218 
four times as abundant to the unit of area in the tree-shrub association 
as in the remainder of the state. In this connection, however, we must 
recall the fact that the woodland area of our survey is not the dense 
and full grown forest of the wilderness, but is rather to be taken as 
equivalent to the forest edge; and the further fact that the limited 
acreage of our tree-shrub formation, 880 acres in all as against 18,408 
of the other habitats, has probably led to a certain concentration of the 
normal forest birds, and especially of those which nest in trees and 
shrubs and seek their food largely in the open fields. 
As a most general outcome of this final discussion, we may con- 
clude that the remaining birds of the Illinois wilderness have adapted 
themselves to civilization, and especially to agriculture and horticulture, 
not so much by a change of choice or of habits, as by searching out 
under the new conditions the places which most nearly resemble their 
original nesting sites and which offer them food the nearest to that 
which, by hereditary inclination, they were impelled to choose; that in 
pastures, orchards, and yards and gardens they have found situations 
more favorable, and in the vast areas devoted to the cereal crops less 
favorable, to their maintenance and multiplication than their original 
habitats; and that while certain species have suffered heavily, in some 
cases nearly or quite to extermination (mainly, however, by the deliberate 
acts of man), others have greatly increased in numbers, the numerical 
make-up of the bird population of the state having thus shifted its 
balance in response to an increase of some resources and a diminution 
of others. Their remarkable success in self-maintenance under changes 
of environment which, from their viewpoint, may be called revolutionary, 
is not due to any flexibility of organization and consequent power of 
physical adaptation to new conditions—for which, indeed, they are much 
too highly and rigidly specialized—but to their remarkable sensory 
endowment and unequaled powers of locomotion, by the use of which 
each species is enabled to search out and occupy the most satisfactory 
ecological situations still to be found in the area of its geographical dis- 
tributiom. 
al is» 
~~ 
