percent of the crop was 
caused on about an acre by the birds, breaking down the stalks so as 
to make it impossible to reap at that point. The extent of the good 
done by sparrows in destroying the seeds of pigeon-grass, ragweed, 
and similar weeds can be definitely ascertained only by visiting the 
field in late winter and observing the proportional extent of destruc- 
tion. Such visits will often show that more than nine-tenths of the 
Beeds produced have been destroyed. Field observation is useful also 
in ascertaining the food habits of nestling birds. Although the work 
