By Keith McKnight, Ph.D.

No feature defines birds quite as well as feathers. Bats can fly, but they have hair. Turtles lay eggs, but they have shells. Ask any kid what makes a bird a bird, and they're likely to say, "it's the feathers." Ask any adult why birds have feathers, and they'll probably tell you, "so they can fly." Both answers are correct partially. Although other features also distinguish birds from other critters, feathers are one of the only characteristics (certainly the most noticeable) completely unique to birds.

But feathers do more than facilitate flight; they also serve to insulate, protect, and even communicate. However, the intrigue of feathers goes far beyond their varied uses. Take a closer look and see what I mean. A feather is a finely tuned collection of branches and sub-branches. Similar to a leaf, the center "stem" is called the rachis. Each branch off the rachis is called a ramus, and each ramus is joined to its neighbor by a Velcro-like system of barbules and barbicels. Through this fairly complex structure, feathers gain strength, flexibility, and light weight-all at the same time.

The description above applies to the standard contour (outer body) or wing feather. However, variations on this structural theme give rise to several specialized feather types. If the function of a feather is insulation, then a softer, fluffier texture is best. Hence, a down feather has a less rigid rachis (or none at all, as in waterfowl) and reduced barbules and barbicels that don't grip one another. Semifiloplume feathers (or simply "plumes" to most of us), as you might find on a breeding snowy egret, are a compromise between contour and down feathers. The long, soft appearance of these feathers made them ideal for decorations in ladies hats around the turn of the century, which proved near disastrous for these birds. Bristles are another variation on the basic feather theme. If you look closely at a flycatcher (such as phoebes, peewees, and kingbirds), you'll see bristles at the base of its bill. Bristles serve to help funnel flying insects into the mouth and are essentially stiff feathers composed of nothing but a rachis.

All feather types need regular care and upkeep. Birds care for their feathers by preening, which usually involves moving, adjusting, or combing feathers with the bill. This activity may accomplish several tasks. First, it serves simply to keep the feathers arranged in the correct position. Flying, walking, and swimming tend to disturb the arrangement of feathers, which is critical to insulation and other functions. As you might guess (or have seen), the rami of feathers can become pulled apart or "unzipped." Recall that the only thing holding them together are the barbules and barbicels. When this happens, the bird can simply pull that portion of the feather through its bill from the rachis outward and zip it back together. If you have a feather lying around, try it. Just pull a portion of the feather apart and then pull it through your fingers to rejoin the rami.

Preening also serves to remove parasites from the feathers. Some of these parasites, such as chewing lice, actually feed on the feather (an obvious problem for the bird). With a list of up to 12 species potentially living on the plumage of a single bird, it is easy to imagine why birds require constant vigilance against these pests. Finally, feathers become brittle over time without the aid of the oils and waxes of the uropygial ("preen") gland. This gland is located near the base of the tail. Therefore, the oils and waxes must be manually applied to the feathers by using the bill. In addition to providing structural treatment, oils of the preen gland also help to regulate the fungal and bacterial floras of the feathers. This discourages growth of detrimental fungi and bacteria and promotes the growth of those that discourage feather lice.

Regardless of how much meticulous care is given to the feathers, they eventually wear out and must be replaced. This regular replacement of feathers is known as molt. Because feathers provide a variety of functions, molt does not serve solely to replace physically worn feathers. In many birds, molt functions to provide the bird with plumage appropriate to the season.

Ducks generally possess two different plumages, a basic (eclipse) plumage, and an alternate (breeding) plumage. For males, these can be quite different. Alternate plumage serves to clearly identify the bird as a member of its species and to attract a mate. Therefore, we see the colorful plumages of males during winter and spring. However, once the job of finding and maintaining a mate is done, males soon find a suitable habitat to replace the wing feathers. During wing molt, ducks cannot fly and are extremely vulnerable to predation. Hence, the males also replace the colorful body feathers with drab basic (eclipse) plumage, enabling them to hide more easily while they cannot escape trouble by flying. There is more than meets the eye with feathers. They are complex structures that perform a variety of important functions. Indeed, feathers are one portion of the anatomy to which birds (and people) give particular attention and one of many avian features that elicit our awe and appreciation.

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.

Color

Ducks vary in color, from the drab browns and tans of a mallard hen to the strikingly colorful wood duck drake to the subtly beautiful king eider. This incredible variation in color is the result of plumage coloration. And depending on the color, the mechanism behind it may be different. Feather color results from either pigment within the feather or from the very structure of the feather itself. Colors like brown, yellow, red, orange, and black are the result of pigments. Some pigments, such as melanin in the black wing tips of snow geese, provide feathers with added resistance to wear. In fact, if you take a look at the primary wing feathers of most waterfowl, you'll find that the tips and/or outer edges are darker than other parts of the wing for this very reason.

On the other hand, iridescent greens, purples, and blues are produced by the way light is reflected and bent within the structure of the feather. Selective absorption and reflection of light results in the glistening blues and purples and greens in the speculum of a dabbling duck, and the green head in a . . . well, greenhead. These colors actually change in appearance depending on your angle of sight. Look at these same feathers in a shadow, however, and it is as if someone flipped off the switch. Without direct light, these feathers look black.