In a previous article, we looked at humanity’s enduring fascination with the mystery of bird migration, beginning with Ancient Greek theories to the radar discoveries of the 1950s. In Part Two, we explore the next major leaps in tracking technology and how these advances are helping scientists uncover the secrets of avian journeys.
As radar technology matured after World War II, scientists turned their attention back to tracking individual birds. The late 1950s and ‘60s saw the emergence of radio telemetry, where birds were fitted with bulky transmitters whose signals were picked up by ground receivers. These early systems were rudimentary, but they laid the groundwork for more sophisticated tracking tools.
A significant breakthrough came with the Argos satellite system, originally developed in the 1970s to monitor ocean buoys. By the ‘80s, it had been adapted to track large birds like Bald Eagles and Trumpeter Swans, offering the first real glimpse into long-distance, high-altitude, and even nonstop migration flights without needing to recapture the birds. Smaller birds, however, posed a bigger challenge. They couldn’t carry these heavy GPS units, but by the ‘90s, researchers had begun using light-level geolocators, devices that estimate position based on sunrise and sunset times, by the amount of light available. Though these devices revealed new information, there were still hindrances to a smooth dataset, as these still required the bird to be recaptured to access the data, and cloud cover and foliage didn’t help matters.
But by this time, GPS-based transmitters were offering pinpoint accuracy. Some also began transmitting data via mobile networks, finally eliminating the need to physically retrieve the devices. Combined with the genius move of using solar-powered transmitters, battery life was also extended dramatically, enabling long-term tracking of globe-trotting species like albatrosses and shorebirds, and over multiple migration cycles.
In 2014, the launch of the Motus Wildlife Tracking System by Birds Canada revolutionized small bird tracking again. By equipping birds with mini radio transmitters and deploying a network of over 1,500 receiver stations worldwide, Motus allowed real-time detection without the need to recapture individuals, filling in migration gaps with unprecedented detail. However, while Motus is effective for tracking movements within range of its terrestrial receiver stations, it was not yet equipped to monitor long-distance, overwater migrations: like those of the Blackpoll Warbler.
Hypothesized for over 50 years, the first direct evidence of the Blackpoll Warbler’s transoceanic migration finally came in 2015. This bird’s 12,400-mile round-trip between the Arctic and South America via the Atlantic Ocean was confirmed by a team of researchers from University of Guelph. As they still needed to use the 0.5 g light-level geolocators, the researchers had to scour the forests of Alaska and western Canada to find the birds and their tiny backpacks; but the immense effort was worth it.
While tagging tells us where individual birds go, broader migration forecasting remained elusive, until recently. Originally designed for meteorology in 1988, the NEXRAD radar network turned out to be a powerful tool for ornithologists, capable of detecting the movement, direction, and density of birds migrating, especially during the night when most migration occurs. In 2018, the Cornell Lab of Ornithology launched a project that has transformed bird migration into a data-driven, actionable science. BirdCast is a visually arresting display combining this NEXRAD weather radar, citizen science data, and machine learning to predict large-scale migration movements in real time across North America. By filtering out weather signals, BirdCast then harnesses this radar data to model the nightly “river of birds” across the continent and provide forecasts that support both research and conservation. Notably, BirdCast’s forecasts aim to help cities prepare for peak migration by implementing “Lights Out” initiatives, efforts to reduce collisions by dimming artificial lights on skyscrapers during migration windows. The project
Despite all this progress, one puzzle remained: most birds migrate at night, making visual tracking nearly impossible. Radar can detect flying biomass but doesn’t distinguish species. Tagging is costly and logistically complex. Scientists needed a better way to track birds efficiently, accurately, and non-invasively. Enter BirdVoxDetect, born in 2024 after an eight-year study building on the original BirdVox created in 2016. Through a collaboration between NYU, Cornell, and École Centrale de Nantes using deep learning, BirdVoxDetect analyzes the species-specific nocturnal vocalizations birds emit while flying. Like “acoustic fingerprints”, these brief calls can be isolated from ambient noise like weather or traffic to identify individual species.
In one test, the system processed 6,671 hours of migration audio and identified over 233,000 flight calls, and it was found that acoustic monitoring can rival radar in estimating migratory biomass. It’s a breakthrough in tracking birds that are otherwise invisible to the naked eye or traditional tech.
Researchers are now working on microphone arrays to triangulate altitude and direction and developing a system that also feeds into the new Nighthawk platform, combining acoustic data with BirdCast and Cornell’s Merlin app to provide even greater insight into nocturnal migration.
This immense journey of bird migration tracking is (almost) as remarkable as the migrations themselves. Each leap in technology has brought us closer to understanding the challenges and marvels migratory birds undergo, and as devices grow smaller, smarter, and acoustic tools become more accurate and accessible, the next breakthrough may already be on the horizon. Who knows what the future will reveal about the migrations we’ve only just begun to understand…
