A beautiful snow-white cat just wandered through my yard, right under a bird feeder.  It reminded me of the American Bird Conservancy's Cats Indoors campaign.   
The U.S. pet cat population tripled from 1970 to 2010. Outdoor and stray cats kill millions of birds and other small creatures each year.  

Cats are thought to be the largest direct source of mortality to birds in the U.S. and Canada.  They annually kill an estimated 2.4 billion birds in the U.S. alone.

Outdoor cats are also a vector for the transmission of parasites and diseases. Cats are the top carrier of rabies among domestic animals in the U.S.  I do not think that is a huge problem but these days we can not be too careful.
You can find more details on the survey from the American Bird Conservancy web site.