195 
birds. The number of these dominant species is thus much larger and 
_ the number of birds to the species is correspondingly smaller than in 
le comparatively level, uniform territory of central Illinois. Although 
e areas covered by the bird survey were approximately equal in 
_ these two sections of the state,* and the number of birds seen was 
nearly 27 per cent. greater in central Illinois than in southern, the 
number of species was 20 per cent. smaller and the dominant species 
_ were much fewer—85 per cent. of all the birds belonging to thirteen 
species instead of twenty-two, and these averaging 553 to the species 
instead of the 250 of southern Illinois. 
The average frequency of all birds in the central section was 925 
per square mile, and 791 of these (86 per cent.) belonged to the thirteen 
more abundant species; while the birds of the remaining fifty-nine 
species, if equally distributed, would each have had about ten acres to 
itself. 
NUMBERS OF PRINCIPAL BIRDS 
: OF CENTRAL ILLINOIS, 
SUMMERS orf 1907 AND 1909 
(13 sPEctiEs) 
(85.5 PER CENT. OF ALL THE BIRDS SEEN) 
PV ETISH SPALTOW aera tes nel - sista apes oo ekeee eels 1865 
MOO DEM io pie cas nels cts psx civze sa¥eva ecole «Sn oiehes. 1744 
BON ZCGs LEACHIE airs cid ain ntanetie are serene 1111 
WVICRUOWIAT Hobe ersten iis shenstiacsousel stowetheote se 630 
IVLGUTITIN ES OVE Wace acc s aye ciaeteros sae civ alers 407 
US ISCINS Eley. Meet al fo peiviclar srehes eveveseslspa cus are 283 
Red-winged blackbird ..........5...... 225 
(Prairies HOLM dy 1ATiS 40/55. Sere tsrale|ss e100 Be 204 
ODTCIOT Dy teterstcsbearecas Raat aeRO Soe. caSteretece 184 
EU ULE hiss halen te eisuslSrecorslisl ekeqeteersl cle cena Soret 170 
DITO MTC UASU GIG, sea thetereie tls occ eicate cee 133 
Red-headed woodpecker ............... 125 
ORO Maver SEG OD OAL 6 er EO OC Smear ace 115 
PDO UEDA ars ara lepesayesaee jaretelove Mioeiole staretomicrace 7196 
Northern Illinois has a somewhat more uneven and diversified sur- 
face than the central section of the state, but much less so than the 
_ southern, and consistently with these facts we find that 15 of its species 
‘made up 84.4 per cent. of its bird population in 1907 and 1909, 79 
5 cs contributing but 15.16 per cent., the former group averaging 698 
birds of each species and 723 to the square mile, and the latter group 
but 21 to the species and 131 to the square mile. 
Illustration from May-beetles—The relation shown in the com- 
plexity of an environment and the number of dominant species of birds, 
which it contains is paralleled in a convincing way by data derived 
from extensive collections of Illinois May-beetles made for the senior 
* 5527.19 acres in southern Illinois and 5823.88 in central. 
