ON 17 March 2007, the bar-tailed godwit known to science as E7 spread her wings and took flight, leaving the northern shores of New Zealand behind her. For the next eight days and nights she flew non-stop, 10,000 kilometres northwards to the coast of China’s Yellow Sea. Five weeks later, after a brief break for refuelling, E7 continued on her way. She headed east, then took a sharp left turn in the featureless mid-Pacific, before arriving six days later at her breeding ground in Alaska. By late August, she was off again, this time on a non-stop journey of nearly 12,000 kilometres – the longest continuous bird flight on record – that ended just 13 kilometres from where she had started.
Until recently, the idea that we could chart a small bird’s migration in such detail was unthinkable. Now, however, biologists are developing tracking technologies that are leading to a revolution in their understanding of migration. Suddenly they have the power to answer questions they could barely guess at before. They are beginning to learn not just where the birds are going, but also the timetables of their migratory flights and their airspeeds and energy costs. These new techniques are also helping to explain the biology that underpins the uncanny navigational abilities of migratory birds.
The new-found opportunities have biologists brimming with anticipation. “Every new instrument you put out is a major breakthrough,” says Martin Wikelski from the Max Planck Institute for Ornithology in Radolfzell, Germany. “I think we’re entering a new, data-rich realm of animal movement. It’s a very exciting time.”
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The old tracking techniques were crude and not very effective. For a century or so, biologists have been putting numbered metal bands on the legs of migrating birds, then waiting to see where they turned up. This helped to sketch the outlines of migration routes, but revealed next to nothing about what individual birds did on the way.
Then, in the 1960s, William Cochran of the Illinois Natural History Survey in Champaign, began mounting home-made radio transmitters on birds. Using hand-held antennas he would track their migratory flights, chasing them by car or – when budgets allowed – by light aircraft. The technique is still used today because it is quite cheap and the tiny transmitters are light enough to mount on even relatively small birds, but the work involved in following them is hard and the hours punishing, especially when tracking songbirds, which migrate at night.
“You sit out there starting at sunset,” says Wikelski. “You’ve filled up the gas tank. You’ve got your food. You’ve got your sleeping bag. You’ve cancelled all your appointments for the next day… And you wait for the bird to take off. Then you race with the bird, try to catch up with it, and after 500 miles, you have to recapture it [to remove the transmitter] and drive back again. You’re up for 35 hours – and then you have to capture another bird, because you only have a short season.”
The pay-off for all those sleepless nights has been a more intimate snapshot of migration than leg bands ever allowed. This, for example, is how Cochran and Wikelski discovered that Swainson’s thrushes often fly all night, but will cut their flight short if they hit a cold front. Radio transmitters also revealed that these thrushes are sensitive to wind speed, and stay put on nights when it exceeds about 10 kilometres per hour. By adding a heart-rate sensor, Wikelski and his colleague Melissa Bowlin found that the birds’ hearts beat faster on windy nights, showing they have to work harder to go the same distance – even with a tailwind. This may be because higher winds cause turbulence that buffets the small birds. Indeed, this may be the reason most small birds choose to migrate at night, when turbulence tends to be lower, says Bowlin, who is now at the University of Montana in Missoula.
Not surprisingly, the rigours of chasing birds by car – and the many failed nights when a bird chooses a course not well served by roads – have meant that very few researchers have taken on the challenge. And, of course, such an approach will not work for birds that migrate over water, such as godwits. No wonder the new tracking devices are causing such excitement.
In-flight transmission
The godwit study, for example, highlights the benefits of satellite tracking. E7 was one of 24 godwits to have a transmitter surgically implanted in its abdominal cavity. This sends out regular beeps that are picked up by a satellite and relayed to researchers on the ground. The global coverage has made it possible to log the godwits’ entire migration itinerary, and has also revealed that their travel plans are extremely precise, with one primary site at each of their three major stops. “No other sites are showing up, and that has got us concerned,” says Robert Gill from the US Geological Survey’s Alaska Science Center in Anchorage, who heads the tracking programme. Although these sites are all protected, godwit numbers have already dwindled to just a few thousand birds, and any damage to any of the sites could be catastrophic to the species, he says.
As well as showing precisely where birds are flying, satellite tracking also gives important information about the timing. Unlike ground-based observers, who can never be certain whether a departing bird is heading to the opposite hemisphere or merely the next marsh, the satellites reveal the moment birds set off on their migratory journeys. In 2007, for example, the godwits timed their southward departure from Alaska to coincide with a low-pressure system about 1500 kilometres to the south-west. This gave the birds tailwinds several days after departure, as they swept across the Pacific. “They ride on the back of that low-pressure system,” says team member Theunis Piersma from the University of Groningen in the Netherlands. “From a Dutch perspective, that’s like using the wind systems in the south of France.” No one knows how the birds manage this feat of long-distance weather forecasting, but enough of them did it last year to suggest it must be more than a lucky coincidence, says Piersma. What’s more, it makes a difference – the few birds that got it wrong and were caught by storms en route took several days longer to complete their journeys.
For all their ease of use, though, satellite tags are not perfect. The transmitters draw a lot of power and their batteries are heavy: the smallest tags available today weigh about 10 grams, which rules out satellite tracking for any birds smaller than about half a kilogram – about the size of a pigeon. What’s more, the batteries only last a few months – even though they are set to transmit just a few hours a day to save power – so it’s rare that researchers can track a single bird through its full migratory cycle, let alone for several years running.
Other researchers are experimenting with ways of improving tracking technologies. One idea is to fit birds with miniature cellphones so that they can be tracked using the existing network of communication towers. Another approach that already shows promise is to fit birds with tiny, light and long-lasting devices that do nothing more than sense and record the time of sunrise and sunset each day. Once these so-called “archival tags” have been recovered from their subjects and their stored data downloaded, researchers can calculate a position fix for each day, determining latitude from day length (except within a week or so of the equinox, when the length of day does not vary with latitude) and longitude from the sunrise time.
Such tags have offered the first opportunity to track the complete migration of sooty shearwaters, small seabirds that spend most of the year roaming the Pacific away from their breeding grounds in New Zealand. Each spring huge flocks of shearwaters turn up in the northern hemisphere, proving the birds must cover vast distances – but little else. “We knew this migration existed, but we didn’t know if the same birds made that migration in a single season, which directions they chose to go, or the routes they followed,” says Scott Shaffer at the University of California in Santa Cruz. “Just within the last couple of years, we’ve really been able to answer these questions.”
It turns out that the shearwaters make a huge figure-of-eight across the Pacific, flying eastward from New Zealand and then north for a stopover off the coast of either California, Alaska or Japan, before finally heading south again. The whole journey lasts more than six months and covers an average of 64,000 kilometres – the longest known migration of any animal (Proceedings of the National Academy of Sciences, vol 103, p 12799). The tracks also show that the shearwaters zoom quickly through the relatively unproductive tropics to spend most of their time in temperate latitudes where food is plentiful.
This new-found ability to plot exactly where a bird goes on its migratory journey has another benefit. It offers biologists a fresh look at one of the great mysteries of migration: how do birds navigate during such long flights, in some cases returning to the very same tree or cliff ledge they left nearly a year earlier? The question continues to be hotly debated among biologists, with some experimental studies suggesting that birds use a magnetic compass, others implying a celestial compass, and still others indicating that odour cues are the key (see “Track the pigeon”). Clearly, evolution has provided birds with multiple back-up systems so that if one fails – if it is too cloudy to see the sun, for example – they can fall back on another. But what are their preferred options? Tracking wild birds may be the best way to answer that question, says Wikelski. “Go wild, and ask the animals how they decide.”
For example, Thomas Alerstam at Lund University in Sweden, and his colleagues used satellite-tracking data to show how ospreys and peregrine falcons navigate northward. If the birds followed a magnetic compass, they reasoned, each day’s trek should follow a slightly different bearing, because the angle between true north and magnetic north changes slightly from place to place. Instead, though, the birds’ daily bearings remained steady, indicating that they navigate using the sun or stars instead (Animal Behaviour, vol 72, p 875). And in tracking studies of Swainson’s thrushes and white-crowned sparrows, Wikelski and colleagues found that they use the position of sunset each night as their fixed compass point. Different species may rely on different systems, he notes.
True navigation requires both a compass and a map. For some birds, at least, evidence suggests that this “map” is learned, not innate. Wikelski’s team captured white-crowned sparrows in Washington state and airlifted them 3700 kilometres eastward to New Jersey on the Atlantic coast. Then they released the birds and tracked them – the old-fashioned way, by aircraft – as they tried to find their wintering grounds in the south-western US and Mexico. Juvenile birds, which had never migrated before, simply headed south as though nothing was wrong. But veteran birds headed south-west, toward their target – a clear sign that their prior migrations had furnished them with a continent-wide map (Proceedings of the National Academy of Sciences, vol 104, p 18115). The researchers are now working to identify which senses the birds use to build their map, with early indications that smell is crucial.
Thinking even bigger, researchers believe that tracking migrations could provide an early-warning system for climate change and other major ecological shake-ups. Ranging over such large areas, migrators are uniquely sensitive to global change, and as more of their wanderings are tracked this may reveal patterns of change that are not obvious from a single study. That is one reason why Wikelski is creating a database called Movebank, due to go live later this year, that will act as a central clearing house for all the information on migration records. “Those big-scale questions are now at our fingertips,” he says. “We can’t do it right now, but we can next year.”
Track the pigeon
Experiments with homing pigeons reveal that birds use several systems to find their way across the landscape. Pigeons flying over familiar terrain settle into habitual routes in which they regularly make the same turns at the same points along the way. Clearly they rely on visual cues, but do they also use their inner compass? To find out, Dora Biro from the University of Oxford and her colleagues altered the timing of light and dark in an aviary, so that released birds would see the midday sun when they expected to see early-morning sun, and so treat south as east. The birds still flew along their usual paths, but shifted slightly to the east – proof that they use both landmarks and a solar compass (Proceedings of the National Academy of Sciences, vol 104, p 7471).
To work out unfamiliar routes pigeons must have an internal map of the area. Like many birds, they can navigate using the Earth’s magnetic field as a guide, but Anna Gagliardo of the University of Pisa, Italy, has found that they seem to rely primarily on their sense of smell. Birds raised in an aviary shielded from winds – so that they could not associate the odours they smelled with the direction the wind was blowing – were unable to navigate properly (Ethology, vol 114, p 95). Her team also found that surgically cutting the olfactory nerve prevented homing, while cutting nerves associated with perception of magnetic fields did not.
In another study, Biro found that when two birds fly together over terrain that each has learned independently, they take an intermediate path between their two individual routes – provided these are reasonably closely matched. This compromise route tends to be more efficient, because it averages out some of the zig-zags that the individual birds fly (Current Biology, vol 16, p 2123). A similar phenomenon may help wild birds collectively find more efficient migratory routes, Biro speculates.
Birds do it…
Migratory insects, such as bees, dragonflies, beetles, butterflies and moths mainly rely on the sun for orientation, but many can also use polarised light as a compass. The monarch butterfly’s journey from the US/Canadian border south to Mexico lasts around four months.
Marine turtles use Earth’s magnetic field to navigate between their egg-laying beaches and feeding grounds, but strong currents can knock them off course. The longest turtle migration recorded to date was 20,558 kilometres, travelled by a leatherback.
Grey whales undertake the longest migration of any mammal, covering up to 19,000 kilometres annually between Baja, Mexico, and the Arctic seas. Caribou hold the record for the longest overland migration, at around 3000 kilometres in a year.
