Bald eagles stun scientists with bizarre migration pattern

How do researchers track the movement of wild animals (especially birds) that can travel hundreds of miles a day? By gently equipping them with GPSs. A team in Arizona equipped bald eagles (Haliaeetus leucocephalus) with the monitors to study their migration patterns. The results proved to be quite intriguing and were recently published in the Journal of Raptor Research.

Between 2017 and 2023, the researchers tracked 24 recently fledged bald eagles and two nonbreeding adults. While breeding Arizona bald eagles tend to stick close to their nesting territories for most of the year, the movement of nonbreeding eagles is more of a mystery. Shedding light on this pattern—as well as any differences of survivorship between nonbreeders and breeders—carries important implications for understanding the state of the species. 

“Emigration and immigration are difficult to observe, so seeing this eagle explore far beyond Arizona state lines was fascinating to see,” Caroline Cappello, a quantitative wildlife ecologist at Cornell University and co-author of the new study, tells Popular Science. 

During the seven-year time period, researchers tracked the individual eagles for as long as the devices lasted. While many North American migrant birds fly south during the nonbreeding season in the winter and up north again to nest during the warmer months, the study’s Arizonan bald eagles followed a different pattern. 

They started nesting in the winter and “moved north from Arizona during the population’s post-breeding period in the boreal spring and summer, and returned south in autumn in advance of the breeding season,” the researchers explained in the study. They documented that eagles of all ages traveled to southern Canada and the northern United States, and their migrations became more precise with age. 

“There are very few instances where bald eagles banded as nestlings in Arizona have been observed breeding outside of the state,” says Capello. “Most that are observed breeding in later years return to Arizona. The female that ranged into northern California showed signs of potentially establishing a breeding territory there, which would have been notable given how rarely this has been documented.”

Most of the eagles in question followed the previously identified Intermountain Flyway, a migratory route that “ranges from Alaska to Mexico east of the Sierra Nevada and west of the Rocky Mountains and Sierra Madre Occidental,” they added. Cappello and her colleagues’ investigation also identified the same bodies of water as migratory pitstops for the bald eagles as those previously found by a study in the 1980s, indicating their multi-generational importance. 

One adventurous young eagle crossed 10  states and four Canadian provinces before settling in California, where researchers thought she might choose a breeding territory. Unfortunately, she was electrocuted, which is not uncommon for large raptors like eagles. This untimely fate was disappointing for the researchers, given that the bird’s unusual movement could have shed rare light on the dispersal prospects for bald eagles hatched in Arizona. 

Nevertheless, the team still had a literal bird’s-eye view into the movements of dozens of bald eagles whose revelations will surely empower conservationists to better protect these majestic animals and their migration routes. 

 

More deals, reviews, and buying guides

The PopSci team has tested hundreds of products and spent thousands of hours trying to find the best gear and gadgets you can buy.