and killed at a television tower near Monticello, Illinois the 
night of September 20-21, 1966. 
Food Habits 
Forbes (1903) examined six specimens of Swainson’s 
thrushes taken in central and northern Illinois in April and 
May, and five taken in September in extreme southern Ili- 
nois. He found the Swainson’s thrush to have the highest 
consumption of insects (98 percent of the diet) of any of the 
thrushes inthe spring. Hymenoptera (31 percent of all 
food, mainly ants) were a primary food, and the intake of 
Coleoptera (30 percent) was typically high for thrushes. 
Caterpillars (22 percent) were also very important. 
In the fall the diet was changed greatly, with wild fruit 
(grapes especially, plus cherries, elderberries, and black- 
berries) making up 60 percent of the food. Gault 
(unpublished notes, 1906-1918) frequently observed 
Swainson’s thrushes eating elderberries in the north, as we 
have also in all parts of the state, but if they have a choice 
the birds will take pokeberries before elderberries. We have 
also seen these thrushes feeding on the fruit of Virginia 
creeper (Parthenocissus quinquefolia). 
Specimen Data 
From the large September and October kills of Swain- 
son’s thrushes at television towers in central Illinois, we 
measured and closely examined nearly 300 specimens and 
saved 93 as study skins. Most of the birds can be placed into 
three fairly distinct color types with no size differences. 
About 16 percent cannot be thus segregated. Each type is 
represented in all sex and age groups. 
About 26 percent of all specimens are definitely gray 
backed (versus reddish) with gray (versus buffy) faces, in- 
distinct spectacles, and dark lead-gray flanks. We believe 
these birds represent the western race, incana. 
Another 24 percent of the specimens are gray backed 
but less gray than incana, with very rich buff faces and dark 
olive-gray flanks. In face and ventral coloration these speci- 
mens match birds from the range of the nominate race, but 
they are much too gray backed. If the group does represent 
a distinct population we cannot identify it. Dr. Earl God- 
frey of the National Museum of Canada examined a few 
specimens of each of the color types and felt that this group 
was best considered a color variant of the eastern popula- 
tion swainsont. 
Specimens in the third group (34 percent) are definitely 
Tas_e 4.—Weights and measurements of Swainson’s thrushes kil 
ber, mainly since 1966. 
1263 
225 
175 
1166 
\4 
125 S 
827 
466 
~I 
on 
SWAINSON’S THRUSHES KILLED 
mR 
SEP. OC 
Fig. 25.—Numbers of Swainson’s thrushes killed at central. 
television towers during night migration. The numbers on the graf 
indicate the total numbers of specimens of all species picked up. 
: : : : nite | 
swainsoni, with reddish backs, distinct spectacles, 
buff faces, and pale gray flanks. 
Bond (1963) also believed there was a strong mig 
of western birds (which he identified as a/mae) throug 
nois. For the present, precise allocation of specimens 
population or the other is less important than the ne 
further corroboration, particularly through bandins 
some western population is an important constituent 
Illinois fauna. The definite westward flights of teler 
Swainson’s thrushes in the spring (Cochran et aller 
further indication of the presence of western birds. 
The sex ratio in the sample of specimens was 
even for adults (Table 4), but immature males ot 
bered immature females about two to one. The age 
(three adult males to one immature male, andsi? 
females to one immature female) do not indicate his 
ductivity. Annan (1962) said that immatures £ 
outnumbered adults, but gave no precise figures: 7 
also gives weights and measurements of the specimer 
led at television towers in central Illinois in September ant 
Bee] 
Gross Weight (grams) 
Wing Length (mm) Tail Lengt! 
Number of ee —— | 
Age and Sex Specimens Range Mean Range Mean Range | 
Adult male 18) 27.8-42.0 Soe 95-105 100.1 66-77 | 
Immature male 41 28.4-43.0 33.5 93-102 97.7 63-73 
Adult female 118 26.0-36.8 BOrO 91-99 95.3 63-74 
Immature female iN DY f= DI) 31.6 90-99 93.4 60-68 
26 
