“Several years ago, a team of Smithsonian Institution and U.S. Fish and Wildlife Service researchers combined data from dozens of previous studies to estimate, as rigorously as possible, how many birds are killed by cats each year in the contiguous United States. Their results were startling. After carefully quantifying cat populations and predation rates (and the uncertainty of both), the scientists calculated that domestic cats pounce on one billion to four billion birds a year in the lower 48 states, as well as 6.3 billion to 22.3 billion small mammals and hundreds of millions of reptiles and amphibians.”
From a Neurologica Blog article about the extent to which Wind Turbines cause bird fatalities.