Scientists in Norway have found that painting one of the three blades on a wind turbine black reduces avian deaths by 72%. If this “contrast painting” were to be implemented at new onshore and offshore wind farms, it could reduce public opposition, speed up permitting processes and enable wind farms to be built at sites previously thought to be too problematic, they write in a scientific paper.

The study by researchers at the Norwegian Institute for Nature Research examined bird death data collected between 2006 and the end of 2016 at Statkraft’s 152.4MW Smøla wind farm on the bird-rich island of the same name off Norway’s west coast. Four turbines at the Smøla project had a single blade painted black in August 2013, so avian fatalities were recorded for seven-and-a-half years before the painting and three-and-a-half years afterwards.

Trained sniffer dogs were used to find bird carcasses and feathers at the bottom of turbines at the wind farm, with dead birds found by wind farm personnel and passers-by also recorded. The data showed that there was “an average 71.9% reduction in the annual fatality rate after painting at the painted turbines relative to the control [ie, unpainted] turbines”.

Why does a black blade reduce bird collisions?

In the paper, the scientists explain why birds are susceptible to flying into rotating turbine blades and why a single black blade helps them to perceive the rotor as an obstacle. “Relative to humans, birds have a narrow binocular [eg, using both eyes to focus on one object] frontal field of view and likely use their monocular [using each eye independently] and high‐resolution lateral fields of view [ie, having eyes on opposite sides of their heads] for detecting predators, conspecifics [ie, birds of the same species], and prey,” the authors write.

“Within an assumed open airspace, birds may therefore not always perceive obstructions ahead, thereby enhancing the risk of collision. To reduce collision susceptibility, provision of ‘passive’ visual cues may enhance the visibility of the rotor blades, enabling birds to take evasive action in due time.” It is thought that birds see the rotating white blades as a “motion smear” — the blur effect humans see when waving a hand quickly in front of their eyes — and do not perceive this blur as a moving object.

Painting one blade black is believed to create motion smear patterns that the bird perceives as a moving object, “as the frontal vision in birds may be more tuned for the direction of movement”. An experimental laboratory study with American kestrels at the University of Maryland in 2003 tested the impact of seven different blade patterns (“striped, staggers and whole black”), as well as coloured blades, to see which was most clearly seen by the birds. The whole black pattern proved to be the most visible.

The Norwegian scientists concluded: “We recommend to either replicate this study, preferably with more treated turbines, or to implement the measure at new sites and monitor collision fatalities to verify whether similar results are obtained elsewhere, to determine to which extent the effect is generalizable. “It is of the utmost importance to gain more insights into the expected efficacy of promising mitigation measures through targeted experiments and learning by doing, to successfully mitigate impacts on birdlife and to support a sustainable development of wind energy worldwide”