Americans love their pets. Americans also love wildlife. Unfortunately, the pets that we love can threaten the wildlife we love.
Cat owners often tell of a dead bird, mouse, or lizard "present" left on the welcome mat or, worse, on the living room rug. Domestic cats have been estimated to kill more than a billion small mammals and hundreds of millions of birds each year. Although the death toll is staggering, it is not surprising when you consider that there are 40 million pet cats allowed to roam free, and millions of additional stray cats.
http://deerflat.fws.gov/pets.htm