By Tina Yerkes, Ph.D.
Duck and goose migrations raise as many questions as there are answers Another beautiful weekend has arrived, and we set off on a canoe trip to a river that we have never visited. The conversation in our car is predictable and goes something like is: To my husband I say, "Do you have the map and the directions?"
His reply: "Kind of." My retort: "What do you mean 'kind of'? You don't have directions, do you?" "I know where I'm going" is his reply. I quietly hold my tongue because in an hour I know we will be lost and he will refuse to stop at a gas station and ask for directions.
If we cannot find our way across the state without a map, how do birds navigate such long distances from their wintering grounds to their breeding areas and back again to the wintering grounds? How do some return to the exact same place where they had a nest the previous year, or to the exact same wintering ground? Are ducks and geese different in their navigation behavior?
Birds use several visual and nonvisual orientation mechanisms to navigate. Some of the visual cues include the sun, polarized light, the stars, and even landmarks. How many of us wear polarized sunglasses when we fish? Do you realize that birds can use the axes of polarized light to determine the position of the sun and perform sun compass orientation? Birds that navigate at night obviously cannot use polarized light, but the stars can provide a good road map for nighttime migrants, and many waterfowl species do utilize star orientation for navigation.
Because of crafty experiments performed in planetariums, scientists know that some birds actually use the stellar map. Presented with the normal night sky in a planetarium, a caged spring migrant bird will orient itself to the north. If you switch the orientation on the planetarium so that the North Star is actually south, the bird moves toward the south instead of moving in the direction of true north as it should.
Landmarks may be important for navigation, not as compasses, but as directional cues. If asked how to get to Ducks Unlimited's national headquarters in Memphis, I would say: "Turn right at Starbucks and make a left at that good Mexican restaurant." If you ask my husband, he would tell you: "Go west on Wolf River Road and head south on One Waterfowl Way."
Using landmarks to give directions makes sense to me and is probably common among nighttime migrants, which respond to major topographic features, such as coastlines, mountain ridges, and major waterways such as the Mississippi River. One of the nonvisual cues that is believed to aid bird navigation is the earth's magnetic field. Now don't try this at home, but when magnets were placed on the heads of captive birds, they did not fly in the correct direction even on sunny days.
One investigator noted a change in direction and altitude of migrating birds when a powerful underground antenna was turned on, interfering with the earth's magnetic field. Even more interesting than a bird's ability to navigate is its ability to "home." Homing is the ability to find home when a bird is released in an unfamiliar place or from an unfamiliar direction. How waterfowl actually do this is not at all clear.
They likely imprint information about their home breeding and wintering areas and use navigational cues to return to them. Ducks and geese differ in their rates of homing. Adult female ducks often return to former breeding sites. As many at 75 percent of adult female canvasbacks return to their breeding area each year, often nesting in the same pothole where they nested the previous year.
This is also true of cavity-nesting species such as wood ducks, buffleheads, and goldeneyes. Blue-winged teal, on the other hand, have one of the lowest homing rates of all ducks: From 5 to 15 percent return to their former home.
Geese are different because they pair for life. In geese, because pair bonds are long lasting, both males and females home to the same breeding area.
Family units including the mother, father, and goslings will stay together for up to a year. They go to the same wintering area and return the next year to the same breeding area as a family. The young goslings likely learn their migration routes and breeding and wintering areas from their parents.
In young, new pairs, the male will follow the female to her birthplace. If re-pairing does occur, the male goose will follow the female to her breeding area.
The ability to navigate over many miles from breeding to wintering grounds is an amazing adaptation. It is likely that most birds use a combination of visual and nonvisual cues, as well as homing. Navigation and migration behavior is very difficult to study and therefore has not been fully resolved, but we quest for answers with great enthusiasm every fall when the birds return to the same wintering ground, or every spring when I see the same female wood duck nesting in her old box from the year before.
Who leads and who follows? The female duck always makes the choice for the breeding area because she is homing to the site of her birth or a site where she successfully hatched a nest. There is very little evidence indicating which sex determines the wintering site. In most duck species, males and females will go their separate ways after the breeding season, each returning to their respective wintering site from the previous year. Female ducks tend to winter farther south, and those that were successful at raising young arrive much later than males.
How do young ducks find their way?
In nearly all waterfowl species, young birds return to breeding areas at much lower rates than adult females: Only about 27 percent of canvasback young return to the area where they were hatched. This is mostly because young do not survive as well as adults. For those that make it through the winter, how do they find their hatch area? This is an unsolved mystery, but they likely use some of the navigation mechanisms mentioned in the text.
What about the wintering grounds?
We know from banding programs and neck collars that birds will return year after year to the same wintering area. Geese and swans are very loyal to specific wintering sites. Ducks are a little more flexible, yet they still home to wintering areas, as well as molting and migration stopover areas. There are important lessons to learn here for managers and private landowners. For birds to return year after year, disturbance and food availability should be managed.