The Denver Post (hard copy, not local work product) reports that about 80% of songbirds are killed by predators, and that in particular 47% of singbirds are killed by cats (about equal numbers of feral and domesticated cats). This works out to 500,000,000 songbirds killed by cats each year v. 400,000 killed by windmills. Thus, windmills kill less than one-half of a percent of all songbirds.
Also on the topic of cats, the U.S. Fish and Wildlife Service has determined that the Eastern Cougar is extinct, and that people who see cougars in the East are merely seeing Western Cougars who have migrated to the East or escapees from captivity. The practical upshot of this (and the reason not to necessarily trust the conclusion) is that Eastern states where the Eastern Cougar was once native, don't have to establish endangered species plans for cougars, because officially speaking, anyway, they don't exist. Habitat protection plans for Eastern Cougars would have been a major blow to timber interests and real estate developers in the forested rural east.
Incidentally, the great scarcity of non-human megafauna, such as cougars and bears, in much of North America means that in much of these region the top non-human predators in the ecosystem are domestic cats, lynx, dogs, foxes, wolves, and a few birds of prey, a status that elevates their importance in the ecosystem relative to the days when larger predators roamed the land. The largest birds of prey are only about 33 pounds, the largest domestic cats and lynxes and foxes aren't all that much bigger, and few dogs and wolves are larger than a deer (about the weight of an adult human). A larger crocodiles and alligators are found in warmer climes, and some aquatic species are also larger (e.g. the Orca and Great White Shark).