The first result on Google for "how many birds are killed by domesticated cats" is a scientific article from the Smithsonian Conservation Biology Institute and the U.S. Fish and Wildlife Service.
We have to keep in mind that this is an article based in google search and bibliography and is just a very raw estimate. Bibliography studies mostly the worse and special cases. There is a risk into taking the worse cases as a sample of the global situation.
Is a guess, much better than having none, but still a guess. Don't overthink it