90. / oN 
ig. 37.—Annual 70 
on in nest box occu- 
by bluebirds for sev- 
yutes in central and 
rn Illinois. Two cen- 
linois routes showed 
me population crash 
(0, and there was a 
il similarity in the 
tion trends in north- 
1 central Illinois. 
HOUSES 
30 
OCCUPANCY OF 
% 
1935 
¢ incidence of cowbird parasitism of bluebird nests is 
Musselman 1942). The rate was exceptionally high 
> species in 1945, 7 nests parasitized out of 268, or 
3 percent (Musselman 1946). 
lere are no extensive data on nesting success for pop- 
ms in either the north or south. Duncan’s 
-1935) small sample of nests in Cook County indicat- 
ging success of 50 percent of the eggs. Our data for 
ith, representing 51 nests, showed fledging success of 
cent in April and May, and 54 percent for June to 
t eggs. 
sting success for the bluebird seems to depend a great 
n weather factors. Evers (1937), in west-central I]li- 
ound fledging success of about 90 percent of the eggs 
first nesting (April and May), but success of second 
8 was down to 50 percent. He attributed the reduced 
to very high late-summer temperatures (114° F. in 
de). In a 7-year period Musselman (1939a) recorded 
evere April freezes that froze many eggs. He has rec- 
esting success rates as low as 38 percent and as high 
ercent of the eggs (Musselman 1935). In general, 
nestings were less successful than those in April and 
nfertility varied from 1 percent to 23 percent, with 
rage of 9 percent, and low hatchability was again 
BLE 7.—Clutch sizes of bluebirds in different regions of 
Percent of Nests 
by Clutch Size 
Number of Average 6 5 4 3 
Months Nests Clutch Eggs Eggs Eggs Eggs 
April-May 642 5.0 16 2 = 42 0 
June-August 57° 4.4 2 oe 2o en .19 
April-May 1694 4.9 Ly Sai 4 
June-August 274 Al 3) 2 43 39 16 
April-May 15 51 eee Ee: 0 
June-August 18 4.2 A ot ay 
€s records from the literature where complete data were provided. 
NORTH» 
CENTRAL — —- 
1945 1955 1965 
correlated with severe heat (26 days over 100° F.) and 
drouth. 
Other frequently mentioned sources of nest mortality 
relate to competition with other species for nesting cavities. 
Hodges (1953) noted that bluebird populations on Credit 
Island near Rock Island declined coincidentally with an 
increase of house wrens (Troglodytes aedon) in the area. In 
other areas of northern and central Illinois bluebirds also 
fared poorly in competition with wrens for housing (Bartel 
fide Smith & DuMont 1944, and Musselman 1946). Wrens 
occupied from 2 to about 7 percent of Musselman’s nest 
boxes, and perforated as many as 7 percent of the bluebird 
€ggs some seasons (Musselman 1935). When the bluebird’s 
first nesting was delayed, the population was thrown more 
directly into competition with the generally later-nesting 
wren (Musselman 1939). 
In extreme southern Illinois we have not observed any 
interference from house wrens, but wren populations are 
low there. In northwestern Illinois, on the other hand, ina 
sample of 33 nest boxes we found 76 percent occupied by 
wrens in June, versus only 9 percent occupied by bluebirds. 
In east-central Illinois, Campbell (1970, and personal 
communication) recorded 9 percent of 23 boxes occupied by 
wrens, versus 35 percent occupied by bluebirds in 1970, but 
in 1971 wrens took over most of the boxes. 
In human residential areas, bluebirds cannot compete 
with house sparrows (Passer domesticus), and there is 
competition even in rural areas. In western Illinois house 
sparrows utilized more nest boxes than did wrens. From 
data provided by Musselman (1934, 1934a, 1935, and per- 
sonal communication) we calculated the incidence of occu- 
pancy for sparrows in rural areas to vary from 1 to 14 per- 
cent, with an average of 6 percent. For redheaded wood- 
peckers (Melanerpes erythrocephalus), tufted titmice 
(Parus bicolor), and black-capped chickadees (Parus atri- 
capullus), the incidence of occupancy averaged about 5 per- 
cent each. Nest boxes in woods and forest edge are more 
likely to attract chickadees and titmice (Musselman 1935). 
In southern Illinois we found Carolina chickadees 
2h) 
