ECONOMIC ORNITHOLOGY Economic ornithology deals with the study of birds in their relation to man and his various activities, such as agriculture, forestry, trade, sport, etc., or in other words "it is the practical application of the knowledge of birds to the affairs of everyday life." It is a difficult and intricate study and of the greatest im portance to mankind.
The scientific study of wild birds from an economic standpoint may be said to date from the publication of a paper by Prevost in 1858, and one by Jenks in 1859. Since then, thanks largely to the workers in the United States Department of Agriculture (Bureau of Biological Survey), great progress has been made and nu merous intensive studies undertaken. As a result it is now almost universally admitted, with very few exceptions, that the wholesale destruction of wild birds as a means of protecting crops, etc., is, economically, an unsound policy. A bird which does a considerable amount of harm at a particular season of the year may more than compensate for it by the nature of the food consumed at another season, and in a like manner a bird which is injurious in one dis trict may be beneficial in another. It must ever be borne in mind that, as a rule, insect-eating birds feed upon those species of in sects that are the easiest to obtain, and that the variation in a bird's diet depends to a very large extent upon the abundance of the supply and the ease with which it may be obtained. Moreover, where destructive irruptions of insect life occur and wild birds concentrate on one particular species of insect, no especial chance is given for the rise of new fluctuations amongst the commoner species eaten, for the species which is unusually numerous is so widely distributed amongst the ordinary food elements. The more numerous insectivorous birds are, both in species and numbers, the fewer and shorter the insect oscillations are likely to be.
The Amount of Food Consumed by Birds.—As a class birds are distinctly beneficial, and experience shows that it is pos sible to learn with very considerable precision the exact amounts of the different kinds of food that each species requires in a year.
This precision is attained by measuring the percentage volumes of the different items of food found in the stomach, crop, etc., during the various months of the year, from a large series of specimens obtained from different localities. This volumetric method, or the expression of the contents in terms of bulk, is much more exact than any numerical count of the different food items, and enables the investigator not only to express himself in exact quantities, but to compare one bird's diet with another; further, it admits of the stating of the definite ratios each ele ment bears to the other. Every bird requires a certain bulk of food per day, not a certain number of insects, seeds, etc., and to esti mate correctly the importance of any item in its diet it is neces sary to know what proportion the insects, seeds, etc., bear to the standard requirements, and to do so it is necessary to use some method of measurement. Take the case of the skylark. This bird requires about 6 lb. of food per year; in other words io,000 larks would require about tons of food in a year. Knowing the per centages of food eaten by this species, it is possible to analyse this figure. Of the total bulk of food consumed in a year 35.5% con sists of injurious insects, 3.5% of neutral insects, 2.5% of bene ficial insects, 9.5% of grain, 1% of leaves, 2% of earthworms, 1% of slugs, 1.5% of miscellaneous animal matter, and 43.5% of weed seeds. In other words, the lark benefits the farmer in regard to 36.5% of its food eaten, is neutral in respect of 50.5%, and injurious only in respect of 13%. If the debit and credit account is further examined it shows on the former side a loss of 21 tons of cereals, and on the latter something like 30,000,00o injurious insects and 30,00o slugs. Such a plague of insects left to them selves would have destroyed many more tons of cereals, root crops, etc. Thus the farmer is undoubtedly the gainer from the activities of this bird by an enormous tonnage of produce.