Tasie 9.— Breeding populations of brown thrashers in various Illinois habitats. 
Birds Per 
‘Type of Region or 
Habitat Acres 1OG Aree Years Cae Gena Reference 
Hedgerows 9 144 1957 Nest Ford (C) Graber, unpubl. data 
8 189 1957-1958 Strip Central Graber & Graber 1963 
Edge shrubbery idl 61 1957-1958 Strip North Graber & Graber 1963 
13 68 Central 
20 76 South 
Thickets 7 ilies Wei Nest Lake (N) Beecher 1942 
13 De 1950 Nest Jackson (S) Brewer & Hardy 1950 
Block shrub areas BW 28 1957-1958 Strip North Graber & Graber 1963 
49 iW Central 
107 2 ; South 
60 2 1949 Nest Richland (S) Stine 1949 
Early shrub nde 24 1966 Nest Vermilion (C) Karr 1968 
Late shrub e 28 1966 
Orchard 78 i 1957-1958 Strip South Graber & Graber 1963 
Second growth or cut-over woods 15 86 1937 Nest Rock Island (N) Fawks 1937 
iG 57 1938 Fawks 1938 
56 36 1941 Nest Sangamon (C) Robertson 1941 
56 3g 1942 Robertson 1942 
56 25 1944 Robertson 1944 
46 14 1948 Robertson & Snyder 1948 
Virgin floodplain forest ad 2 1948 Nest Sangamon (C) Snyder, et al. 1948 
Grazed bottomland 93 2 1955 Nest Macon (C) Chaniot & Kirby 1955 
Forest (all types including edge) 177 1 1957-1958 Strip North Graber & Graber 1963 
214: 8 Central 
340 0) South 
Di iS) 1937 Nest Lake (N) Beecher 1942 
24 +b 1967 Nest Hancock (C) Franks & Martin 1967 
Forest edge 55 0-2 1927-1946 Nest Champaign (C) Kendeigh 1944, 1948 
55 (0-8 birds/mile) 1949-1968 Nest Champaign (C) Kendeigh & Fawver 1949 
Kendeigh & Forsyth 1959 
and Kendeigh & Barnett 
1968 
Modified woodland 28 a, IDS) Nest Lake (N) Beecher 1942 
Parkland estates 100 18 1915 Nest Cook (N) Eifrig 1915 
Residential areas 160 1 19a 5195S Strip North Graber & Graber 1963 
75 4 Central 
98 3 South 
Swampy prairie 67 g 1941 Nest Sangamon (C) Robertson 1941 
a All figures were converted to read birds per 100 acres or birds per mile of edge (number of territorial males or nests X 2). 
» Less than one. 
22-24. Eggs in the first clutch never developed embryos, 
but the second clutch was successful. 
Cowbird parasitism is probably not a significant prob- 
lem for the thrasher, although the incidence may be high 
at times. Miller (1955) reported two nests parasitized (of 
seven found) in farmland in Champaign County, and 
Hensley (1948) reported one parasitized (of two found) 
in forest edge habitat of Piatt County. In the northern 
region only 2 percent of 83 nests that received full clutches 
were parasitized. The incidence for all central Illinois 
nests with adequate data also was 2 percent. We have 
data on only 39 nests for southern Ilinois, 1 of which was 
parasitized. Parasitized nests fledged thrashers but not 
cowbirds, although in one nest a cowbird survived 6 days. 
Silloway, in a typed manuscript in 1924 entitled “Birds 
of the Peoria Parks,’ and Hensley (1948) reported the 
selective removal of cowbird eggs from thrasher nests, 
30 
presumably by the adult thrasher. The incidence of pare 
sitism was higher for the thrasher than for the other tw 
mimids, and thrasher eggs more closely resemble cowbit 
eggs (Fig. 25). 
Juvenile thrashers may be seen out of the nest as eal! 
as April 27 in southern Illinois (Ridgway 1925), and Ma 
18 in central and northern Illinois. 
Nesting Success 
The laying curve for the thrasher (Fig. 23) sugges 
that most thrashers are not double brooded. Because ‘ 
parasitism, predation, infertility, and other causes, the 
appear to have trouble enough producing one broo 
(Table 12), and we suspect that most June-July nes 
represent repeated nestings after earlier failures. In nes 
that we observed, only about a third of the eggs laye 
produced fledglings (Table 12). These data refer 
