If you can track a satellite, you can track a Swainson’s thrush.
“Birds fly South for the winter and North for the summer,” has historically proven to be only slightly less reliable a maxim than the sun always rising in the East and setting in the West. Humanity has been fascinated by the comings and goings of our avian neighbors for millennia, but the why’s and how’s of their transitory travel habits have remained largely a mystery until recent years. In Flight Paths, science author Rebecca Heisman details the fascinating history of modern bird migration research and the pioneering ornithologists that helped the field take off. In the excerpt below, Heisman recalls the efforts of Dr. Bill Cochran, a trailblazer in radio-tagging techniques, to track his airborne, and actively-transmitting, quarry across the Canadian border.
HarperCollins
From Flight Paths, Copyright © 2023 By Rebecca Heisman. Reprinted here with permission of Harper, an imprint of HarperCollins Publishers
Follow That Beep
Swainson’s thrush looks a bit like a small brown version of its familiar cousin the American robin. Its gray-brown back contrasts with a pale, spotted chest and pale “spectacle” markings around its eyes. These thrushes are shy birds that forage for insects in the leaf litter on the forest floor, where they blend in with the dappled light and deep shadows. Birders know them by their fluting, upward-spiraling song, which fills the woods of Canada and the northern United States with ethereal music in summer. But they don’t live there year-round; they spend the winters in Mexico and northern South America, then return north to breed.
On the morning of May 13, 1973, a Swainson’s thrush pausing on its journey from its winter home to its summer home blundered into a mist net in east-central Illinois. The researchers who gently pulled it from the net went through all the usual rituals—weighing and measuring it, clasping a numbered metal band around its leg—but they added one unusual element: a tiny radio transmitter weighing just five- thousandths of an ounce. They carefully trimmed the feathers from a small patch on the bird’s back, then used eyelash glue to cement the transmitter, mounted on a bit of cloth, in place against the bird’s skin (Generations of ornithologists have learned exactly where to find the eyelash glue at their local cosmetics store. Designed to not irritate the delicate skin of the eyelids when attaching false eyelashes, it doesn’t irritate birds’ skin, either, and wears off after weeks or months.)
When the thrush was released, it probably shuffled its feathers a few times as it got used to its new accessory, then returned to resting and foraging in preparation for continuing its trek. At only around 3 percent of the bird’s total body weight, the transmitter wouldn’t have impeded the bird noticeably as it went about its daily routine. Then, around 8:40 that evening, after the sun had dipped far enough below the horizon that the evening light was beginning to dim, the thrush launched itself into the air, heading northwest.
It would have had no way of knowing that it was being followed. Bill Cochran — the same engineer who, a decade and a half earlier, had rigged up a tape recorder with a bicycle axle and six thousand feet of tape so that Richard Graber could record a full night of nocturnal flight calls — had been waiting nearby in a converted Chevy station wagon with a large antenna poking out of a hole in the roof. When the thrush set out into the evening sky, Cochran and a student named Charles Welling were following on the roads below.
All they could see in the deepening night was the patch of highway illuminated by their headlights, but the sound of the wavering “beep . . . beep . . . beep” of the transmitter joined them to the thrush overhead as if by an invisible thread. They would keep at it for seven madcap nights, following the thrush for more than 930 miles before losing the signal for good in rural southern Manitoba on the morning of May 20.
Along the way, they would collect data on its altitude (which varied from 210 to 6,500 feet), air and ground speed (eighteen to twenty-seven and nine to fifty-two miles per hour, respectively, with the ground speed depending on the presence of headwinds or tailwinds), distance covered each night (65 to 233 miles), and, crucially, its heading. Because they were able to stick with the bird over such a long distance, Cochran and Welling were able to track how the precise direction the bird set out in each night changed as its position changed relative to magnetic north. The gradual changes they saw in its heading were consistent with the direction of magnetic north, providing some of the first real-world evidence that migrating songbirds use some sort of internal magnetic compass as one of their tools for navigation. Today Bill Cochran is a legend among ornithologists for his pioneering work tracking radio-tagged birds on their migratory odysseys. But it wasn’t birds that first drew him into the field of radio telemetry; it was the space race.
From Sputnik to Ducks
In October 1957, the Soviet Union launched the world’s first artificial satellite into orbit. Essentially just a metal sphere that beeped, Sputnik 1 transmitted a radio signal for three weeks before its battery died. (It burned up in the atmosphere in January 1958.) That signal could be picked up by anyone with a good radio receiver and antenna, and scientists and amateur radio enthusiasts alike tracked its progress around and around Earth.
It caused a sensation around the world — including in Illinois, where the University of Illinois radio astronomer George Swenson started following the signals of Sputnik 1 and its successors to learn more about the properties of Earth’s atmosphere. Around 1960, Swenson got permission to design a radio beacon of his own to be incorporated into a Discoverer satellite, the U.S. answer to the Sputnik program. In need of locals with experience in electrical engineering to work on the project, he recruited Bill Cochran (who still had not officially finished his engineering degree — he wouldn’t complete the last class until 1964) to assist.
Cochran, as you may recall, had spent the late 1950s working at a television station in Illinois while studying engineering on the side and spending his nights helping Richard Graber perfect his system for recording nocturnal flight calls. By 1960, no longer satisfied with flight calls alone as a means of learning about migration, Graber had procured a small radar unit and gotten Cochran a part-time job with the Illinois Natural History Survey helping operate it. But along the way, Cochran had apparently demonstrated “exceptional facility with transistor circuits,” which is what got him the job with Swenson. It was the transistor, invented in 1947, that ultimately made both the space race and wildlife telemetry possible.
The beating heart of a radio transmitter is the oscillator, usually a tiny quartz crystal. When voltage is applied to a crystal, it changes shape ever so slightly at the molecular level and then snaps back, over and over again. This produces a tiny electric signal at a specific frequency, but it needs to be amplified before being sent out into the world. Sort of like how a lever lets you turn a small motion into a bigger one, an amplifier in an electrical circuit turns a weak signal into a stronger one.
Before and during World War II, amplifying a signal required controlling the flow of electrons through a circuit using a series of vacuum-containing glass tubes. Vacuum tubes got the job done, but they were fragile, bulky, required a lot of power, and tended to blow out regularly; owners of early television sets had to be adept at replacing vacuum tubes to keep them working. In a transistor, the old-fashioned vacuum tube is replaced by a “semiconductor” material (originally germanium, and later silicon), allowing the flow of electrons to be adjusted up or down by tweaking the material’s conductivity. Lightweight, efficient, and durable, transistors quickly made vacuum tubes obsolete. Today they’re used in almost every kind of electric circuit. Several billion of them are transisting away inside the laptop I’m using to write this.
As transistors caught on in the 1950s, the U.S. Navy began to take a special interest in radio telemetry, experimenting with systems to collect and transmit real-time data on a jet pilot’s vital signs and to study the effectiveness of cold-water suits for sailors. These efforts directly inspired some of the first uses of telemetry for wildlife research. In 1957, scientists in Antarctica used the system from the cold-water suit tests to monitor the temperature of a penguin egg during incubation, while a group of researchers in Maryland borrowed some ideas from the jet pilot project and surgically implanted transmitters in woodchucks. [ed: Although harnesses, collars, and the like are also commonly used for tracking wildlife today, surgically implanting transmitters has its advantages, such as eliminating the chance that an external transmitter will impede an animal’s movements.] Their device had a range of only about twenty-five yards, but it was the first attempt to use radio telemetry to track animals’ movements. The Office of Naval Research even directly funded some of the first wildlife telemetry experiments; navy officials hoped that radio tracking “may help discover the bird’s secret of migration, which disclosure might, in turn, lead to new concepts for the development of advanced miniaturized navigation and detection systems.”
Cochran didn’t know any of this at the time. Nor did he know that the Discoverer satellites he and Swenson were building radio beacons for were, in fact, the very first U.S. spy satellites; he and Swenson knew only that the satellites’ main purpose was classified. Working with a minimal budget, a ten-pound weight limit, and almost no information about the rocket that would carry their creation, they built a device they dubbed Nora-Alice (a reference to a popular comic strip of the time) that launched in 1961. Cochran was continuing his side job with the Illinois Natural History Survey all the while, and eventually someone there suggested trying to use a radio transmitter to track a duck in flight.
“A mallard duck was sent over from the research station on the Illinois River,” Swenson later wrote in a coda to his reminiscences about the satellite project. “At our Urbana satellite-monitoring station, a tiny transistor oscillator was strapped around the bird’s breast by a metal band. The duck was disoriented from a week’s captivity, and sat calmly on the workbench while its signal was tuned in on the receiver. As it breathed quietly, the metal band periodically distorted and pulled the frequency, causing a varying beat note from the receiver.”
Swenson and Cochran recorded those distortions and variations on a chart, and when the bird was released, they found they could track its respiration and wing beats by the changes in the signal; when the bird breathed faster or beat its wings more frequently, the distortions sped up. Without even meaning to, they’d gathered some of the very first data on the physiology of birds in flight.
An Achievement of Another Kind
Bill Cochran enjoys messing with telemarketers. So, when he received a call from a phone number he didn’t recognize, he answered with a particularly facetious greeting.
“Animal shelter! We’re closed!”
“Uh . . . this is Rebecca Heisman, calling for Bill Cochran?”
“Who?”
“Is this Bill Cochran?”
“Yes, who are you?”
Once we established that he was in fact the radio telemetry legend Bill Cochran, not the animal shelter janitor he was pretending to be, and I was the writer whom he’d invited via email to give him a call, not a telemarketer, he told me he was busy but that I could call him back at the same time the next day.
Cochran was nearly ninety when we first spoke in the spring of 2021. Almost five decades had passed since his 1973 thrush-chasing odyssey, but story after story from the trek came back to him as we talked. He and Welling slept in the truck during the day when the thrush landed to rest and refuel, unwilling to risk a motel in case the bird took off again unexpectedly. While Welling drove, Cochran controlled the antenna. The base of the column that supported it extended down into the backseat of their vehicle, and he could adjust the antenna by raising, lowering, and rotating it, resembling a submarine crewman operating a periscope.
At one point, Cochran recalled, he and Welling got sick with “some kind of flu” while in Minnesota and, unable to find a doctor willing to see two eccentric out-of-towners on zero notice, just “sweated it out” and continued on. At another point during their passage through Minnesota, Welling spent a night in jail. They were pulled over by a small-town cop (Cochran described it as a speed trap but was adamant that they weren’t speeding, claiming the cop was just suspicious of the weird appearance of their tracking vehicle) but couldn’t stop for long or they would lose the bird. Welling stayed with the cop to sort things out while Cochran went on, and after the bird set down for the day, Cochran doubled back to pick him up.
“The bird got a big tailwind when it left Minnesota,” Cochran said. “We could barely keep up, we were driving over the speed limit on those empty roads — there aren’t many people in North Dakota — but we got farther and farther behind it, and finally by the time we caught up with it, it had already flown into Canada.”
Far from an official crossing point where they could legally enter Manitoba, they were forced to listen at the border as the signal faded into the distance. The next day they found a border crossing (heaven knows what the border agents made of the giant antenna on top of the truck) and miraculously picked up the signal again, only to have their vehicle start to break down. “It overheated and it wouldn’t run, so the next thing you know Charles is out there on the hood of the truck, pouring gasoline into the carburetor to keep it running,” Cochran recalled. “And every time we could find any place where there was a ditch with rainwater, we improvised something to carry water out of the ditch and pour it into the radiator. We finally managed to limp into a town to get repairs made.”
Cochran recruited a local pilot to take him up in a plane in one last attempt to relocate the radio-tagged bird and keep going, but to no avail. The chase was over. The data they had collected would be immortalized in a terse three-page scientific paper that doesn’t hint at all the adventures behind the numbers.
That 1973 journey wasn’t the first time Cochran and his colleagues had followed a radio-tagged bird cross-country, nor was it the last. After his first foray into wildlife telemetry at George Swenson’s lab, Cochran quickly became sought after by wildlife biologists throughout the region. He first worked with the Illinois Natural History Survey biologist Rexford Lord, who was looking for a more accurate way to survey the local cottontail rabbit population. Although big engineering firms such as Honeywell had already tried to build radio tracking systems that could be used with wildlife, Cochran succeeded where others had failed by literally thinking outside the box: instead of putting the transmitter components into a metal box that had to be awkwardly strapped to an animal’s back, he favored designs that were as small, simple, and compact as possible, dipping the assembly of components in plastic resin to seal them together and waterproof them. Today, as in Cochran’s time, designing a radio transmitter to be worn by an animal requires making trade-offs among a long list of factors: a longer antenna will give you a stronger signal, and a bigger battery will give you a longer-lasting tag, but both add weight. Cochran was arguably the first engineer to master this balancing act.
The transmitters Cochran created for Lord cost eight dollars to build, weighed a third of an ounce, and had a range of up to two miles. Attaching them to animals via collars or harnesses, Cochran and Lord used them to track the movements of skunks and raccoons as well as rabbits. Cochran didn’t initially realize the significance of what he’d achieved, but when Lord gave a presentation about their project at a 1961 mammalogy conference, he suddenly found himself inundated with job offers from biologists. Sharing his designs with anyone who asked instead of patenting them, he even let biologists stay in his spare room when they visited to learn telemetry techniques from him. When I asked him why he decided to go into a career in wildlife telemetry rather than sticking with satellites, he told me he was simply more interested in birds than in a job “with some engineering company making a big salary and designing weapons that’ll kill people.”
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