The obvious solution would be to either make the windmills more visible, like the ones that used to dot the landscape 150 years ago. There's one along those lines located just off the I-25 freeway, just south of 6th avenue which seems to have been installed as an art project. It's highly visible, having a large number of blades, closely spaced like the intake of a jet engine, but painted a very visible white color so as to be hard to miss.
This is too easy.
Overthinking the problem to a fare-the-well, folks have concluded, with the help of government monies, that the best way to prevent bird kills is to fit each 400 foot high windmill with a radar set which can detect when a large bird is flying towards it, and add software to stop the mill until the bird is safely past the device.
And you thought the availability of power from these things was spotty before.