According to a Nature.com article, domestic and free-ranging cats are responsible for 47% of bird deaths per year and many scientists believe they are destroying important eco-systems.
Domestic cats have contributed to multiple wildlife extinctions on various islands around the world.
Source: i.imgur.com
The magnitude of cat-related deaths that happen to wildlife each year in the United States is still unclear.
Source: veryhilarious.com
It's estimated that they kill somewhere between 1.4–3.7 billion birds...
Source: i.imgur.com
...and 6.9–20.7 billion mammals in the U.S. annually.
Source: i.imgur.com