BROWN THRASHER 
Annual Variation in Egg Production 
e |—— CENTRAL 1959 
a ie 
Fig. 23. — Annual Zz 
1 regional variation wW 6.0 
egg production by 2 
pwn thrashers. Note © 5.0 
ecially the very early Fa 
ting cycle in southern O 
nois. Lines are inter- Oa 4.0 
‘ted where data have = 
been collected. st 3.0 
O 
aee2.0 
uu 
5 Ne 
ae 
0 
[OR 200 9 19 29 S) EF ie) Bie) Ye lO 20 
MAR APR MAY JUNE JULY 
ishers in hedgerows or similar linear habitats, and BROWN THRASHER 
. . . . . ii... 
Ing Success may be very different in other situations. Nest Height 
Although in general there was little difference in nest 12 
‘ival between regions (Table 12), our observations in- 
‘te that the nesting problems are actually very different N 
n place to place. For example, in the Ford County 
ulation the incidence of “infertility,” i.e., no embryonic 10 
lopment following incubation, was 11 percent in eggs 
1 full clutches, but it was only 1 percent in our sample 
gs in northern Illinois, and 3 percent in the southern 
on sample. In another 3-5 percent of the eggs (all 
ons), the embryos died before hatching. In the 
ples of central and southern IIlinojs nests, we found 
w nests in which the young birds perished, not from 
ation but from some other cause, possibly low tem- 
tures. Such nestlings accounted for about 3 percent 
€ eggs in nests with full clutches, 
Jur judgment on the causes of mortality in the re- % 
ider (the majority) of nest failures is based on 
mstantial evidence. We suspect that in the northern 
central study areas, the fox squirrel (Sciurus niger) is 2 
nificant predator on thrasher nests. We have never 
SOUTH - 60 nests 
= CENTRAL - 190 nests 
i 
ye 
NORTH - 87 nests 
NEST HEIGHT IN FEET 
o 
ABLE 10.— Seasonal variation in clutch sizes of brown 
hers in Illinois. 0 
0 10 20 30 40 50 
nthe Total Percent of Nests by Clutch Size 
Se era PERCENT OF NESTS | 
Fig. 24. — Percentages of brown thrasher nests at different 
May 203 6 68 25 1 heights in roadside habitats in different regions of the state. 
luly 64 1 34 63 1 Heavy solid line represents central, light solid line southern, and 
dotted line northern Illinois. 
