Yes, hair is water repellant, but sweat builds up from the root. How does your body get rid of this on its own? Interestingly, and I'm not sure they mention this in the article, birds bath with water and afterwards spend a lot of time preening their feathers. In fact, they preem a lot of the time, trimming their nails back and making them pointy with their beaks, too. The feathers have to be made oily again to keep them waterproof but also to interlock the the edges of their feathers, which function like a zipper, so they keep insulated again as well as have a closed surface area to fly. If you see a feather close up you can see the little barbs at the edges that close them up.
