Alpine golden eagles fly like adults months before they leave. Skill is not what moves them
Six years of one-second GPS data on 92 young golden eagles found that no measure of flight skill predicted when a bird left home. Competence arrives early; departure waits on something else.
The Scientist · Science desk
What happened
- Researchers GPS-tagged 92 young golden eagles in the Alps between 2019 and 2024, following each bird from about 50 days old to the day it left home.
- Within a few months of fledging the birds soared as well as adults, climbing fast and circling tightly in thermals.
- Many nonetheless stayed in the parental territory until almost a year after tagging, usually leaving only once winter was over.
- No flight-skill measure predicted the timing of first long excursions or final departure.
Why it matters
- What it limits constraint One-second telemetry of soaring performance turns out to carry no information about when a bird will disperse, which limits what high-resolution flight data can be used to forecast.
- What doesn't add up contradiction The natal range is described as a safety net while the offered mechanism for staying is a shortage of vacant territories; the reported results do not distinguish being supported from having...
- Who's exposed exposure If dependence on the natal range extends across roughly two thirds of a juvenile's pre-dispersal life, disturbance in that range reaches birds long after the point at which monitoring usually...
- What it sets up precedent The predictor that worked bundles motivation, competition, food and weather, so follow-on work has to unpick a composite rather than refine a skill metric.
The negative result is the load-bearing one here, and it is hard to wave away. The tags sent bursts at one location per second, which let the team reconstruct how tightly each bird circled in a thermal, how fast it gained altitude and how it coped with wind [9]. That is the resolution at which a skill effect should have shown up if there were one. None of those measures predicted when a bird made its first long excursion, or when it left for good [10]. What did predict both was cruder: how far the bird strayed on ordinary days, and how high it got [11].
The arithmetic of the gap is worth doing, because the paper's own framing leaves it implicit. Birds were tagged at around 50 days, shortly before leaving the nest [2]. First excursions came within about three months of tagging, some of them 77 kilometres out and several days long [7], and work in other regions puts full adult flying ability at precisely that age [22]. Departure came close to a year after tagging, usually after winter had ended [8]. So competence lands near day 140 and emigration near day 415, leaving roughly 275 days of being able and staying anyway [19] - about two thirds of the bird's life at the moment it finally goes [20].
That residency is where the two available explanations diverge. The account of the study opens by calling the natal territory the only safety net the bird has ever known [18], which implies the year buys something: tolerance from the adults, and access to their food. The authors' stated mechanism is different. The Alpine population is saturated, with almost every suitable territory occupied and few vacancies to move into [16], and they suggest this is why capable flyers stay [17]. One story is about what home provides. The other is about what is unavailable elsewhere. As reported, nothing in the study separates them.
Elham Nourani, one of the lead authors, is explicit that the working predictor is a composite: daily movement, she says, reflects motivation, competition, food availability and weather all at once [13]. That is an honest description and also a limit. A variable that bundles four drivers cannot tell you which one is doing the pushing, so the finding is that skill is not the trigger rather than that scarcity is.
For anyone who reads telemetry for a living, the practical content is that soaring performance is not a readiness signal. Hester Brønnvik of the Leibniz Institute for Zoo and Wildlife Research puts it as two different things: being able to survive alone, and taking the step [14]. The prior assumption ran the other way, that eagles left as soon as they could fly well enough to make it [15]. Ninety-two birds over six years [1], roughly fifteen a year [21], is enough to retire that assumption. It is not enough to say what replaces it.
What to watch
- Whether the same tracking design in a non-saturated golden eagle population shows readiness driving departure, which would isolate the effect of vacant territories.
- Whether the full Royal Society Open Science paper reports per-bird data on parental provisioning or on nearby territory vacancies.
- Whether post-departure survival differs between early and late leavers, which would put a price on the year spent at home.
Clarity's read
What the record supports and how the coverage leans. The claims behind it follow.
Reality
- Evidence58
- Adoption
- Not scored
- Hype gap+18
- Incentives42
- Confidence52
Claim ledger
Ranked by verification strength, evidence, and original report placement.
- [1]
A new study tracked 92 young golden eagles in the Alps using GPS transmitters over six years, between 2019 and 2024.
- [2]
Each bird's movements were tracked from the age of around 50 days, shortly before it left the nest, through its first forays outside the parental territory and, in most cases, up to the day it emigrated.
- [3]
Hester Brønnvik is lead author of the study and a scientist in the Department of Evolutionary Ecology at the Leibniz Institute for Zoo and Wildlife Research.
- [4]
The paper is published in the journal Royal Society Open Science.
- [5]
Nourani is affiliated with the Max Planck Institute for Animal Behavior and the University of Konstanz, and is now an assistant professor in the Department of Ecology and Evolution at the University of Lausanne.
- [6]
Just a few months after fledging, young eagles can fly as skilfully as adults: climbing rapidly, circling in tight radii while soaring on thermals, and making use of shifting winds.
- [7]
Most eagles made their first journey outside their parents' territory within about three months of being tagged, covering distances of up to 77 kilometres (48 miles), sometimes on excursions lasting several days.
- [8]
Many birds did not leave their parents until almost a year after being tagged, often only after winter had ended.
- [9]
The transmitters sent short bursts of GPS data at a resolution of one location per second, letting scientists reconstruct how tightly each eagle circled in thermal updrafts, how quickly it gained altitude and how well it coped with wind.
- [10]
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 home or when they would leave for good.
- [11]
What predicted both was how far eagles strayed from the territory during daily forays and how high they soared; birds that covered greater distances and reached greater heights were far more likely to make exploratory flights soon after and to leave the territory months later.
- [12]
Elham Nourani, one of the study's lead authors, says the flight skills "proved to be necessary, but were by no means sufficient" and that the decision to leave depends on much more than flight and hunting ability.
- [13]
Nourani says the eagles' daily movements "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."
- [14]
Lead author Hester Brønnvik says golden eagles seem able to fly well long before they leave their parents' territory, and that "being able to survive on one's own and actually taking that step are clearly two different things."
- [15]
Until now biologists had assumed that young eagles left the parental territory as soon as they could fly well enough to survive on their own.
- [16]
The Alpine golden eagle population studied is saturated: almost every suitable territory is already occupied and there are only a few vacant territories young eagles could exploit.
- [17]
The authors suggest that population saturation could explain why so many young eagles delay leaving the territory even when they are already fully capable flyers.
- [18]
The account of the study describes the parental territory as the only safety net a young golden eagle has ever known, which it leaves behind on emigration.
- [19]
Adult-grade competence is reached at roughly 140 days of age and emigration at roughly 415 days, leaving about 275 days in which a bird is capable of independence but stays in the natal territory.
- [20]
The capable-but-resident period accounts for about two thirds of the bird's life up to the day it emigrates.
- [21]
The study tagged an average of about 15 young eagles per year.
- [22]
Studies from other regions suggested that golden eagles reach their full adult flying ability at precisely the age at which the tracked birds made their first excursions.
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Sources
1 independent publisher whose own reporting we read for this story.
- phys.orgYoung golden eagles leave their parents' territory late
1 article · August 25, 2026
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Topics
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- Golden eagleFollow
- Royal Society Open ScienceFollow
- University of KonstanzFollow
- Kamran SafiFollow
- Max Planck Institute of Animal BehaviorFollow
- Elham NouraniFollow
- University of LausanneFollow
- European AlpsFollow
- Leibniz Institute for Zoo and Wildlife ResearchFollow
- New Caledonian crowFollow
- Hester BronnvikFollow