The blades were turning fast enough that their tips blurred into the sky behind them.
To a white-tailed eagle quartering a Norwegian island for fish, they were effectively open sky.
Not hidden by fog or darkness. Invisible in plain daylight, in open air.
At the Smøla wind power plant on Norway’s central coast, eagle after eagle from one of the country’s densest breeding populations had been found dead beneath the rotors.
Radar, curtailment and detection systems had all been weighed. Then someone picked up a paintbrush.
Why a spinning rotor can read as open sky, and what one dark blade changes
The effect is called motion smear, the same optical trick that makes a helicopter rotor vanish on video. A three-bladed rotor turning at speed stops reading as three solid objects and starts reading as a faintly translucent disc, open air as far as a hunting bird’s visual system is concerned. Paint one of the three blades black and the disc is no longer uniform.
That explanation is the leading one rather than a settled one. Laboratory work on avian vision by William Hodos, reported for a national renewable energy laboratory, indicated that painting one of three blades black minimizes motion smear and recommended field tests of exactly that single blade, solid black pattern. Other researchers counter that straightforward contrast enhancement is the more likely explanation, since motion smear may only matter when a bird is already close to the blade tip.
The island where eagles and turbines share the same air
Smøla’s plant sits on the northwest side of the main island and has consisted of 68 turbines with roughly 230-foot hubs and 130-foot blades, on flat, treeless coastal heath. The plant was built to supply Norway’s national grid, and the archipelago is rich in birdlife, having been designated an Important Bird Area by BirdLife International. Behind a spinning rotor there is little but grey water and low ground, about as uncluttered a background as a raptor will ever get.
The eagle problem there was not something siting could undo after the fact. Roel May, the senior researcher who led the work, told the BBC that six to nine white-tailed eagles are killed annually within the plant, and that this had caused opposition and conflict. What the site did have was years of carcass records, turbine by turbine, which is the rarest ingredient in this kind of test.
Four turbines, one blade each, and dogs working the ground below
Between August 1 and August 8, 2013, one of three rotor blades was painted black at four turbines, with four neighboring turbines left white as controls. Fatalities were recorded for seven and a half years before painting and three and a half years after, with trained sniffer dogs used to find carcasses and feathers beneath the turbines. The comparison used a before, after, control, impact design across 2006 to 2016.
The result was an average reduction of nearly 72 percent in the annual fatality rate at the painted turbines relative to the controls, with a 95 percent confidence interval of 61.8 to 79.1 percent. The treatment had the largest effect on raptors, and no white-tailed eagle carcasses were recorded after painting. Seasonally, the reductions were strong in spring and autumn but fatalities rose in summer.
The work itself is awkward to carry out. Applying black blade paint requires sanding, cleaning and painting blades in place roughly 330 feet above the ground on fully assembled turbines. That is part of why the first real test covered only a handful of machines.
Where the effect travels, what it costs, and where it fails
At a wind farm in Cádiz, Spain, one of three blades was painted black at four of 20 turbines, and bird fatality rates fell 74 percent relative to unpainted controls. There, though, the largest reductions were among passerines and large waterbirds rather than birds of prey.
At Eemshaven in the Netherlands, one blade was painted on seven of 14 turbines and the analysis found no significant effect. Researchers suspect the black blade simply did not contrast enough against a busy industrial background. One Dutch ornithologist has also suggested local birds may be habituated to constant vehicle activity, though both remain proposed explanations rather than demonstrated ones.
In Wyoming, a team has painted one blade black on each of 36 turbines at a facility near Glenrock, with the US Geological Survey, Oregon State University and other partners involved. Twenty-eight blades were done in 2023 and the remaining eight in 2024, at an estimated cost of $500,000 to $700,000, working out near $15,000 a turbine. The study tracks eagles, other daytime birds and bats, and results are still being gathered.
What the painted turbines mean for the next farm built near a flyway
Two long-running field tests at opposite ends of Europe now point the same way, which is the strongest argument that something real is happening. The Spanish authors note the measure has been shown to function across a wide latitudinal range and argue the remaining task is clearing implementation hurdles and testing other site and species conditions. They also stress the results should not be extrapolated blindly to other settings.
The caution is shared in North America. “This was a dramatic effect, but it was a relatively small sample size,” said Christian Hagen of Oregon State University, explaining why industry and scientists wanted replication before policy moved. A Kansas blade reuse program addresses the end of a rotor’s life; paint addresses the years it spends turning. A study in Ecology and Evolution remains the most detailed published account of what the Norwegian trial actually found.
The open question is bats. Researchers hypothesize that eagles and other day-active birds can detect the painted blades, while bats, which rely more on auditory cues, may not perceive them as effectively. If Wyoming confirms that, the industry will still need a separate tool for the hours after dark. But for large raptors hunting in daylight over open coast, the answer may be one blade and one color.
