High above the mountains of Castellón, Spain, a golden eagle soars, its flight now meticulously documented by a tiny GPS transmitter. This device, recently fitted to the bird, is set to provide researchers with a staggering more than 40,000 location points every single day, offering an unprecedented real-time window into the behaviour of one of nature's most powerful apex predators.
The Unseen 'Weekend Effect' on Wildlife
Contrary to long-held assumptions, the data streaming from these advanced trackers reveals that eagles are far from indifferent to human presence. Dr Pascual López-López from the University of Valencia, who helped capture the bird after 18 attempts between 2024 and 2025, admits previous thinking was flawed. "We thought before that eagles would just ignore people in the forest," he says. The GPS evidence tells a different story.
Researchers have identified a clear "weekend and holiday effect." On days when more people visit wilderness areas for hiking or beach trips, eagles significantly expand their hunting ranges. The disturbance scares not only the birds of prey but also their prey, forcing the eagles to travel further to find food. In severe cases, this human pressure can lead breeding pairs to abandon their nests entirely.
Electrocution: A Leading Cause of Death
The tracking technology also delivers sombre data, pinpointing the exact moment a bird's movements cease. Since 2015, GPS studies on the endangered Bonelli's eagle in Valencia have exposed an alarming mortality rate linked to human infrastructure. Collision and electrocution with power lines emerged as a primary killer. "We weren't aware of the magnitude of the problem prior to this research," states López-López.
This is not an isolated issue. Across Spain, it is estimated that at least 33,000 raptors die annually from electrocution. The problem extends beyond eagles to species like storks, bustards, and flamingos. Furthermore, despite legal protections, the GPS data confirms that Bonelli's eagles are still being shot and poisoned by hunters, farmers, and pigeon fanciers.
Renewable Energy Presents a New Challenge
While historic threats like deforestation persist, a modern form of development is creating new risks. The rapid expansion of renewable energy infrastructure, particularly wind farms, is having a severe impact. Research published in December highlighted that 9,000 griffon vultures have died from turbine collisions in Spain over two decades.
"I'm in favour of renewable energy, and the energy transition is necessary," clarifies López-López. "But the problem is the cost of these developments – we need better planning." Globally, studies show 18% of species threatened with extinction are negatively affected by power lines, while 11% are impacted by wind facilities.
Data-Driven Solutions Offer Hope
However, the precise data from GPS tags is empowering concrete conservation actions. For instance, a decade-long study of red kites found that a 25-metre increase in wind turbine rotor diameter led to a five-fold rise in mortality, allowing scientists to give exact design recommendations.
Collaboration with utility companies is also proving effective. In Doñana National Park, retrofitting dangerous pylons has reduced electrocutions of the Spanish imperial eagle by 97%. In the US, experts estimate that focusing mitigation efforts on just 15% of the most high-risk pylons in golden eagle habitat could control a vast majority of the problem.
As Duncan Eccleston of EDM International notes, electricity companies have a vested interest in solving this, as bird electrocutions often cause power outages. "There is an opportunity to do things differently and do things better," he says. The story of the GPS-tagged eagle in Castellón is a microcosm of a global challenge, where technology is finally revealing the true scale of our impact on the wild skies above.
