What I’ve read suggests trees and telephone poles kill more birds than wind mills. (Birds run into trees and poles because, well, they are stupid.) Since bird droppings can kill trees, there’s a parity there at least.
If you want to worry about the effect of the human population on birds, start by address loss of habitat, then predation on the bird population from feral cats. Down there in the noise, probably not event at ecologically relevant numbers, lies the effect of windmills on birds.