They are intentionally built into the blade. I am fanning a hard time remembering if they are leading our trailing edge without finding the picture I took at the time. I am sure there is someone who had worked with these and knows as in sure they serve a purpose. I put positive flair because I am guessing it has some physics reason. Thanks in advance!
In: Physics
One set of holes allow air to leak into the low pressure side to reduce condensation of the water there. Wet blades weigh more and don’t work as effectively.
Another set at the blade tip disturb the tip vortex shedding. This can reduce drag but I think it’s mostly to avoid whipping birds into the next blade tip.
Is it all windmills with [blades like this](https://previews.123rf.com/images/celli07/celli071803/celli07180301608/99580561-close-up-of-windmill-blades-in-a-cloudy-day-at-kinderdijk-situated-in-a-polder-has-the-largest-conce.jpg) you are talking about?
The answer can be seen in the image. The wooden structure with hols is there as a support for fabric that catches the air. It is a way so you can change the catchment area of the blades so you can regulate the speed the rotate it. When not in operation you can roll op the fabric and there is a lot less braking force needed to keep it still.
Modern wind turbines have a blade that can be rotated to change the angle of attack for the same reason, some even has tips that can rotate independently so they provide a force in the other direction as a way to keep the blades stationary if needec.
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