RESOLVING THE LEDGER
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Add as preferred source When a young golden eagle (Aquila chrysaetos) finally leaves its parents' territory, it leaves behind the only safety net it has ever known. Until now, biologists had assumed that eagles took this step as soon as they could fly well enough to survive on their own.
However, a new study, in which 92 young golden eagles in the Alps were tracked using GPS transmitters over six years, shows that flying ability has almost nothing to do with it. Just a few months after young eagles fledge, they can fly just as skillfully—climbing rapidly, circling in tight radii while soaring on thermals and making use of shifting winds—as adult eagles.
Nevertheless, many remain in their parents' territory for almost a full year afterward. "Golden eagles seem to be able to fly well long before they leave their parents' territory," says Hester Brønnvik, lead author of the study and a scientist in the Department of Evolutionary Ecology at the Leibniz Institute for Zoo and Wildlife Research. "Being able to survive on one's own and actually taking that step are clearly two different things."
The paper is published in the journal Royal Society Open Science .
Between 2019 and 2024, the team fitted young golden eagles in the Alps with GPS tags and tracked each bird's movements from the age of around 50 days—that is, shortly before it left the nest—during its first forays outside its parents' territory and, in most cases, right up to the day it finally emigrated.
Most eagles undertook their first journey outside their parents' territory within about three months of being tagged, covering distances of up to 77 kilometers (48 miles), sometimes on excursions lasting several days. Studies from other regions suggested that golden eagles reach their full adult flying ability at precisely this age.
However, this was not followed by emigration; many birds did not leave their parents until almost a year after being tagged, often only after winter had ended. "These young eagles were clearly capable of surviving outside their parents' territory long before they actually left it," says Brønnvik. "They were already undertaking flights of dozens of kilometers and staying away for days at a time. They simply kept coming back."
To analyze their flying abilities, the transmitters sent short bursts of GPS data at a resolution of one location per second. This enabled the scientists to reconstruct exactly how each eagle soared: how tightly it circled in thermal updrafts, how quickly it gained altitude and how well it coped with the wind. The team wanted to know whether the birds that mastered flying in thermals the quickest were also the ones that left home the earliest.
"These skills proved to be necessary, but were by no means sufficient," says Elham Nourani of the Max Planck Institute for Animal Behavior and the University of Konstanz, one of the study's lead authors. "The decision to leave the parental territory depends on much more than just whether one has the necessary skills for flight and hunting." None of the detailed measures of the eagles' flying skills allowed the scientists to predict when eagles would undertake their first long journeys away from their home territory or when they would leave it for good.
What predicted both was another factor: how far eagles strayed from their territory during their daily forays and how high they soared into the sky. Young eagles that covered greater distances and reached greater heights were far more likely to undertake exploratory flights shortly afterward and were the ones that eventually left the territory months later.
"These daily movements capture something that soaring skill alone cannot," says Nourani, who is now an assistant professor in the Department of Ecology and Evolution at the University of Lausanne. "They reflect motivation, competition, food availability and weather all at once, and it is clearly the experience of these factors that actually determines the decision to leave."
The Alpine population of golden eagles studied by the team is unusual: It is saturated, meaning that almost every suitable territory is already occupied and there are only a few vacant territories that young eagles could exploit.
The authors suggest that this could explain why so many young eagles delay leaving the territory, even when they are already fully capable flyers: They remain in their parents' territory for as long as possible, rather than venturing into a competitive, overcrowded landscape where there is no guarantee of finding a place of their own.
This decoupling of skills and independence may not be limited to eagles. Similar patterns have been described in New Caledonian crows (Corvus moneduloides), which take up to a year to master tool use but don't leave home for another year after that. This has also been observed in several other species with long periods of parental care.
"Independence is not necessarily a race to leave home as soon as an animal is capable of surviving," says Kamran Safi of the Max Planck Institute of Animal Behavior and the University of Konstanz, one of the study's lead authors. "For a young eagle, the parental territory may remain the safest available place for months after it has mastered flight.
Staying there, while parents still tolerate it, can provide time for small but meaningful improvements in performance and experience before entering a landscape where space is scarce and risks are high. This might eventually increase the odds of outliving competitors and surviving long enough to challenge a territory holder when the time comes to take over what they will defend, not uncommonly to the death. Skill acquisition might just be the wrong yardstick for predicting independence in a lot of ...
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