For Ken Able, the first bird songs he
hears each spring are both an ode to joy and a summons to work.“To all of a sudden hear a bird singing after a
long dreary winter is very dramatic,” says Able, an avid observer of birds since the age of 11. “Growing up in
a temperate climate and being interested in birds, I’ve always regarded the spring arrival and fall departure of
migrating birds as big events.” But Able, a biologist at the University at Albany for 26 years, is more than just a practiced birdwatcher. He is one of the country’s leading experts on bird migration. And most every spring, as the birds he loves to watch return, his research moves into high gear. Over the years, his research has taken many forms, ranging from aiming tracking radar at flocks of birds to hand-raising as many as 90 baby birds in one summer, as he’s sought to answer the question of how birds manage their amazing migratory feats. The picture that has emerged—through his work and the research of others—is one of intricate orientation and navigation capabilities. Birds, it appears, depend on a variety of cues: the stars, the sun, polarized light patterns, and the earth’s magnetic field. How birds weigh those cues and sense the earth’s magnetic field are among a host of questions that are yet unanswered. Still, scientists have come a long way in their understanding of migration, and Able, his wife Mary, and the Savannah sparrows they’ve worked with deserve a fair share of the credit. “When I first got into the field, it was an area ripe for investigation. The conventional wisdom was that migratory birds had a star compass and a sun compass, by which we mean they can tell north, east, south, and west, by looking at the sun and stars. But a lot about migration was simply not known,” says Able. He chose migration as his research focus when he joined the University’s Department of Biological Sciences in 1971. Able started his investigations using National Weather Service radar systems to track songbirds, most of which migrate at night, along the Gulf of Mexico. Those initial investigations provided broad brush strokes of information. “Because birds’ bodies have a lot of water in them, weather radar can detect birds and thus you can watch birds’ movements on the radar screen. You can tell something about what kinds of birds you’re looking at—not what species—but you can tell if they are big birds in flocks as opposed to little birds. You can tell something about how fast they’re flying, about their direction, and about how many are flying,” he explains. Able next narrowed his focus. He used an Army surplus radar system to track individual birds, gaining quite precise information on the speed, altitude, and direction of each bird he tracked. But with this approach, he notes, “you lose the big picture.” About 15 years ago, Able, with crucial help from his wife Mary, and funding from the National Science Foundation, took an entirely new tack in his study of bird orientation and navigation. “We decided to try to ask questions of birds we raised in captivity. We wanted to take a developmental approach to the question of how birds orient and how they find their way around,” he says. He chose Savannah sparrows as the birds to whom he would pose his questions. Savannah sparrows migrate between the Northeast and the Deep South or Mexico. Like most songbirds, they’re night migrants and they make their migration journeys largely on their own. That means that young, inexperienced Savannah sparrows must somehow figure out which way to go when the time comes for their first journey from their birthplace in the Northeast to a wintering place in the South. (Birds like geese and cranes, by contrast, migrate in groups where youngsters could learn from the “old folks,” notes Able.) Savannah sparrows are also “reasonably common, amenable to laboratory rearing and housing, and they perform for you in the orientation cages” that are now widely used in experimental studies, says  Able.
“They’ve taught us much,” he says. For these experiments, Mary Able hand-raised between 50 and 90 birds a season. Once they were able to eat on their own, the birds were moved into cages. By varying the conditions affecting the cages, Able “asked” the birds different questions and got his answers from the birds’ hopping patterns. “With this kind of approach, we’re able to control and manipulate the cues used in bird orientation—the sun, skylight polarization patterns, the stars, the magnetic fields—and see how the birds’ orientation mechanisms respond,” says Able. In one set of experiments, for example, the Ables raised Savannah sparrows entirely indoors with no exposure to the visual orientation cues of stars, sun, and polarized light patterns. When the time came for the birds to migrate south, their restless hopping movements were recorded on the paper floors of their cages by their ink-smeared feet. The birds faced in the appropriate migratory direction, southeast. These experiments, as well as similar ones performed by other researchers, demonstrated that migratory birds have a magnetic compass, an ability to orient in the appropriate migratory direction based on the earth’s magnetic field. “We now know that magnetic orientation is widespread in animals. We know it occurs in many species of birds and in all the other vertebrate organisms,” says Able. “The other important compass scientists have learned about is one based on polarized light patterns in the skies. Way back in the ’40s, it was discovered that honeybees use such information, but we discovered its use by birds here,” says Able. “The earth’s atmosphere acts as a polarizing filter scattering sunlight, and the patterns of the polarized sunlight change as the position of the sun in the sky changes,” explains Able. “If you learn the patterns associated with different positions of the sun, you can figure out where the sun is even if the sun is behind clouds and you can see only a little patch of blue sky.” To explore birds’ use of polarized light at sunset, the Ables exposed young Savannah sparrows to clear daytime skies in cages covered with sheet polarizers so that each of three groups of birds observed a different set of light patterns. “In their first autumn, under clear skies (no Polaroids) and in the normal magnetic field, the migratory orientation at dusk of these birds showed that they had learned compass directions relative to manipulated patterns of polarized light. “They selected the directions indicated by this polarized light compass in preference to magnetic directions and seemed unable to select appropriate directions when sunset position was the only available visual cue,” wrote the Ables, in the The Journal of Experimental Biology in 1996. “We have no idea at all how birds see polarization patterns in the scattered light of the blue sky, but it’s clear they sense it somehow,” says Able. The importance of polarized light patterns has been bolstered by subsequent experiments by the Ables and others. “Birds seem to use their polarized light compass especially—though not exclusively—at dawn and dusk,” explains Able. “Dusk is a particularly important time for birds who migrate at night. Presumably it’s a big decision time. ‘Am I going to migrate tonight or not? If I’m going to migrate, which way am I going to go when I leave this tree?’ “Most birds take off within a half hour to 45 minutes after sunset, and that’s the very time of day—as well as sunrise—when polarized light patterns are the most conspicuous.” All this new information about magnetic compasses and polarized light compasses raises, of course, new questions. An intriguing one that the Ables have explored is how these two kinds of compasses work together. In one series of experiments, a large magnetic coil surrounding a test cage was turned on to shift the magnetic field felt by the birds. The field was shifted 90 degrees so that the artificial magnetic north imposed by the coil would cause a compass needle in the cage to point to the west. In a variation of this test, a transparent plastic sheet was placed over the cage to remove polarization from the daylight reaching the birds. 
The Ables found that under the plastic depolarizing filter, a shift in the artificial magnetic field around the
cage caused the birds to shift their preferred direction correspondingly; they ignored the visible sky as a navigational
guide and relied on the false magnetic cue. But when the depolarizing filter was removed so that the birds could
see unfiltered daylight, they ignored the misleading magnetic field and oriented themselves instead toward true
south, as indicated by the naturally polarized daylight. “It appears that the magnetic navigational sense in migrating species is used as a backup when celestial cues are lacking,” Able says. “When the birds are forced to rely on the magnetic field, they try to calibrate their magnetic sense using other cues, especially the polarization of daylight at sunset. “In the hierarchy of navigation cues, it’s my view that sunset information, which seems to be polarized light, takes precedence. Next is the magnetic compass and then the stars —which is a very different perspective than we had when I started in this business. The dogma was that birds that migrate in the day use the sun as a compass and birds that migrate at night use the stars.” Stars are indisputably important, particularly in a young bird’s learning and development. Birds learn that stars appear to rotate except for the star that we know to be the North Star, and birds seem to be genetically programmed to know in what direction they should go in relation to the North Star, says Able. But birds face a wide range of conditions as they migrate, and it’s not surprising that their navigation systems have a lot of flexibility. “On cloudy nights, there may be virtually no visual information; a bird may be able to rely only on its magnetic compass. Even on clear nights, the visual information is changing. A bird that flies very far south, for example, will find that the star patterns it’s familiar with from the northern sky are going to disappear below the horizon behind it and new ones are going to pop up in front of it. If this is its first trip, it will have never seen those stars. And besides all that, there’s variability in the earth’s magnetic field,” says Able. Able’s work has clearly shed new light on how songbirds manage to navigate through such conditions. In the last ten years, he has published more than 35 articles in scientific journals, including the prestigious Nature and Science magazines. In 1996, the American Ornithologists Union bestowed its highest award, the William Brewster Memorial Award, on him for what it called “his success in unraveling some of the mysteries of bird orientation and navigation and his record of excellence in field and laboratory research.” Still, there’s much to be learned. What makes a bird migratory? What are the differences—physiologically, genetically and so on—between migratory and non-migratory species? Any day now, as migrants again start showing up around his home, Ken Able’s delight will remind him of all the work ahead. |
