The Quiet Strength of a Territorial Eagle
A 19-year photographic study charts how Bonelli’s Eagles hold their ground and pass territories from one generation to the next.
A long-term look at a threatened raptor
For 19 years, researchers in south-eastern Spain followed one of Europe’s most vulnerable raptors – the Bonelli’s Eagle Aquila fasciata – using an innovative, non-invasive method. By identifying individual eagles from detailed photographs, they were able to track survival, recruitment and territory turnover across 41 breeding sites.
The study built a remarkably complete demographic picture for 142 identifiable birds. It shows that, despite ongoing pressures in Mediterranean landscapes, adult survival remains high, and population stability is strongly underpinned by the species’ fidelity to territories.
Younger birds are taking up territories
One of the clearest trends is a shift in recruitment patterns. Increasingly, younger eagles are entering the breeding population, with more individuals settling at one to three years of age rather than delaying until four years or older.
This change appears alongside a slight rise in overall recruitment rates and very rapid replacement of missing birds – typically within just six weeks. Such fast turnover suggests a pool of non-territorial adults waiting for vacancies, a behaviour common in long-lived raptors.
Adult survival remains strong – especially for females
Survival estimates were consistently high across the population. Adult females showed the highest apparent survival, followed closely by adult males, with slightly lower rates for younger birds of both sexes.
The differences likely reflect the division of labour during the breeding season. Females spend more time at or near the nest, while males range more widely to hunt and defend territories, increasing their exposure to natural and human-related risks.
Experience brings advantages
The study also found that survival improved markedly during an eagle’s early years on territory, stabilising once individuals had accumulated around eight seasons of experience.
As eagles become more familiar with local topography, food resources and weather patterns, they appear to forage more efficiently and conserve energy more effectively – benefits that strengthen both survival and breeding performance.
Early breeding doesn’t reduce future survival
A longstanding question in raptor ecology is whether breeding too early reduces later survival. Here, first-time breeders – whether young or old – showed similar survival rates.
This supports the idea of reproductive restraint: Bonelli’s Eagles appear to begin breeding only when prepared to do so without compromising their long-term prospects. For the population, this means even young breeders can contribute offspring without incurring survival costs.
Conservation implications
The study provides rare, long-term evidence that high adult survival continues to anchor this regional population. However, the shift towards younger recruits may signal increasing losses of experienced adults, potentially driven by pressures such as electrocution, illegal persecution or habitat change.
Crucially, the research demonstrates that photographic capture–recapture is a powerful and non-invasive tool for monitoring raptors with distinctive plumage. With high resighting rates and minimal disturbance, it offers a practical alternative to tagging and telemetry for species that show strong fidelity to nesting areas.
December 2025
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