The University of Arizona tracked recently fledged and non-breeding bald eagles with GPS transmitters and found many delay northbound returns until late winter and summer, arriving long after many other migrants. Researchers classified movements into post-fledgling dependence, migration, and breeding-season return and mapped stopovers using a 12.5-mile rule. Eagles moved quickly out of Arizona, slowed and meandered at northern summer sites, then sped up for the return south. The study highlights human-caused hazards—electrocution, light pollution, turbines and poison—and the need for ongoing monitoring.
Why Some Bald Eagles Migrate 'Backwards': New GPS Study Explains the Timing
These Birds Are Migrating in the Wrong DirectionElizabeth W. Kearley - Getty Images
Every spring, boreal skies fill with returning geese, corvids and songbirds—but bald eagles from Arizona often arrive much later than those migrants. A new study using satellite GPS transmitters reveals that many Arizona bald eagles delay their northbound movements until late winter and into summer, a pattern that helps explain why these large raptors are absent from early spring displays.
Study Design and Methods
Researchers from the University of Arizona equipped a group of recently fledged bald eagles (Haliaeetus leucocephalus) and two slightly older, non-breeding adults with GPS transmitters and tracked their seasonal movements across the Pacific Flyway. The team classified each GPS location into three behavioral phases: post-fledgling dependence (when young birds remain near their natal territory), migration (sustained shifts in location), and breeding season (return movements toward Arizona that coincide with nesting timing for breeding birds).
Key Findings
Using speed, departure and arrival points, and elapsed time, the researchers reconstructed frequent routes and identified stopovers (consecutive locations less than 12.5 miles apart where a bird paused during the same day). The tracked eagles showed a clear pattern: they made a rapid, relatively direct northbound push out of Arizona, slowed and adopted less-direct, meandering movements at their northern summer destinations (with multiple stopovers), and then accelerated on the final leg back south to Arizona. Importantly, these timing patterns were observed in non-breeding individuals as well as fledglings, indicating the behavior is not limited to established breeders.
'Our results contribute to a growing understanding of avian migration routes within the Pacific Flyway and the diverse migration strategies exhibited across raptor populations,' said Caroline D. Cappello, lead author and wildlife ecologist at the University of Arizona.
Human-Caused Risks And Conservation Implications
The study documents several human-related threats to migrating eagles. Juvenile raptors frequently collide with or are electrocuted on power lines and other electrical infrastructure; the team recorded at least one electrocution incident. Light pollution can disorient birds and increase collision risk, while wind turbines pose another collision hazard. Indirect poisoning is also a concern when eagles feed on rodents exposed to lead or rodenticides. Finally, habitat loss and climate change could alter seasonal cues and stopover resources, potentially disrupting migration timing.
Although the bald eagle is a recovered species in the United States, continued monitoring of movement ecology—especially for non-breeding and juvenile birds—remains vital for identifying threats and guiding conservation measures along the Pacific Flyway.
