Barometric geolocation is helping scientists unravel the complex journeys of small songbirds, including yellow-rumped warblers, by capturing detailed movement and flight data that older tracking tools could not provide.
On a recent field day in Anchorage, Alaska, a researcher recaptured a banded male yellow-rumped warbler that had worn a Tic Tac-size multisensor geolocator for a full year. The device, fitted like a tiny backpack, had traveled with the bird from its breeding grounds to its wintering range and back, storing a trove of data that could only be retrieved when the bird was caught again.
According to the research team, the warbler was one of dozens outfitted with these new tags during a study led with evolutionary biologist David Toews. The aim was to document, at fine scale, the full migration of high-latitude breeding warblers, a task long out of reach for birds this small.
How can scientists track small birds with barometric geolocation?
Early tracking in the late 1960s relied on radio telemetry, which required field crews to follow tagged birds with receivers, sometimes using aircraft. GPS satellite tags later enabled remote tracking with high accuracy, but their batteries are still too heavy for most songbirds, which weigh only a few grams.
To cut weight, researchers adopted light-level geolocators, which store sunrise and sunset data on board and infer latitude and longitude from day length and solar noon. These devices opened new research avenues but come with notable uncertainty. Position estimates can be off by hundreds of miles, and accuracy drops near the equinoxes when day length is similar worldwide.
A newer approach couples pressure sensors with geolocation. As a bird climbs to migrate, air pressure decreases sharply. Barometric geolocation logs these changes to mark departures and arrivals at stopovers, while stationary periods reflect local elevation.
Analysts then compare the pressure time series to global weather data using tools such as GeoPressureR. This creates a location “fingerprint” from distinctive pressure patterns, which can be further refined by incorporating light and wind data.
New tech answers an old question
The study focused on myrtle warblers, the northern subspecies of yellow-rumped warbler, that breed across the boreal forest. While northeastern populations are known to winter along the Atlantic and Gulf coasts, the destination of northwestern breeders has been less certain.
An 1899 account by ornithologist Richard McGregor documented wintering myrtle warblers in coastal California, suggesting they might breed in Alaska and British Columbia. With multisensor tags on warblers breeding in Anchorage, the researchers tested whether these birds take the shortest path to California.
The data showed the opposite. Every tracked bird moved east across boreal Canada, then turned south toward the Gulf Coast, bypassing the Pacific route.
The round trip spanned roughly 6,800 miles, or 10,900 kilometers, which is long for a species typically labeled a short- or medium-distance migrant. The work builds on a growing body of migration research from institutions such as the Smithsonian Migratory Bird Center.
Why take such a circuitous route?
This indirect pathway mirrors patterns seen in some other northwestern songbirds, such as blackpoll warblers and Swainson’s thrushes. One explanation points to historical glaciations that compressed northern bird populations southward, then gradually allowed range reexpansion as ice retreated and forests returned.
As habitat reopened, breeding ranges extended north and west in stages, stretching the distance to southern wintering grounds. Because migration routes are thought to be genetically encoded, present-day birds may still follow routes laid down during this incremental expansion.
The research team notes that the precise breeding origins of myrtle warblers wintering in California remain uncertain. Some Anchorage breeders may go to California, but many could originate farther west or north in Alaska or Yukon.
Future tagging on California wintering grounds could clarify the picture and explain why some birds choose a different path. Similar questions about how environmental change reshapes movement are also emerging in areas such as space weather and atmospheric dynamics.
Barometric geolocation provides new data
The barometric geolocation method strengthens scientists’ ability to pinpoint key stopover and wintering sites for protection and to assess hazards along the flyway, including collision risks with buildings or wind turbines tied to flight altitude. Because the tags capture pressure changes for each migratory leg, they also enable precise timing analyses.
Researchers have already pooled data from more than 50 species to examine whether and when birds migrate by day or night. Ongoing work at the Smithsonian Migratory Bird Center is integrating multisensor geolocator outputs with satellite imagery to explore how stopover arrivals and departures track seasonal vegetation changes, aided by global datasets from agencies like NASA.
With dozens of species now tracked using multisensor geolocators, and more added each year, each new map of a migratory journey sharpens understanding of how small birds move across continents, what shapes their routes, and how conservation can be targeted for the greatest effect.