Conceptual
and geographic layout of the study system. Space weather from the sun, such as
coronal mass ejections, disturb Earth’s magnetic field, causing the auroras and
potentially decreasing the magnetic field’s reliability for migrating birds.
Credit: John Megahan, University of Michigan, from Gulson-Castillo et al. in
PNAS, October 2023. DOI:10.1073/pnas.2306317120
It's
well-known that birds and other animals rely on Earth's magnetic field for
long-distance navigation during seasonal migrations.
But how do periodic disruptions of the
planet's magnetic field, caused by solar flares and other energetic outbursts, affect the
reliability of those biological navigation systems?
University of Michigan researchers and
their colleagues used massive, long-term datasets from networks of U.S. Doppler
weather radar stations and ground-based magnetometers—devices that measure the
intensity of local magnetic fields—to test for a possible link between
geomagnetic disturbances and disruptions to nocturnal bird migration.
They found a 9–17% reduction in the
number of migrating birds,
in both spring and fall, during severe space weather events. And the birds that
chose to migrate during such events seemed to experience more difficulty
navigating, especially under overcast conditions in autumn.
The new findings published in Proceedings of the National Academy of Sciences,
provide correlational evidence for previously unknown relationships between
nocturnal bird migration dynamics and geomagnetic disturbances, according to
the researchers.
"Our findings highlight how animal
decisions are dependent on environmental conditions—including those that we as
humans cannot perceive, such as geomagnetic disturbances—and that these
behaviors influence population-level patterns of animal movement," said
study lead author Eric Gulson-Castillo, a doctoral student in the U-M
Department of Ecology and Evolutionary Biology.
An animation of birds taking off near a NEXRAD
Doppler radar station in Wichita, Kansas. Nocturnal migratory birds depart for
migration in the evening. On a night of heavy migration, they will appear as a
cloud around the radar station. Credit: Kyle Horton.
Earth's magnetic field is regularly
impacted by solar outbursts that can trigger colorful auroras and that
sometimes disrupt satellite communications, human navigation systems and power
grids.
But little is known about how those
disturbances affect animals that depend on Earth's magnetic field for migratory
orientation and navigation. Previous experimental studies over several decades
provide strong evidence that birds, sea turtles and other organisms key into
small changes in magnetic inclination, intensity and declination when making
orientation decisions and developing navigational maps.
One recent study examined millions
of bird banding records and found that geomagnetic disturbances were associated
with increased incidence of migratory bird "vagrancy," that is, birds
becoming lost during migration.
But most previous studies were
narrowly focused in geographic extent, duration and the number of species
examined. The newly published study, in contrast, uses a 23-year dataset of
bird migration across the U.S. Great Plains to provide new insights at population
and landscape levels.
The researchers used images
collected at 37 NEXRAD radar stations in the central flyway of the U.S. Great
Plains, a major migratory corridor. The flyway spans more than 1,000 miles in
the U.S., from Texas to North Dakota.
The research team selected this relatively flat region to minimize influences from mountainous topography or oceanic and Great Lakes coastlines. Their final datasets included 1.7 million radar scans from the fall and 1.4 million from the spring.
Distribution
of NEXRAD radar stations (dark blue circles) and SuperMAG inventory
magnetometer stations (purple crosses) used in the study in relation to
topography (grayscale). Researchers used the three closest and active
magnetometer stations surrounding each radar station to interpolate ΔBmax, or
maximum change in the magnetic field from quiet conditions, every hour. From
Gulson-Castillo et al Proceedings of the National Academy of Sciences (2023). DOI: 10.1073/pnas.2306317120
The
community of nocturnally migrating birds in this region is primarily composed
of a diverse set of perching birds (Passeriformes, 73% of species) such as
thrushes and warblers; shorebirds (Charadriiformes, 12%) such as sandpipers and
plovers; and waterfowl (Anseriformes, 9%) such as ducks, geese and swans.
The NEXRAD radar scans detect groups of
hundreds to thousands of migrating birds. Migration intensity—meaning the
number of birds in each cluster—can be estimated and direction of flight can be
measured.
Concurrent geomagnetic measurements were
accessed through superMAG, a worldwide collection of geomagnetic ground
stations. Data were collected from magnetometer stations near weather radar
sites.
The researchers matched data from each
radar station with a customized, spatiotemporally explicit index of geomagnetic
disturbance that represents the maximum hourly change from background magnetic
conditions.
U-M space scientist Daniel Welling and
former University of Texas at Arlington undergraduate Michelle Bui compiled the
space weather data and designed the geomagnetic disturbance index. Welling and
Bui are co-authors of the new study.
"The biggest challenge was trying
to distill such a large dataset—years and years of ground magnetic field observations—into a geomagnetic disturbance
index for each radar site," said Welling, assistant professor in the
Department of Climate and Space Sciences and Engineering at the U-M College of
Engineering. "There was a lot of heavy lifting in terms of assessing data quality and validating our final data product to ensure
that it was appropriate for this study."
The data trove was fed into two
complementary statistical models to measure the putative effects of magnetic
disturbances on bird migration. The models controlled for the known effects of
weather, temporal variables such as time of night and geographic variables such
as longitude and latitude.
"We found broad support that
migration intensity decreases under high geomagnetic disturbance," said
study senior author Ben Winger, assistant professor in the U-M Department of
Ecology and Evolutionary Biology and a curator of birds at the U-M Museum of
Zoology.
"Our results provide ecological
context for decades of research on the mechanisms of animal magnetoreception by
demonstrating community-wide impacts of space weather on migration
dynamics."
The researchers also found that
migrating birds appear to drift with the wind more frequently during
geomagnetic disturbances in the fall, instead of expending great effort to
battle crosswinds.
"Effort flying" against the
wind was reduced by 25% under cloudy skies during strong solar storms in the
fall, suggesting that a combination of obscured celestial cues and magnetic
disruption may hinder navigation.
"Our results suggest that fewer birds migrate during strong geomagnetic disturbances and that migrating birds may experience more difficulty navigating, especially under overcast conditions in autumn," said Gulson-Castillo, who conducted the study as part of his doctoral dissertation. "As a result, they may spend less effort actively navigating in flight and consequently fly in greater alignment with the wind."
Source: Space weather disrupts nocturnal bird migration, study finds (phys.org)
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