Each Spring and Fall, millions of songbirds migrate up and down the central flyways of North America using navigational cues from both celestial targets and Earth’s North-South geomagnetic field lines.
Under overcast skies, the migrators must rely on their own built-in magnetoreceptors to follow Earth’s geomagnetic compass. Thought to be located in the bird’s eyes, there’s evidence that some bird species actually sense Earth’s magnetic field lines in their vision.
But things can all go wrong when space weather and solar flares wreak havoc on these field lines.
We do see that birds are navigating less well during magnetic disturbances and there’s evidence that the magnetic compass becomes more important when stars are not available, Eric Gulson-Castillo, a PhD student in ornithology and evolutionary biology at the University of Michigan in Ann Arbor, told me by phone.
Space Weather Is Notoriously Unpredictable
A magnetic field is not completely stable; they are regularly disrupted by bursts of energy from the Sun that destabilize them and make them less reliable for navigation, Gulston-Castillo, the lead author of a paper appearing this week in Proceedings of the National Academy of Sciences, told me.
That’s why occasionally, researchers will see what are known as vagrants, birds that show up well out of their normal migratory range looking lost and disoriented.
But sometimes even manmade obstacles, such as lights, which naturally attract birds, can cause disastrous results as happened last week in Chicago. That’s when almost 1000 songbirds met their death when weather pushed the birds off course when flying over Lake Michigan and into the lighted windows of Chicago’s McCormick Place convention center.
The threat posed to birds by high rise buildings, lighthouses and telecommunications towers is well known. The extent of geomagnetic disruption to a given bird species’ navigatory abilities remains more of a global unknown.
For their study, Gulson-Castillo and colleagues used datasets from networks of Doppler weather radar stations and ground-based magnetometers—that measure the intensity of local magnetic fields—to test for a possible link between geomagnetic disturbances and disruptions to nocturnal bird migration, the University of Michigan notes.
Across Spring and Fall, we found a 10 to 17 percent decrease in bird migration, says Gulson-Castillo. Fewer birds migrate under strong magnetic disturbances, he says.
The team’s data ran from periods ranging from March to June and August to mid-November.
For each radar sampling event, we built vertical profiles of migration intensity and migrant speed and direction at 100-m intervals between 100-m and 3000-m above ground level, the paper’s authors note. We used data out to a 100-km radius in the construction of these profiles, note the authors. And at the mid-latitudes that encompass our study area, large geomagnetic storms cause local decreases in the magnetic field strength, they write.
The study included 1.7 million radar scans from the Fall and 1.4 million from the Spring, using a 23-year dataset of bird migration across the U.S. Great Plains, the University of Michigan notes. The researchers used images collected at 37 NEXRAD weather radar stations in the central flyway, spanning more than 1,000 miles from Texas to North Dakota, the university notes.
They Travel By Night
Most bird species also only migrate at night, says Gulson-Castillo. There are very notable exceptions, such as hawks, but most small songbirds only migrate at night, he says.
As to why?
At night they are thermally more efficient; they don’t overheat as much, says Gulson-Castillo. And they are less likely to be detected by predators like hawks, he says. There’s less wind resistance; the air is calmer and so the birds find it’s more energetically efficient to fly at night, says Gulson-Castillo.
Most nocturnally migrating birds in this region are composed of perching birds such as thrushes and warblers, followed by shorebirds such as sandpipers and plovers as well as a minority of migrators made up of waterfowl, like ducks, geese and swans, the University of Michigan notes.
For each radar station, the team made a unique disturbance index to know at any given moment how disrupted the magnetic field was at each radar station. They then correlated this data with the amount of localized bird migration and the height at which the birds fly.
We cannot discriminate between different species, but we’re essentially getting a signal from any species that’s migrating at night over the radar station, says Gulson-Castillo.
As For The Paper’s Main Takeaways?
The team found that the threshold at which the migrators were negatively impacted to be around 500 nanoteslas which is considered a large geomagnetic disturbance.
“Our work further implies that longer-term impacts to Earth’s magnetic field could also affect bird migration, such as the 11-year solar cycles that affect the frequency of geomagnetic disturbances,” the authors write.
But we never found a situation where all birds were grounded; so, there were still birds migrating, says Gulson-Castillo.
Gulson-Castillo is also still blown away by the fact that a tiny songbird’s navigational capacities are impacted by space weather and solar activity, particularly since we humans are physiologically incapable of sensing our planet’s geomagnetic fields.
It’s amazing that something that we humans cannot sense is impacting birds on a continental scale, says Gulson-Castillo.
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