46 ILLINOIS ACADEMY OF SCIENCE. 



Otherwise seem incredibly exaggerated. The insects eaten by an 

 insectivorous bird are evidently to be numbered in hundreds per 

 day ; and the weed-seeds eaten by a seed eater, in thousands. 



With respect to the number of birds inhabiting any extensive 

 area, the only systematic work attempted is that done in recent 

 years by the Natural History Survey of this State. Thanks to 

 this w^ork, we now know something of the numbers of birds of 

 various species in our area in different situations and at different 

 seasons of the year. Time wall not permit more than a brief ref- 

 erence to this matter, especially as our data have not yet been 

 completely studied. I have generalized them, however, to the 

 effect that, taking the State as a whole and the year as a unit, our 

 bird population averages something over a bird per acre for the 

 open area of the State — excluding, that is, forests and other simi- 

 lar tracts on which accurate counting of birds is impracticable. 

 This population varies greatly, of course, in density, according 

 to the character and vegetable covering of the surface. Pastures 

 contain, on the whole, more birds than any other open surface, 

 and corn fields the fewest of all. The tendency of birds to con- 

 centrate where their food is especially abundant, and even to 

 change their habits temporarily, in order to take advantage of an 

 unusual profusion of certain elements of their food, greatly 

 increases their economic value, since it makes them most efficient 

 where their efficiency is most important. Nevertheless, the very 

 fact that insectivorous birds thrive best when insects are most 

 numerous must lead us to doubt that nature has really committed 

 the blunder of producing birds in such numbers, and endowing 

 them with appetites so vigorous, as to keep the natural sources of 

 their own food supply much below the line of the highest possible 

 productivity. 



That birds are economically useful, scarcely admits of serious 

 question, but the ultimate value of their services can scarcely be 

 guessed at intelligently until much more work has been done on 

 the problem. In the meantime, there are abundant reasons, both 

 economic and esthetic, for their protection, and to this erd tlie 

 most important pending measure is the passage of national laws 

 forbidding the destruction of migrant species — an interest which 

 I would like to have this Academy promote by a resolution favor- 

 ing the passage of the Anthony bill, now under consideration in 

 the national House of Representatives. 



The fur-bearing mammals of the State are by no means so 

 nearly extinct as most of us would be likely to suppose. The 



