( 



riELD WOEK. 11 



is probably adequate. But these conditions are seldom realized, and 

 under 'dny 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 

 pests 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- 

 all}^ 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 

 seeds produced have been destroyed. Field observation is useful also 

 in ascertaining the food habits of nestling birds. Although the work 



