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.