Four turbines on a Norwegian island had one of their three blades painted black in the first week of August 2013, and the 6 white-tailed eagle carcasses found under them in the years before dropped to 0 after, while the 4 unpainted turbines next door went from 7 bodies to 18.

On the windswept island of Smøla, off Norway’s central coast, a small experiment with a can of black paint has become one of the most cited pieces of evidence that wind turbines and birds of prey do not have to be permanently at odds. Researchers from the Norwegian Institute for Nature Research (NINA) painted a single blade black on four turbines at the Statkraft-operated Smøla wind farm in August 2013, then spent the next three and a half years counting carcasses underneath them. The results, published in 2020 in the journal Ecology and Evolution, are striking enough that they are still being cited by wind developers and conservation groups six years later. They are also, by the authors’ own account, based on a small enough sample that they should be read as a promising lead rather than a proven fix.

Why Smøla became a test site

Smøla’s wind farm, which reached its full complement of 68 turbines in 2005 after opening with 20 in 2002, sits on an island that is also home to one of Norway’s densest breeding populations of the white-tailed eagle, Haliaeetus albicilla. Long before the paint experiment, the site had already become a well-documented case study in raptor collision risk: NINA researchers had spent years tracking eagle deaths there using satellite telemetry and systematic carcass searches, establishing Smøla as one of the best-monitored wind farms for bird mortality anywhere in the world.

That monitoring history is what made the painted-blade experiment possible. Because NINA already had years of baseline carcass-search data for specific turbines, the team could compare fatality patterns before and after a change to the turbines themselves, rather than relying on a single before-and-after snapshot at an unfamiliar site.

The experiment, precisely

In the first week of August 2013, technicians painted one of the three rotor blades black on four turbines at Smøla. Four adjacent, unaltered turbines served as controls. Researchers led by Roel May then compared carcass counts at the eight turbines across two periods: 7.5 years of monitoring before the paint job (2006 through July 2013) and 3.5 years after it (August 2013 through the end of 2016), using standardized searches with trained detection dogs conducted throughout both periods.

The topline numbers, excluding willow ptarmigan (a ground-dwelling grouse tracked separately in a companion study), were as follows. At the four painted turbines, researchers found 11 bird carcasses in the pre-treatment period and 6 afterward. At the four unpainted control turbines, carcasses rose from 7 before to 18 after. That divergence — fatalities falling at the painted turbines while more than doubling at their unpainted neighbors — is the core finding the paper is built on, and it is what let the authors calculate an estimated 71.9% reduction in the annual fatality rate at the painted turbines relative to the controls, with a 95% confidence interval spanning roughly 62% to 79%.

For white-tailed eagles specifically, the pattern was sharper. Before the blades were painted, six eagle carcasses were recovered at the four turbines that would later be painted, and none at the four control turbines. After painting, no eagle carcasses were found at either the painted or the control turbines through the end of the study. It is that eagle-specific figure — six deaths dropping to zero — that has circulated most widely in press coverage of the study, sometimes without the context that the accompanying rise in fatalities at the control turbines (from 7 to 18) reflects all recorded bird species combined, not eagles alone; no white-tailed eagle carcasses were found at the control turbines in either period.

Why a black blade might matter

The mechanism the researchers propose is straightforward: motion smear. A spinning rotor blade, especially one of a uniform light color against open sky, can become difficult for a bird’s eye to resolve as a solid, moving object — an effect sometimes likened to how a fan blade seems to disappear at speed. Painting one of the three blades black breaks up that uniform rotor disc, creating a visual contrast that the researchers hypothesized would make eagles and other birds more likely to notice and avoid the turbine while approaching it. The idea builds on decades of work on “motion smear” and object visibility in bird flight behavior, though the Smøla study did not directly test eagle flight paths or avoidance behavior — it measured fatalities as an outcome.

What the authors themselves caution

The paper is unusually candid about its limits, and that caution is worth carrying into any account of the findings. The experiment covered four treated turbines compared against four controls at a single wind farm — a small sample that the authors say requires care in interpretation. They write plainly that they “must therefore be careful what we deduce from the experiment given the limited number of turbine pairs,” and they recommend the study be replicated, ideally with more treated turbines or at additional sites, before treating the approach as generically effective. The authors also note they cannot rule out that broader shifts in local bird abundance over the decade-long study period contributed to the pattern, since they lacked independent data to test for that possibility. And they flag a practical constraint that matters for anyone hoping to apply the fix at existing wind farms: painting turbines that are already built and spinning is comparatively expensive and logistically demanding, since blades have to be painted in place; the cost would fall substantially if paint were applied during manufacturing or installation instead.

How the finding has traveled since

Despite those caveats, the Smøla result has proven influential. It has been cited by outlets including Audubon and the Wildlife Society, and follow-up interest has come from wind operators themselves — Vattenfall has explored blade-painting at other sites, and some offshore developments have experimented with alternative high-visibility color schemes rather than solid black, partly out of concern that black blades absorb more heat. No large-scale, multi-site replication with a sample comparable to a definitive trial has yet been published confirming the Smøla effect size across different bird communities, turbine designs, or climates. The companion Smøla study on willow ptarmigan, which tested painting turbine towers rather than blades, is a reminder that different species and different parts of the turbine may call for different interventions — a single paint scheme is unlikely to be a universal answer even if the blade-painting result holds up under further testing.

The bottom line

The Smøla data offer an encouraging signal: a low-cost, non-invasive modification correlated with eagle deaths falling from six to zero at the turbines where it was applied, alongside a broader relative reduction in bird fatalities compared to untreated turbines nearby. But it remains a single-site study built on eight turbines and a modest number of carcasses, and its own authors have asked for replication before it is treated as a settled solution to bird collisions at wind farms. As wind capacity continues to expand globally, what happens when other operators test blade-painting at their own sites — with their own bird communities, turbine models, and the added cost of repainting blades already in the air — will determine whether Smøla’s numbers hold up anywhere else.