Just do a quick search on songbirds and cats and you'll come up with a load of info.
From the U.S. Fish and Wildlife Service:
Cat Predation
Americans keep an estimated 60 million cats as pets. Let's say each cat kills only one bird a year. That would mean that cats kill over 60 million birds (minimum) each year - more wildlife than any oil spill.
Scientific studies actually show that each year, cats kill hundreds of millions of migratory songbirds. In 1990, researchers estimated that "outdoor" house cats and feral cats were responsible for killing nearly 78 million small mammals and birds annually in the United Kingdom.
University of Wisconsin ornithologist, Dr. Santley Temple estimates that 20-150 million songbirds are killed each year by rural cats in Wisconsin alone.
Feline predation is not "natural." Cats were domesticated by the ancient Egyptians and taken throughout the world by the Romans. Cats were brought to North America in the 1800's to control rats. The "tabby" that sits curled up on your couch is not a natural predator and has never been in the natural food chain in the Western Hemisphere.
Cats are a serious threat to fledglings, birds roosting at night and birds on a nest. Research shows that de-clawing cats and bell collars do not prevent them from killing birds and other small animals. For healthy cats and wild birds, cats should not be allowed to roam free.
Work with your local humane society, veterinarians and state wildlife agency to enact and enforce free-roaming cat regulations. For more information:
Free Roaming Cats. American Backyard Bird Society, PO Box 10046, Rockville, MD 20849.
Cats: A Heavy Toll on Songbirds. by Rich Stallcup. Point Reyes Bird Observatory, 4990 Shoreline Hwy., Stinson Beach, CA 94924.
Is there a Killer in Your House? by George Harrison, National Wildlife Magazine (October/November 1992).
Beware of Well-Fed Felines. by Peter Churcher and John Lawton, Natural History Magazine (July 1989).