Areas near where I live where we could photograph eagles, osprey, hawks, owls, antelope, deer, etc., only a few years ago are now devoid of that wildlife because wind farms have been built nearby. The windmills may not directly kill these animals, but they certainly drove them away. One can only speculate as to how this loss of habitat has affected the populations.
Domestic cats, on the other hand, have been in those neighborhoods as long as I can remember, and those particular species were present.
Earlier this spring, I was driving home from work, and saw something rather large in the street. When I turned around and approached it from the other direction to get my headlights on it, guess what it was.
It was an owl eating a housecat.
Anecdotal, I know. But this is what I've seen in areas I frequent.
I don't see housecats out on the open prairie or in the mountains or in other areas not inhabited by humans. Instead, they seem to stay in town. But in places with milder climates, perhaps there are large populations of cats out away from towns and cities killing birds in wild areas. I just never see that anywhere near here.
And the distribution of species and population densities of birds I see in town seems quite different from what I see out in the wild areas near here, too. So I believe that the very existence of the towns warps the bird population away from what it would be if the towns (and therefore the domestic cats) were not present.
Again, I question whether we can blame cats for any effect on what bird populations would be without also examining the effects the towns and cities responsible for the cats' presence have on those bird numbers.
I'd like to see non-biased statistics for bird deaths by species by cats versus wind turbines along with statistics about bird and cat species distribution and how they are affected by the very presence of towns and cities.
I suspect that a lot of what humans do, including harboring cats, has increased populations of some birds while causing others to decline.
Looking for statistics briefly just now, I ran across various sources showing wildly different estimates of bird death causes. But I didn't see any that broke it down by species.
Some sources placed impacts with windows far above deaths from cats. It's hard to know whose statistics to trust. Everyone seems to have an agenda.