Illinois Natural History Survey Reports

              1
R e p o r t s
July/
August 1996
No. 340
I N S I D E
The Distributions of
Aquatic Organisms in
the Central Highlands
2
Trophic Cascades in
Ecosystems
4
Using Natural Enemies
for Pest Control
5
Species Spotlight:
Cecropia Moth
6
The Naturalist's
Apprentice:
Chemical Cues
7
Eastern Bluebirds: Effects of Environment on
Reproductive Strategies
Continued on back page
I LLINOIS
NATURAL
HI STORY
S U R V E Y
vival and fi tness. For birds,
habitat structure determines
the availability of nest sites,
availability and accessibility of
food resources, and the types and
densities of predators and nest-site
competitors. In addition,
because environmental conditions
vary among habitats over time,
and may alter the success of a
given reproductive strategy,
there must be some degree of
behavioral fll exibility involved.
All of these factors infl uence
the decisions that breeding birds
make, as well as their ultimate fii t-ness.
By examining links between
reproductive performance and the
breeding environment, this de-cision-
making process and the
development of life histories can
be better understood.
A study is currently under
way to determine how environ-mental
factors infl uence the repro-ductive
behavior and success of
the eastern bluebird, a cavity-nest-ing
songbird, in central Illinois.
From 1992 to 1994, reproductive
traits, behavior, and success of
box-nesting bluebirds in three dif-ferent
habitats (early successional,
agricultural, and forest edge) were
monitored. We found that nest-ing
habitat did not seem to affect
reproductive strategies and there
was no difference in the propor-tion
of boxes used in each habitat
group. In addition, neither clutch
size nor nestling mass (an indica-tor
of nestling quality) differed
across habitat groups. The groups,
Researchers capturing an incubating
female bluebird to be weighed and
banded.
Photo by Jeff Brawn, INHS Center for Wildlife Ecology
Anyone who studies bluebirds
hears the same comments: “I
haven’t seen a bluebird in thirty
years!”, or “Are bluebirds still
around?” In the early 1900s,
bluebirds were a common sight
in suburbs and rural areas. By the
middle of the century, however,
populations had shrunk to one-tenth
of their original numbers.
Several factors contributed to this
decline, including loss of nest sites
due to new agricultural practices
(trimming dead snags and replac-ing
wood fence posts with metal)
and competition with introduced
starlings and house sparrows. Only
with the construction of bluebird
trails, consisting of nest boxes
erected and monitored for bluebird
use, have populations rebounded in
recent years.
The history of the bluebird is a
good example of how the physical
habitat and biotic community in
which animals live affect their sur-
Male eastern bluebird (Sialia sialis).
however, did differ in terms of
productivity. Early in the season,
nests along forest edges produced
a lower proportion of fl edglings
than those in the other groups.
This is probably due to nest-site
competition from house wrens
that set up territories during this
time, and because shrubby and
forest edge habitats are a known
source of nest failure in blue-birds.
Late in the season, nests
in agricultural areas were less
productive than those in other
habitats. During this time, wren
aggression decreases as they raise
their own young. Also, vegetation
in agricultural habitats is tall and