While the fact isn't well known, house cats are incredibly effective predators, and one of the leading causes of extinction worldwide.
Observation of free-ranging domestic cats shows that some individuals can kill over 1000 wild animals per year, although smaller numbers are more typical. . . . Recent research  suggests that rural free-ranging domestic cats in Wisconsin may be killing between 8 and 217 million birds each year. The most reasonable estimates indicate that 39 million birds are killed in the state each year. Nationwide, rural cats probably kill over a billion small mammals and hundreds of millions of birds each year. Urban and suburban cats add to this toll. . . .As another example, house cats have wiped out many New Zealand's many native bird species (see also here and many of Australia's native marsupial species (but see an opposing view here). The case of cat v. bear shows why this is true.
Worldwide, cats may have been involved in the extinction of more bird species than any other cause, except habitat destruction. Cats are contributing to the endangerment of populations of birds such as Least Terns, Piping Plovers and Loggerhead Shrikes. In Florida, marsh rabbits in Key West have been threatened by predation from domestic cats. Cats introduced by people living on the barrier islands of Florida's coast have depleted several unique species of mice and woodrats to near extinction.