Research led by the University of Michigan (UM) has found a link between periodic disruptions of Earth's magnetic field and intensity of nocturnal bird migration.
The study used long-term datasets from US Doppler radar weather stations and ground-based magnetometers to investigate a possible connection between geomagnetic disturbances and interruptions to normal nocturnal bird migration.
Images collected at 37 NEXRAD radar stations across the central US flyway on the Great Plains were used to measure bird migration intensity from 3.1 million radar scans. This comparatively flat and expansive region was selected to minimise influences from coastlines and mountains.
Indigo Bunting is a typical bird of the Great Plains flyway, where radar-station images allowed migration intensity and direction to be compared with space weather data (Steve Bell).
The majority of the birds passing through the Great Plains flyway are passerines, making up 73% of species migrating through the region, while waders represent 12% and wildfowl 9%. The scans allow overall migration direction to be recorded, as well as estimates of the number of birds in the air.
A 9-17% reduction in the number of migrating birds was associated with severe space weather events during both the spring and autumn migration periods. The reduced number of birds that did migrate despite the magnetic disruption appeared to have difficulty navigating, particularly during overcast conditions in autumn.
Eric Gulson-Castillo, doctoral student at UM and lead author of the study, said: "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."
Solar outbursts are noticeable to humans in the form of auroras and occasional problems with satellite communications, power grids and other infrastructure.
Previous studies have demonstrated how birds and other animals use the planet's magnetic field to navigate during their migrations, but understanding of the impact of space weather has been poorly understood.
Daniel Welling, UM space scientist, and former University of Texas undergraduate Michelle Bui, compiled the space weather data.
Welling said: "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.
"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."
Two statistical models were used to measure the impact of magnetic disturbances on bird migration, these controlling for the effect of weather, time of night and latitude and longitude.
Ben Winger, senior study author, said: "We found broad support that migration intensity decreases under high geomagnetic disturbance.
"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 team found that birds drifted with the wind more frequently during geomagnetic disturbances in the autumn migration period, rather than fighting to correct for crosswinds.
Reference
Gulson-Castillo, E R, Van Doren, B M, Bui, M X, Horton K G, Li, J, Moldwin, M B, Shedden, K, Welling, D T, and Winger, B M. 2023. Space weather disrupts nocturnal bird migration. Proceedings of the National Academy of Sciences 120 (42) DOI: https://doi.org/10.1073/pnas.2306317120
