"Wind turbines kill between 214,000 and 368,000 birds annually ... compared with the estimated 6.8 million fatalities from collisions with cell and radio towers and the 1.4 billion to 3.7 billion deaths from cats, "
Well, the data is definitely questionable - but unclear whether the Ornithological Society (or TreeHugger) cites unbiased studies, I'll give you that. Here is a Stanford Study with some sources:
What I like about it, is that it cites avian mortality in terms of Mortality per MW, (though one might suggest MWh is a better number, but we can extract that by assuming 20% Capacity Factor) which is the more relevant number than total deaths, because are going to see a lot more MW in the future, and presumably deaths will scale linearly.
Total Average US was 4.12 Avian Deaths / MW. With California really leading the pack at a high 18.76 per MW.
Presuming a 20% capacity factor, and assuming the united states uses about 10 million MwH/day, that means the United States will need 10 million * 5 / 24 MW in capacity, or about 2 million MW capacity (Sanity Check - 2 million MW = 2000 GW which is about 400 GW of nuclear power plants @ 100% Capacity factor - about 400 Reactors,- seems within the realm of reason given the US has about 100 Reactors right now), which means that on average, we'll see about 8 million bird deaths in the future because of wind farms.
That compares to (in 2009) - 4 million due to communication towers, 14 million due to fossil fuels, 72 million due to Pesticide, 97 million due to Building Windows, and 110 million due to feral cats.