Tiny backpacks reveal the secret, sometimes circuitous migrations of small songbirds

Tiny backpacks, known as multisensor geolocators, have revolutionized our understanding of the migrations of small songbirds. These lightweight devices, weighing less than a spoonful of sugar, are affixed to the backs of birds such as the yellow-rumped warbler, also affectionately called "butter-butt" by bird-watchers.

Stephanie Szarmach, a researcher, placed identifying bands on a specific individual of the bird species during her Ph.D. research the previous year. This bird, now carrying a multisensor geolocator backpack, was captured using a mist net below a speaker playing the bird's signature "warble" song. The geolocator, about the size of a Tic Tac, was then successfully installed on the bird's back.

Over the course of a year, this warbler utilized the device to record its migratory journey from breeding grounds in Anchorage, Alaska, to an unknown winter refuge and the return journey. Collaborating with her Ph.D. adviser, evolutionary biologist David Toews, Szarmach later analyzed the data collected by the geolocator.

Traditionally, scientists used radio telemetry and satellite tags to track migrating birds, but these methods were limited by the weight of the equipment. Light-level geolocators, containing a battery, clock, computer chip, and light sensor, addressed this issue by minimizing weight. However, these devices store data internally and require recapture of tagged birds a year later to analyze the data.

A more recent advancement in geolocation technology uses atmospheric pressure to determine a bird's location. As the bird ascends during migration, the tag records a significant drop in air pressure, capturing the timing of departure and arrival for each stop along the migration route. By comparing the change in pressure over time with global weather data using the GeoPressureR program, researchers can estimate the bird's location with increased precision.