The sky is never pure white, and i'm not saying all birds will fly into them if the sky and the windmill are a perfect match. But at the same time, if it were a bright colour less birds would fly into it. Collisions will still happen. It also has to do with the blades rotating, but also that is easier to calculate when the blades are very visible. Still the best thing is to do turn them off sometimes. Best case scenario would be that you can switch over to a source that is also renewable.