It's not necessarily the raw numbers of birds killed, it's the type and the impact on their population. If your cat kills 1000 pigeons, the pigeon population won't notice. A handful of golden eagles every year or two though, and we get closer to losing another species.
[1]http://www.audubon.org/news/will-wind-turbines-ever-be-safe-...
At least over here in Germany the organisations working to get more renewables deployed and studying these things massively overlap (e.g. nature clubs lobbying/advertising for renewables, and at the same time providing the boots on the ground counting dead birds). It has been recognized as a problem and is now managed, just as other environmental influences of new construction projects are (e.g. evaluating a new location includes a comparison with known migratory patterns/populations and restrictions and monitoring requirements can be set accordingly)
Stop quoting this bullshit "fact." It is a meme made up by conservative think tanks to discredit wind farms. As other posters have already pointed out, other human-related factors such as domestic cats, buildings and towers, and most importantly pollution and loss of habitat are far more significant dangers to birds. If you actually care about birds, take your cats to the vet and put them down (this will also help save the oceans: a large part of fish in cat food comes from massive and illegal trawling in South-east Asia often done with slave labor), and donate any rural/suburban property you own to a land trust, to be given back to natural habitat.