33.—Bluebird carrying food near 
1 in Illinois is that of Fawver (19472), 3.5 acres. 
man (1935) noted that bluebirds did not nest closer 
‘than one-fourth of a mile in west-central Illinois, 
tel (Smith & DuMont 1944) effected a 40 percent 
Ina bluebird population near Blue Island when he 
d the number of nest boxes from 10 to 27 in a one- 
© square area. In the south we have found contem- 
us nests as close together as 380 feet. 
a nest box, in Pope County, Illinois. 
Major periodic crashes in the bluebird population (Fig. 
37) is a phenomenon of long standing. Butler (1896) noted 
a sharp decline in northern Illinois and Indiana in 1895 
and attributed it to severe cold in April. Later studies have 
also indicated that such crashes are, to a large degree, cor- 
related with adverse weather, especially in the winter and 
spring (Barnes 1912, Musselman 1941, and James 1960). 
Musselman (19412) estimated that 50 percent of the blue- 
oo 
