Geomagnetic disturbances disrupt nocturnal bird migration across the U.S. great plains

It’s widely acknowledged that birds and various animals depend on Earth’s magnetic field for long-distance navigation during their seasonal migrations. However, there’s been a curiosity about how occasional disruptions in the planet’s magnetic field, caused by events like solar flares, impact the reliability of these biological navigation systems.

Researchers from the University of Michigan, alongside their collaborators, used extensive, long-term data gathered from networks of Doppler weather radar stations and ground-based magnetometers in the United States. They aimed to investigate a potential connection between geomagnetic disturbances and disturbances in nocturnal bird migration.

Their findings revealed a reduction of 9–17% in the number of migrating birds during severe space weather events in both spring and fall. Interestingly, birds that chose to migrate during these events seemed to struggle more with navigation, especially when faced with overcast conditions in the autumn.

Published in the Proceedings of the National Academy of Sciences, these new findings offer correlational evidence of previously unknown relationships between the dynamics of nocturnal bird migration and geomagnetic disturbances, according to the researchers.

Eric Gulson-Castillo, the lead author of the study and a doctoral student in the U-M Department of Ecology and Evolutionary Biology, highlighted the significance of environmental conditions, including those imperceptible to humans like geomagnetic disturbances, in shaping animal decisions and influencing population-level patterns of animal movement.

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 affected by solar outbursts, which can lead to colorful auroras and occasional disruptions in satellite communications, human navigation systems, and power grids. Yet, little has been known about how these disturbances affect animals that rely on Earth’s magnetic field for their migratory orientation and navigation.

While previous experimental studies have provided strong evidence that birds, sea turtles, and other organisms are sensitive to small changes in magnetic inclination, intensity, and declination when making orientation decisions, one recent study examined millions of bird banding records and found a link between geomagnetic disturbances and increased instances of migratory birds getting lost during their journey.

Most prior studies focused on specific geographic areas, durations, and examined a limited number of species. In contrast, this newly published study utilizes a 23-year dataset of bird migration across the U.S. Great Plains, providing fresh insights at the population and landscape levels.

To gather data, the researchers used images obtained from 37 NEXRAD radar stations in the central flyway of the U.S. Great Plains, a significant migratory corridor stretching over 1,000 miles from Texas to North Dakota. This flat region was chosen to minimize the influence of mountainous terrain, oceans, and Great Lakes coastlines. The final datasets included 1.7 million radar scans from the fall season 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

In this region, the community of nocturnally migrating birds consists primarily of diverse perching birds (73% of species), including thrushes and warblers, as well as shorebirds (12%), such as sandpipers and plovers, and waterfowl (9%), like ducks, geese, and swans.

To detect these migrating bird groups, NEXRAD radar scans are used. These scans allow us to estimate migration intensity, which refers to the number of birds in each cluster, and measure the direction of their flight.

Concurrently, geomagnetic measurements were obtained from superMAG, a global network of geomagnetic ground stations. Data were collected from magnetometer stations situated near the weather radar sites.

To correlate the bird migration data with geomagnetic disturbances, the researchers developed a customized, spatiotemporally explicit index of geomagnetic disturbance, representing the maximum hourly change from the normal magnetic conditions. This index was matched with data from each radar station.

The task of creating this geomagnetic disturbance index was challenging, involving the distillation of extensive ground magnetic field observations collected over many years. It was crucial to assess data quality and validate the final data product to ensure its suitability for this study, according to Daniel Welling, a space scientist at the U-M College of Engineering.

The collected data were then analyzed using two complementary statistical models to understand the potential impact of magnetic disturbances on bird migration. These models accounted for various factors, including weather, temporal variables (such as time of night), and geographic variables (such as longitude and latitude).

The study’s senior author, Ben Winger, an assistant professor in the U-M Department of Ecology and Evolutionary Biology and a curator of birds at the U-M Museum of Zoology, noted that their findings support the idea that migration intensity decreases during high geomagnetic disturbance, shedding light on the ecological implications of space weather on bird migration dynamics.

Furthermore, the research suggests that during geomagnetic disturbances in the fall, migrating birds tend to drift with the wind more often instead of expending significant effort to battle crosswinds. Specifically, “effort flying” against the wind was reduced by 25% under cloudy skies during strong solar storms in the fall. This hints at a combination of obscured celestial cues and magnetic disruption potentially hindering navigation.

In summary, the study indicates that fewer birds migrate during strong geomagnetic disturbances, and those that do may face navigation challenges, particularly under overcast conditions in the autumn. As a result, they might opt for more wind-aligned flight paths with reduced active navigation.

Source: University of Michigan

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