From a bird's perspective, windmills move in novel and unpredictable ways. Birds have not evolved to be concerned with gigantic, sword like objects swinging perpendicular to the plane if flight.
When the windmills are stationary (the blades that is), birds very rarely collide with them. When the blades do move, they are often a lot faster than these drones (120-180mph is not uncommon).
"Dr. Shawn Smallwood’s 2004 study, spanning four years, estimated that California’s Altamont Pass wind “farm” killed an average of 116 Golden Eagles annually... Smallwood also estimated that Altamont killed an average of 300 red-tailed hawks, 333 American kestrels and 380 burrowing owls annually – plus even more non-raptors, including 2,526 rock doves and 2,557 western meadowlarks"
"With regard to bird fatalities, it says: "Wind turbines represent an insignificant fraction of the total number of bird deaths caused by man-made objects or activities (eg building structures, transmission lines, and keeping domestic cats)." According to the CSE, for every bird killed by a turbine, 5,820, on average, are killed striking buildings, typically glass windows."
