days. The development of young to fledging took another 
13 days. Nest-building may generally take less time later 
n the season; we found several early nests that looked 
complete but remained empty 3-4 days before laying 
egan. 
In southern Illinois, plant species used for nesting were 
ibout the same as those used by thrashers (Table 1). The 
veight of mockingbird nests varied with the season, the 
werage being just under 5 feet in April and May (28 
ests), and nearly 7 feet in June, July, and August (32 
ests). We have no explanation for the change. No 
nstances of ground nesting have been reported for this 
pecies. 
Clutch size in 52 nests in southern Illinois averaged 
.8 eggs, with clutch distribution as follows: 6 eggs, 2 
ercent; 5 eggs, 6 percent; 4 eggs, 59 percent; and 3 eggs, 
3 percent. There was little decline in clutch size from 
pril to July, in marked contrast to the nests of both 
atbirds and thrashers. The majority of mockingbird 
ests in central Illinois also had 4-egg clutches. (For 
1ore information on eggs, see Fig. 6.) 
Cowbird (Molothrus ater) parasitism of mockingbirds 
as not been recorded in Illinois. 
Mockingbirds had the poorest nesting success of the 
wee mimids. In April-May only 9 percent of the egos 
id survived to produce fledglings (Table 2). On the 
isis of largely circumstantial evidence, we suspect that 
uch of this mortality came from black rat snakes 
Elaphe obsoleta). We saw this predator near nests 
ten, but rarely saw the actual predation. Mockers often 
tacked the snakes, pecking them vigorously, but with 
tle apparent effect. The mockingbird’s nesting efforts 
ere more successful later in the season (Table Zand 
ost of the production of young came in July and August. 
ore years of study and larger samples of data are needed 
see if this is a consistent pattern. Thrashers nesting 
the same area as mockingbirds had better nesting suc- 
ss early in the season. Mockingbird nests were generally 
ore conspicuous than those of the thrasher, and the 
ocker population may have served as a cushion for nest- 
3 thrashers by absorbing the brunt of the predation. 
1e rate of infertility, ie., no embryonic development, 
‘s only 2 percent in the mockingbird population we 
died, but one form of mortality more apparent in the 
cker than the other mimids was the death of half- 
wn nestlings from unknown causes, but possibly cold 
d exposure. Such deaths accounted for 6 percent of all 
TABLE 2. — Nesting success of mockingbirds in southern 
nois.* 
April-May June-August 
Neat Percent Percent 
Pe Fledged ass Fledged 
sts Eggs Nests Eggs Nests Eggs Nests Eggs 
28-100 9 ¢ 33 108 28 26 
* Nesting success computed by exposure-day method (Mayfield 1961). 
eggs in April-May nests, but only 1 percent in later nests. 
Because there is so much similarity in habitat between 
nesting thrashers and mockingbirds, the question of pos- 
sible competition deserves study. We witnessed one fierce 
fight between birds of these two species near a mocking- 
bird nest in Pope County. 
There are virtually no data on nesting success of 
mockers north of southern Illinois. 
Fall Migration 
Summer and winter distribution records for the mock- 
ingbird show that a definite population shift occurs be- 
tween the two seasons (Fig. 2), but the precise timing 
and nature of that migration are unknown. The appear- 
ance of mockers in northern Illinois from September to 
November (Clark & Nice 1950, Ford et al. 1934, and 
others) may represent fall migration, but from where? 
There are many records of mockingbirds “out of place” 
in the north in November (Lewis 1925, Perkins 1927, and 
others). These records suggest that there may be much 
random wandering by mockers in the fall, and that the 
migration patterns are not yet fully developed. 
Our censuses of mockingbirds in central and southern 
Illinois showed that fall populations were two times 
higher than spring populations and three to five times 
higher than summer populations, day to day. The level 
in October was particularly high (Fig. 2). Do the high 
populations from August to November represent pro- 
ductivity, migration, or both? So far as we know, no 
mockingbird has been found among the thousands of 
night migrants killed in fall at television towers in Illinois, 
but the towers in southern Illinois where we would most 
expect kills are rarely checked. To resolve the problems 
of the mockingbird’s migration, its timing and manner, 
intensive banding studies are needed. 
Winter Populations 
The distribution of winter records of mockingbirds in 
Illinois is shown in Fig. 7. The most extensive winter 
population data for the mockingbird and other species 
come from the annual Audubon Christmas counts. The 
counts indicate great variation in the population from 
year to year (Fig. 8), but the long-term trend has been 
upward in more recent years in all three regions of the 
state. In northern Illinois the frequency of occurrence 
of mockers on Christmas counts averaged less than 1 per- 
cent between 1900 and 1948, but 5 percent in the next 
20 years. The censuses for southern and central Illinois 
show a marked increase in the number of mockingbirds, 
beginning especially in the 1950’s (Fig. 8). To some 
extent this increase may represent increased participation 
in the censuses, but many observers in many different 
areas reported the same kind of change in the Midwest 
and North (see especially Andrews 1967), and we believe 
the increase is real. The statewide censuses show the same 
trend (Graber & Graber 1963). The Christmas counts 
also emphasize the variation in mockingbird populations 
from place to place. In southern Illinois, Richland 
Bh 
