Finding the Way

By J. Dale James, Ph.D., and Andi Cooper

Many of us have used a GPS (global positioning system) to find a "duck hole" in the timber, a pothole off the beaten path, or simply an out-of-town destination.

And before GPS, we depended on maps or road atlases for our navigational needs. Waterfowl, on the other hand, have no need for any of these. They've been winging their way from their breeding grounds to wintering areas and back again for eons. How do they find their way?

While our understanding of navigational mechanisms is limited by the difficulty of studying migrating waterfowl in the field, there are several things we do know. To begin, navigation during migration is primarily an extension of the homing ability found to various degrees in all birds. This ability enables birds to locate their nests or frequently used feeding and roosting sites. In long-distance migrants such as waterfowl, this ability allows them to find specific wintering and breeding sites from year to year with remarkable precision.

We also know that the navigational ability of birds is the result of both innate (genetic) and learned influences. This combination of behaviors provides birds with an extremely adaptable navigational system that can be adjusted based on their environment. This enables them to find their way remarkably well in highly variable conditions and across great distances.

The first basic element of avian navigation is orientation, or the innate ability to adjust direction. Orientation alone, however, cannot get a bird to a precise destination.

Precision in navigation-how a duck or goose gets from Point A to Point B-is generally a learned behavior, which is why immature birds get lost from time to time.

Most research to date has been aimed at answering questions about how birds find their way via these two concepts. In general, these studies indicate that birds employ a variety of cues to aid their navigation, including visual landmarks, the sun, the stars, and geomagnetism.

Landmarks provide birds with directional cues rather than north-south orientation. Prominent geographic features like mountain ranges, coastlines, and major waterways are visible to migrating waterfowl day and night. If you take a close look at the major geographic flyways proposed by Frederick Lincoln in the mid-1930s, you can easily identify the significant landscape features that define each path. The Atlantic and Pacific flyways roughly follow their respective coastlines. The Mississippi Flyway follows the Mississippi River, and the Central Flyway roughly follows the eastern slope of the Rocky Mountains, swinging out through the southern Great Plains along the Platte River and on to the Gulf Coast at its southern terminus.

Research has revealed that birds use the sun to migrate by using the axes of polarized light to determine the position of the sun. This information is then processed to achieve a solar compass orientation. Likewise, nighttime migrants often navigate by stellar maps. Remarkably, experiments conducted with birds in darkened planetariums have shown that spring migrants will orient themselves to the north according to the visual representation of the stars. If the night sky image is rotated so that the North Star is actually in the south, birds move toward the south rather than true north.

Scientists have determined that birds also may use nonvisual cues such as magnetic fields for orientation, although how birds do this is poorly understood. Researchers have shown that magnets placed on the heads of captive birds interfered with the birds' ability to fly in the correct direction, even on sunny days. Another study showed that migrating birds changed both direction and altitude when a powerful underground antenna was turned on, interfering with the earth's magnetic field.

The instinct and capability to return to the same location and area has important ramifications for waterfowl. It makes sense for a hen to return to a location where she has experienced nesting success, but what if that location has been converted from native prairie to corn production while she was away?

Waterfowl face the same problem on their wintering grounds. Northern pintails, for example, exhibit high site fidelity (the tendency to return to a specific site) to wintering areas. Research has shown that survival rates vary greatly from one wintering area to the next. In the San Joaquin Valley of California, survival rates of female pintails are much lower than for those wintering in the Sacramento Valley to the north. This difference is likely due to the greater availability of flooded rice fields and refuge habitats in the Sacramento Valley. For pintails winging their way to the Gulf Coast, this loyalty to habitat may soon spell disaster as coastal marshes and rice agriculture are both disappearing at rapid rates.

The tendency of waterfowl to migrate to and from the same areas year after year is one of the many reasons Ducks Unlimited uses the best available science to guide its habitat conservation efforts. Identifying and conserving the most important and imperiled waterfowl habitats across this continent is essential to sustaining the birds' traditional migrations and maintaining their populations at healthy levels now and in the future.

Homeward Bound

Some ducks return to the precise location where they nested the previous spring, while others return to the same wintering area year after year. The ability of migratory birds to find these specific locations after being away for several months is a form of navigation known as homing.

Homing rates vary among waterfowl species. For example, up to three-quarters of adult female canvasbacks return to the same pothole where they nested the previous year.

Cavity-nesting species like wood ducks, buffleheads, and goldeneyes also return at high rates to prior nesting sites. In contrast, blue-winged teal have one of the lowest homing rates of all ducks. Only 5 to 15 percent of female bluewings return to their former nesting grounds the next year.