Illinois Natural History Survey - University of Illinois

Bird Migration: How Much Fuel Does a Songbird Need?

Scientists at the Illinois Natural History Survey pioneered research on migration using tiny radio transmitters attached to birds during natural flight. Tracking the birds is accomplished by driving beneath them in a vehicle fitted with special electronic equipment and chasing as long as possible. Because most migration takes place high above the ground at night, radio tracking is one of the few ways of learning about this critical phase in the life of birds. A new use of this method promises to help conservation of migrating songbirds.

Birds migrating through Illinois require the right habitat for daytime stopovers. In particular, they need fuel--a rich food supply to support the energetic demands of long migratory flights. For the majority of forest-breeding birds this means wooded habitat, which is scarce in both urban areas and the rich agricultural areas covering much of Illinois. We do not know whether migrating birds actually suffer because they have trouble finding suitable wooded areas for stopover or because they are packed in too densely, but we do know that wooded areas swarm with hungry birds during migration.

Are woodland "gas stations" in Illinois up to the job of providing fuel for millions of migrating songbirds during a few weeks in spring and fall? To answer the question one needs to know not only the quantity and quality of fuel available and the number and sizes of birds needing it, but also the mileage attained by birds flying long distances at night. Flight efficiency during migration is presently estimated using simple aerodynamic theories of how birds fly (assuming they are like little aircraft) and measurements of metabolism of birds on short flights and confined in wind tunnels.

A typical woodlot in east-central Illinois where Survey researchers captured, measured, and released radio-tracked songbirds.

To get substantial data on this subject, a team of researchers in the Center for Wildlife Ecology is combining radio tracking with an isotopically labeled water technique on freely migrating birds. Researchers catch a migrant bird while it is feeding in a wooded area, inject it with a tiny quantity of an inert but detectable water isotope, and attach a radio transmitter. Then they follow the bird's transmitter wherever it flies, all night if necessary, and recapture the bird as soon as possible to determine the energy used in the flight by analyzing the water isotope. The work is being done in collaboration with scientists at the University of Illinois and in the Netherlands. The results should help us evaluate and manage remaining forested areas in Illinois so they can support birds that depend on them.

During planning, the researchers had to explain that they were basing an entire research project on being able to follow a 1-ounce migrating bird anywhere it decided to fly and then catching that same bird again right away. Although no one said, "You plan to WHAT?," there were many polite smiles and vague wishes of good luck. Fortunately, followed by two vehicles and an occasional radio-tracking aircraft, many birds are cooperating. One recent thrush plopped softly into a net in a patch of forest 40 minutes after sunrise, none the worse for wear, its radio transmitter beeping steadily.

If you are a property owner, tired scientists may knock on your door early one morning in the fall or spring explaining that an important small bird has chosen your land for stopover habitat. They will catch the bird, peel off its little transmitter, take a drop of blood from a pinprick, and release the bird to fuel up and continue its migratory journey. Be understanding, and be proud that you are providing an essential resting and refueling place for insect-eating migrating birds!

Martin Wikelski, Ronald P. Larkin, Arlo Raim, Philip Mankin, Robert H. Diehl, Center for Wildlife Ecology

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