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Birds of feather don't always flock together, despite the adage. A study in the Journal of Raptor Research tells the story of Arizonan bald eagles (Haliaeetus leucocephalus) engaging in intriguing migrations by traveling north rather than south, contrary to most migratory birds in temperate zones.

The paper, titled "Northward Migrations of Nonbreeding Bald Eagles from Arizona, U.S.," also highlights one eagle who chose to leave Arizona and settle in California, a behavior that challenges previous understandings about emigration in this population.

Their findings illustrate the exploratory nature of southwestern bald eagles, which is likely an adaptation to take advantage of ephemeral food sources. As the planet continues to change, studies demonstrating the adaptive movements of wildlife are crucial to informing conservation practices and protecting travel corridors for species in need.

Between 2017 and 2023, the Arizona-based research team satellite-tracked 24 recently fledged bald eagles and two non-breeding adults to investigate eagle movements within the state. While breeding Arizona bald eagles are thought to remain near their nesting territories through much of the year, knowledge of nonbreeding eagle movements is limited.

Survival rates differ for eagles at different stages of life, and therefore, comparing movement behavior and survivorship of nonbreeders versus breeders can provide a more robust picture of how a species fares across its range, in a manner that attends to population-level influences.

The team tracked eagle movements for as long as the devices allowed, with a maximum of four years over the seven-year study.

Researchers and non-researchers alike are used to thinking of North American migrant birds as moving south during the nonbreeding season, typically our "winter," to pursue more hospitable winter weather and food. However, these Arizonan bald eagles begin nesting in the winter, so their non-breeding season occurs in the summer and fall months.

Lead author Dr. Caroline D. Cappello was fascinated by this alternate approach to seasonal movement, and says "northward migratory behavior raises interesting questions about the recent and historical pressures that shape these movement strategies."

The authors note that their tracked eagles stopped at the same bodies of water documented as important stopovers for bald eagles in a study conducted in the 1980s. This finding lends credence to the importance of these habitats for multiple generations of eagles.

In addition to confirming a concentration along the Intermountain Flyway and the eagles' reliance on lakes and rivers on their journeys, the team found that eagles across age classes migrated to the northern U.S. and southern Canada.

A co-author of the paper, Dr. Javan M. Bauder, a USGS researcher, Assistant Unit Leader at the Arizona Cooperative Fish and Wildlife Research Unit and Assistant Professor at the University of Arizona, underscores the importance of this, saying "managing and conserving wildlife populations requires that we understand the movement of patterns of individuals of all age classes within those populations."

The team also reported that as eagles aged, their routes became more fine-tuned, and that most of the birds tracked for at least two years returned to Arizona in the autumn of their hatch year after a summer wander. The authors theorize that these eagles may be taking advantage of seasonal food sources such as spawning salmon, nesting waterfowl, or ungulate carcasses. Confirmation of this will require future study.

Bald eagles, like other raptors, face a barrage of threats in today's human-dominated landscape. One of the eagles in this study traveled through ten states and four Canadian provinces in her early years of life, before finally settling in northern California, where it seemed she might establish a breeding territory.

Then, at age four, she was electrocuted, a sadly frequent fate for large raptors. Her death was disappointing, says Cappello, not only because it "underscored the ongoing risks these birds face," but also because emigration is a difficult population variable to measure.

This young eagle's apparent choice to settle in California would have offered a lens into the dispersal potential for Arizona-hatched bald eagles, which has rarely been documented.

Lead and rodenticide poisoning, windmill collisions, habitat destruction, and climate uncertainty are additional perils for a young raptor. Studying raptors at all life stages, including their exploratory chapters, allows researchers to make evidence-based suggestions about which stopover sights might be protected, and which infrastructure adjustments, including electrical, could be implemented for better raptor survivorship.

The research team suggests future studies on route fidelity because, as Cappello says, "if consistent patterns exist it could help focus conservation efforts on specific, high-use areas along their migratory route."

They also emphasize the value of comparative studies of other wildlife populations that migrate north from the southern extent of their range to further understand the costs and benefits of this strategy.