One Reason Migrating Birds Get Lost Is Out of This World

During the first week of September 1958, keen observers spotted hundreds of rare birds in the British Isles. According to a report at the time, they saw melodious warblers, tree pipits and, most surprisingly, a “remarkable influx” of red-breasted flycatchers. Very few of these flycatchers typically visit the United Kingdom annually as they make their way from central Europe to their wintering grounds in South Asia. Those who do wind up in the U.K. are termed “vagrants,” since they are well outside their expected range.

The appearance of red-breasted flycatchers, let alone at least 30 of them, so far off their migratory course that autumn baffled birdwatchers and ornithologists at the time. Analyzing weather maps for recent storms did little to explain the phenomenon. Kenneth Williamson, a migration research officer for the British Trust for Ornithology, wrote that the phenomenon was extremely difficult to understand, considering that the weather was “well-nigh perfect for orientation.”

Now, 65 years later, new research into avian navigation gives scientists another hunch about what might have happened. They’ve found that Earth’s weather isn’t the only thing that can cause birds to veer off course—space weather seems to impact birds’ internal GPS. Bursts of energy from the sun in the form of sunspots, solar flares and coronal mass ejections are becoming more frequent and may affect how birds navigate.

“There is a signal across very different data sources, very different methods, suggesting that these extraterrestrial phenomena have real-time impacts on the organisms that can sense them,” says Eric Gulson-Castillo, a University of Michigan graduate student who led a study into the effects of space disturbances on bird migration. “We ourselves cannot directly sense them, but the birds can.”

“Space weather” and “Earth weather” both describe natural and often unpredictable phenomena, but the similarities stop there. While “Earth weather” can represent a range of water, wind, earth and solar events, “space weather” simply refers to variations in energy flow from the sun, explains Gulson-Castillo.

These blasts of magnetic solar energy disrupt Earth’s geomagnetic field, which humans can only feel secondhand as disturbances to our technological systems. Solar storms and other space weather cause discharges of static electricity that can overwhelm satellites, which power GPS, and knock out power lines. In 1989, for instance, a geomagnetic storm led to a nine-hour blackout in Quebec.

Our terrestrial magnetic field is formed when heat energy from molten iron in Earth’s outer core is converted into electrical and magnetic energy. This ever-changing, fluid process leads to a slightly variable magnetic field and, at times, results in an odd outcome: Every 300,000 years or so, Earth’s magnetic poles flip locations—the North Pole becomes the South Pole and vice versa. When space weather enters this equation, it temporarily puts Earth’s magnetic field on the fritz.

Even though humans don’t have magnetoreception, or the ability to sense magnetic fields, many animals do, including whales, turtles, fish and birds. Scientists have known for decades that birds use magnetic fields to find their bearings, but researchers didn’t understand what happens when these fields are unexpectedly disturbed. Using large data sets, two studies published earlier this year —one in Scientific Reports, and the other in Proceedings of the National Academy of Sciences—paint a clearer picture of how birds respond to geomagnetic disturbances.

Gulson-Castillo, the lead author of the latter study, combined radar data from 1995 to 2018 with geomagnetic measurements during the same period of time. Radar beams that are used for weather detection also bounce off migratory birds when they travel in large flocks at night. By analyzing scans roughly half an hour apart, researchers can isolate bird movements from other signals. Concurrently, observatories across North America measure the Earth’s magnetic field, picking up any geomagnetic disturbances due to space weather. Combining these two data sources allowed Gulson-Castillo and his co-authors to measure the effects that transitory disturbances due to space weather would have on birds migrating each night.

The researchers found a significant decrease in bird migration numbers—around 10 percent—during high geomagnetic disturbance. They took that finding to mean that birds were increasingly hesitant to migrate when Earth’s magnetic field had been considerably altered by space weather. Additionally, the birds in the fall that did choose to migrate behaved strangely: More than usual, they chose not to struggle against the wind and instead drifted where it pushed them. Since a greater proportion of birds would otherwise fly against the wind, the researchers speculated that submitting to the wind could throw these birds off course—eventually leading them to become vagrants, found far outside their expected range.

In the other study, in Scientific Reports, ecologist Morgan Tingley of the University of California, Los Angeles, correlated bird vagrancy with geomagnetic disturbances and solar activity. Over the course of 60 years, scientists at the Bird Banding Laboratory captured and banded two million birds in the United States and Canada, recording their species and location. Using these records, Tingley calculated a vagrancy index for each capture by juxtaposing the bird’s documented location with the expected range of its species for that season. He took the number of sunspots and a daily measure of geomagnetic disturbance and averaged each over the three weeks prior to a bird’s capture, to account for the lag period between a bird veering off course and being caught.

Tingley found that during fall migrations, geomagnetic disturbances were strongly associated with vagrancy. Armed with this correlation, Benjamin Tonelli—the first author of the research and Tingley’s graduate student—zeroed in on the peculiar birds found in the U.K.

“Ben went back and actually looked at the geomagnetic history preceding September 1958 and found that like there was a massive, massive surge of geomagnetism just preceding that event that would have been particularly strong in that area and very well could have led to the vagrancy of all of these birds,” Tingley says.

In both studies, the disruptive effects of altered geomagnetism were strongest during fall migration. In the fall and not the spring, more birds drifted with the wind, and geomagnetic disturbances were associated with vagrancy. The seasonality of these effects may not be a coincidence, Gulson-Castillo says, but rather a reflection of a younger, less-savvy migratory population in the fall. Juvenile birds that hatched in the spring would be taking their first migratory journeys then, and their internal sense of direction might not be as fine-tuned as for older birds that have flown the route before. Still, scientists don’t have a clear reason why these younger birds would leave more up to the wind, or how small errors could compound and lead to a vagrant bird.

The two studies raise questions about the navigational decisions that birds make, says University of California, Irvine, biophysicist Thorsten Ritz, who was not involved in either study.

“If you could sit down and interview the birds—‘How do you make your decision?’—then you would find out,” he says. “These are very difficult studies to do, so we may not know for a while why birds do it this way—we can just see they do it.”

The studies both offer evidence that contradicts findings from controlled-lab experiments. In some of these prior studies, researchers concluded that magnetic disturbances 1,000 times stronger than the natural geomagnetic ones had no effect on birds’ internal bearings.

“It is fascinating to see that if you now look at the large-scale behavior, it seems that fields of that magnitude actually do matter, that maybe birds get disturbed by them,” Ritz says.

But the question of exactly how birds navigate is difficult to answer because it involves imagining oneself with another organism’s sensory capabilities. And the nature of other animals’ reality will, at some level, remain a mystery to us, Ritz says.

“Ultimately, we don’t really know what it feels like to be a bird.”

Maddie Bender | READ MORE

Maddie Bender is a freelance science journalist whose work has appeared in STAT, Scientific American and Vice, among others.