Illinois Natural History Survey Reports

              1
I LLINOIS
NATURAL
HI STORY
S U R V E Y R e p o r t s
September/
October 1997
No. 347
I N S I D E
PEET: A Training
Effort That is Paying
Dividends
2
Understanding
Large-river Fish Com-munities
3
Complex Life-cycle
Puzzles
4
Waterfowl Research
Center Named to Honor
Bellrose
5
Species Spotlight:
Eastern Mole
6
The Naturalist's
Apprentice:
Stream Habitats-
Leaf Packs and Surface
Film
7
Continued on back page
Wing Beats Over Illinois
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Each spring migrating birds
that winter in South and Central
America make their way back to
breeding sites in North America.
Most of these migrants are pas-serines
(perching birds, such as
thrushes or warblers) that fl y
thousands of kilometers before
reaching their destinations. Pas-serines
generally migrate at night,
and on quiet nights it is possible
to hear them calling as they pass
high overhead. During the day-light
hours between these migra-tory
fl ights they rest and feed in
preparation for the next nocturnal
migration.
Migration is a critical time
for these birds and many are
thought to perish during the trip.
Nonetheless, little is known about
the natural history of many pas-serines
during the weeks they
spend in migration to and from
their breeding sites. When in the
late evening do migrants take off
for a night’s migration? What
is the specifi c route these birds
take during the night? How high
and fast do passerines fl y during
migration? When and where do
they stop for the day? William
W. Cochran, a retired INHS sci-entist,
has been instrumental in
answering these questions about
one group of passerine migrants,
the thrushes.
Cochran pioneered monitor-ing
bird behavior using radio
telemetry—the remote monitor-ing
of animals fi tted with radio
transmitters that broadcast audi-ble
tones. Investigators extended
Cochran’s work by attaching tiny
radio transmitters to the backs of
thrushes (Veeries and Swainson’s
Thrushes) captured near Ur-bana,
Illinois, in mid-migration.
These transmitters produced
continuous, not pulsing, audible
tones that warbled in response
to movement by the thrushes,
revealing these birds’ migratory
behaviors both before and during
fl ight. During daylight hours, er- ratic warbling in the radio signals
indicated the thrushes were mov-ing
about the forest vegetation,
probably feeding. Later at dusk,
just prior to the onset of noc-turnal
migration, a nonwarbling
tone suggested the thrushes had
entered a calm period, orienting
themselves and assessing condi-tions
for migration. Then after
sunset, the thrushes took off,
the tones from their transmitters
warbling synchronously with the
birds’ beating wings.
Following a migrating bird in
a radio tracking vehicle is chal-lenging.
Migrants pushed by
strong tailwinds can fl y at speeds
approaching 60 kph (36 mph)
and their paths are not limited by
roads. Many birds would have
The radio transmitters these thrushes carried weighed about been lost shortly after takeoff if
half as much as a dime.
A Swainson's Thrush, one of many Illinois migrants.
Photo by Ron Larkin, INHS Center
for Wildlife Ecology
Photo from INHS image archives