A 9-year test in Norway found that painting just one wind turbine blade black can decrease birds deaths by about 72%. Via Ars Technica:
At the Smøla wind farm, regular checks of four particular wind turbines—each 70m tall with three 40m-long blades—found six white-tailed eagle carcasses between 2006 and 2013. In total, the four turbines killed 18 birds that flew into the blades over those six years, along with five willow ptarmigans that are known to collide with the turbine towers rather than the blades. (Another four turbines selected as a control group were responsible for seven bird deaths, excluding willow ptarmigans, over the same timeframe.)
And so, in 2013, each of the four turbines in the test group had a single blade painted black. In the three years that followed, only six birds were found dead due to striking their turbine blades. By comparison, 18 bird deaths were recorded by the four control wind turbines—a 71.9-percent reduction in the annual fatality rate.