Illinois Natural History Survey - University of Illinois

Wingbeats Over Illinois

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 passerines (perching birds, such as thrushes or warblers) that fly thousands of kilometers before reaching their destinations. Passerines generally migrate at night, and on quiet nights it is possible to hear them calling as they pass high overhead. During the daylight hours between these migratory flights 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 passerines 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 specific route these birds take during the night? How high and fast do passerines fly during migration? When and where do they stop for the day? William W. Cochran, a retired INHS scientist, has been instrumental in answering these questions about one group of passerine migrants, the thrushes.

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A Swainson's Thrush, one of many Illinois passerine migrants.

Cochran pioneered monitoring bird behavior using radio telemetry--the remote monitoring of animals fitted with radio transmitters that broadcast audible tones. Investigators extended Cochran's work by attaching tiny radio transmitters to the backs of thrushes (Veeries and Swainson's Thrushes) captured near Urbana, 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 flight. During daylight hours, erratic warbling in the radio signals indicated the thrushes were moving about the forest vegetation, probably feeding. Later at dusk, just prior to the onset of nocturnal migration, a nonwarbling tone suggested the thrushes had entered a calm period, orienting themselves and assessing conditions 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 challenging. Migrants pushed by strong tailwinds can fly at speeds approaching 60 kph (36 mph) and their paths are not limited by roads. Many birds would have been lost shortly after takeoff if we were not able to pick-up their transmitters over distances exceeding 10 kilometers (6 miles).

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Veery

On some nights, thrushes covered more than 450 kilometers (270 miles) on continuous flights lasting over 8 hours. Other flights lasted only a few minutes and may represent the thrushes making exploratory forays into the air column, perhaps testing for appropriate weather conditions at migration altitudes (usually several hundred meters above the ground). In one mysterious 5-minute flight, a Veery took off from a small patch of woods, flew a large circle around our tracking vehicle, and landed in the same patch. Another Veery flew nearly straight west out of Urbana, landing near Springfield, Illinois, 2.5 hours later. Northbound spring migrants do not always fly north!

The special radio transmitters enabled the thrushes' individual wing beats to be counted throughout nocturnal flights. These data are the first of their kind ever collected. It was revealed these thrushes beat their wings about 10 times every second in typical migratory flight and as many as 15 times a second during takeoffs; therefore, an 8-hour thrush flight requires about 300,000 wing beats. These unique wing beat data will help unravel questions about bird flight.

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The radio transmitters these thrushed carried, weighed about half as much as a dime.

After a night of flight, the thrushes landed in forested habitats to spend daylight hours feeding and resting for the next migratory flight. The importance of conserving forested stopover habitat for migrating passerines was recently highlighted by INHS scientist Scott Robinson (New York Times, June 10, 1997). Perhaps apropos of Robinson's conclusions, one Swainson's Thrush was tracked to a tiny forest patch (about 15 m across) among miles of corn stubble, where it spent 3 days before departing on another nocturnal flight. Even the smallest forest fragments may be important to these migrants.

Robb Diehl and Ron Larkin, Center for Wildlife Ecology



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