Finding Beautiful Facts in Feathers
Through new studies of bird plumes, science sleuths are solving some perplexing mysteries about avian migration patterns and the effects of pollution
- Cynthia Berger
- Aug 01, 1997
Perched in a tall maple tree at New Hampshire´s Hubbard Brook Experimental Forest, a tiny bird looses a series of high-pitched notes and then falls silent. But even with its bill firmly closed, the creature still sends a message. Its sleek body and the bright red patches on its wings and tail serve as a kind of avian shorthand that says, "male American redstart in prime breeding condition."
This message, written in feathers, is clear to anyone who knows a little about bird identification. Recently, however, new research techniques have allowed scientists to read other, more subtle messages from the feathers of the redstart and other birds. Some of these messages are visible to the naked eye, but others can be read only in translation--by disassembling the fibrous, sulfur-containing protein called keratin, of which feathers are composed, into its molecular components. The first rough translations of these feather communiques promise new insights into avian ecology and may help us to set priorities for bird conservation, to monitor environmental pollutants and even to protect our own species.
In upstate New York, in the warehouselike building that houses Cornell University´s Vertebrate Collections, researcher Kevin McGowan opens a tall, metal cabinet, slides out a shallow drawer packed with cotton-stuffed birds and selects the prepared skin of a Florida scrub jay. He holds the specimen belly-up so that the underside of its long, blue tail catches the light from fluorescent bulbs overhead. Narrow stripes shimmer across the width of the tail, looking something like the teeth of a comb. "All birds have these growth bars, but they´re really easy to see in scrub jays," says McGowan, who studies jays and their relatives. Seen close up, the bands are distinct--clear enough to count and measure. At a distance, they blur together and the tail looks solid blue.
Growing evidence: African ostrich farmers, selling feathers to milliners in the early 1900s, were the first to describe feather bars. In the late 1930s, two amateur bird banders in California trapped, measured and released the same backyard house finches day after day until they confirmed a hunch that each bar--one light band plus one dark band--represents 24 hours of feather growth. From that modest experiment, scientists have since confirmed more precisely the rate at which such avian plumes grow.
Most birds molt, or replace, their flight feathers once a year; some do it more often. The alternating bands of lightly pigmented and heavily pigmented feather material are laid down while each feather is growing--a process that usually takes two to four weeks. The new feathers sprout from follicles in the skin, somewhat like human hair. But whereas hair keeps growing almost indefinitely, feathers grow until they are fully formed, then stop. The blood supply to the follicle cuts off, and the feather maintains its form until the next molting cycle.
Growth bars, scientists have learned, are like a dieter´s diary, a terse account of past meals. The bars are narrow when food is scarce, wide when there is plenty to eat. Ohio State University biologist Tom Grubb, Jr. was the first person to realize that he could read the nutritional record etched into the growth bars.
Surprise discovery: Grubb chuckles as he explains that he made the discovery in 1987 while trying to answer a completely different question. "I had collected tail feathers from some tufted titmice because I wanted to test the idea that the shape of the feathers could reveal a bird´s age," he said. "There was evidence for other species that first-year birds have more sharply pointed feathers than older birds. But I put the feathers in an envelope in my pocket and forgot about them. When I finally looked at them, I suddenly noticed the banding--I guess I´d never looked closely before." For Grubb, the discovery was like a light bulb going on.
"I did a literature search and came up with that article about the house finches and the 24-hour banding," he says. "And I thought, if I trap a bird and pull one tail feather out, the bird will grow a new one to replace it. I´ll know exactly when the new feather started to grow, and it will be like a little computer printout--a complete record of how well the bird ate while the feather was growing."
Grubb calls his biological bar code reading "ptilochronology" (literally, "the study of feather time"). In one study, he collaborated with researcher Bob Mauck to temporarily handicap storm petrels by clipping a few feathers so the birds could not fly as well as an unclipped control group--a painless process that did not harm the birds. Because of the hampered flight, the clipped birds could not collect as much food as the unclipped petrels.
Depriving young: At the beginning of the experiment, Grubb and Mauck pulled a single tail feather from each bird. At the end, they pulled the new feather that had grown to replace it and measured the bars. The handicapped parents grew new feathers with bars as robust as those of the control group, indicating they were eating normally. However, the offspring of the handicapped birds gained weight more slowly than did the control nestlings, a sign nestlings of the modified parents were not getting as much to eat. Evolutionary survival dictated that petrel parents put themselves first, eating a normal diet while depriving their young.
Another way to use ptilochronology, says Grubb, is to measure habitat quality. Take the loggerhead shrike, a gray-and-black bird the size of a stocky starling, with a stout, downcurved beak. Nicknamed "butcherbird" for its habit of impaling on thorns and barbed-wire fences the grasshoppers and mice it catches, this bird once was common throughout North America, from the southern Canadian provinces down through Mexico. Like many grassland species, however, loggerhead shrikes are now declining in their range. They are listed as either threatened or endangered in many states and in the past 15 years have virtually disappeared from the northeastern United States and Ontario, Canada.
The reasons for these declines are not clear, but modern farming practices seem to be a factor. Grubb and Israeli ornithologist Reuven Yosef discovered this while studying shrikes in Florida, where the bird´s scrubland habitat is being converted to citrus groves. The researchers compared the growth bars of shrikes from four types of habitat: undisturbed scrublands, citrus groves, pastures and urban areas. Shrikes in citrus groves, which are routinely sprayed with insecticides, had the narrowest bands, indicating they were finding the least food of the birds studied. Citrus groves, the feather study suggested, were trouble spots for this declining species.
Hoarding behavior: Swedish researchers Jan-Ake Nilsson, Hans Kallander and Owe Persson have used ptilochronology in another way: to study hoarding behavior in the Eurasian nuthatch. Like several North American bird species, these nuthatches often store food in hidden caches. It seems reasonable to think that this behavior is adaptive--surely it helps birds survive when times are lean. But how can we be sure?
Ptilochronology provided the answer. In the Swedish study, birds that hoarded extra seeds provided by the researchers grew back tail feathers that had been experimentally plucked much faster than those of the unsupplemented birds. The hoarders also grew feathers that were longer, with wider growth bars than those of the non-hoarding birds--direct evidence that they were better nourished.
Ptilochronology lets scientists read a feather message that, although subtle, is visible to the naked eye. But at Dartmouth College a team of scientists is decoding an invisible feather message by adopting an analytical technique used mostly for geological research. It is a message spelled out in the alphabet of isotopes.
Geography lesson: Isotopes are stable forms of chemical elements that differ in their atomic weights. The exact isotopic composition of rocks, soil and plants varies in predictable patterns from region to region. This variation often allows geologists to determine what region a rock came from just by measuring its isotope ratios. Similarly, when a redstart eats a caterpillar that fed on a maple that grew in the thin, granitic soil of Hubbard Brook, the bird incorporates a geographic tag into its bones, muscles and feathers.
Dartmouth geochemists Page Chamberlain and Joel Blum, and their colleague Dick Holmes, think a new application of isotope studies to birds has the potential to sort out populations of migratory birds over vast geographic areas. Their ultimate goal is to map northern breeding populations to the exact tropical locations in which they winter. This research is of special interest to conservationists, because many forest-nesting songbirds are declining. Redstart populations in the Adirondacks, for example, have plummeted 38 percent in the past 10 years. Wood thrushes, scarlet tanagers, cerulean warblers and many other migrants also are dwindling.
Scientists debate the cause of these declines. Though the news media have spotlighted tropical deforestation as the likely villain, Holmes and others think the biggest threats may be in North America. Isotope studies could help resolve the debate. "Imagine two songbird populations that breed in different places in the north," says Chamberlain. "And one breeding population is holding steady, but the other is declining. It could be that problems right here on the breeding grounds are contributing to the decline. But problems on the wintering grounds might also be a factor. We can´t sort things out unless we know where the birds spend the winter."
Migration mystery: If only birds arrived in the Neotropics the way tourist luggage comes off a plane, with a tag that shows where the flight originated. Unfortunately, many songbirds are too small to carry radio transmitters, one useful tag. Tiny warblers can be tagged with metal ankle bands, but so far, even labor-intensive banding studies have not been able to put many pieces of the puzzle together. "We searched all the U.S. Fish and Wildlife Service banding records to see if we could match where certain redstarts bred with where they went in winter," says Holmes. "But only a dozen banded redstarts had ever been recovered in the tropics--and they were banded on migration, not on their breeding grounds. So we don´t know where they came from." Adds study collaborator Tom Sherry of Tulane University: "The beauty of using isotopes is that you don´t have to catch the bird to mark it--it´s marked for you."
The Dartmouth team has been doing some preliminary studies using feathers from a migratory species that is not in trouble--the black-throated blue warbler. This warbler, whose numbers are holding steady, makes an ideal test case, says Chamberlain, because a complete feather collection already exists. Gary Graves, a researcher at the Smithsonian Institution, has collected black-throated blue specimens across the bird´s North American range for his own studies, and he generously contributed feather samples to the project. "If we´d had to start with redstarts or some other species," says Holmes, "it would have been a major effort just to get the feathers to work with."
To analyze feathers, Chamberlain uses a custom-built apparatus that sorts a feather´s isotopes by weight. The results are promising. The team has found clear geographic trends in carbon and hydrogen isotope ratios: highest in feathers from Georgia, which is the southern limit of the black-throated blue´s breeding range, and lowest at the northern limit of the range, in southern Canada. "We´ve also analyzed some feathers collected on the wintering grounds," says Holmes. "It looks like birds from separate breeding populations in the northernmost part of the breeding range do mix together in the Caribbean in winter." Now that the team has shown that feather isotopes can identify bird populations from different regions, the scientists hope soon to conduct a similar study using redstart feathers. "Once we can link breeding and wintering grounds and identify critical habitats, we can set priorities for habitat protection," says Holmes.
Pollution record: If Holmes and his colleagues see feathers as being a little like luggage tags, Joanna Burger of Rutgers University finds them more like the lapel badges some scientists wear to document exposure to hazardous elements. Bound up in each feather´s structure is a record of the bird´s exposure to heavy metals and other dangerous pollutants. River and ocean water may contain toxic heavy metals. These pollutants become concentrated as they move up the food chain. Birds are often the top predators in ocean food chains. When they eat contaminated fish, the metals circulate in their blood and accumulate in some tissues--including feathers. So feather samples can serve as an index of environmental pollution.
Heavy-metal pollution could be monitored by analyzing seawater or fish, says Burger. But there are problems with both of these approaches. The oceans are so vast that collecting representative water samples is nearly impossible. And some fish migrate, following watery routes that are still mostly unknown to science. "If you don´t know where the fish has been, you don´t know how to interpret the measurements you get," Burger says.
In contrast, scientists know a great deal about the migration paths of birds and about bird life cycles. Burger and colleague Michael Gochfeld usually sample breast feathers from young birds in the nest. "This way, we know the only food they´ve eaten is what their parents collect locally," she says. "So we´re sure we´re measuring only local contamination."
Dangerous effects: Burger wants to know the natural concentrations of heavy metals in feathers so that if problems develop, they can be detected. One of her projects involves measuring the manganese content of herring gull feathers. "I wanted to find what levels of manganese are normal, and what levels might be problematic," she says. "I´m finding that manganese affects birds much like lead. It changes how well they can find food, and it affects their ability to avoid predators." Although manganese has been a legal gasoline additive in the United States since last fall, its potential health effects are not yet established.
In the case of lead, a known toxic, Burger has found that even exposure to very low levels can delay by a week a young gull´s ability to recognize its parents. "That´s a critical problem," she says. "The young birds walk around the colony, and if they go up to the wrong adult, they´re going to get hit over the head or even eaten."
If you are tempted to collect some feathers yourself, says Burger, remember this: Under the Migratory Bird Treaty Act, almost all native North American birds are protected--and so are their eggs and feathers. Collecting feathers, even from common backyard birds, requires both state and federal permits. For now, most of us will have to be satisfied with reading feather messages in the scientific press.
Cynthia Berger is a senior editor with Finger Lakes Productions, Inc., in Ithaca, New York.