There are 60 to 90  million pet cats in the United States today [1], and it is estimated that 25 to 66% of cat owners allow their cats outdoors [2].   Add to that the tens of millions of strays and feral cats, and there is certainly a sizable population of free-ranging cats in our country.  Studies [3] have indicated that 60 to 70% of a cat's prey is small mammals, 20 to 30% birds, and 10% other animals including reptiles, amphibians, and insects.

Free-roaming cats clearly have an impact on wildlife!  If each outdoor cat only killed one bird per year, it would equal over 60 million birds annually.  Here are some references:

• The University of Wisconsin's Dr. Stanley Temple estimates that rural cats kill 39 million birds every year in Wisconsin alone.
• Here's is another well-reasoned calculation, by one of Audubon's senior scientists,  where the estimate is  over a billion birds per year.
• This January 2003 paper to the U.S. Fish and Wildlife Service offers detailed discussion on the impact of free-roaming cats in Florida (requires Adobe Acrobat reader to view).  
• Here is the abstract of another paper, written by Michigan State University researchers:

Fluctuations of bird abundances have been attributed to such factors as supplemental feeding, landscape change, and habitat fragmentation. Notably absent from consideration, however, is the role of private landowners and their actions, such as owning free-ranging domestic cats (Felis catus; cats allowed free access to the outdoors). To understand the impacts of cat predation on birds, we surveyed all 1694 private landowners living on three breeding bird survey (BBS) routes (~120 km) that represent a continuum of rural-to-urban landscapes in Southeastern Michigan, where the majority (>90%) of land is privately owned. Our data indicate that among the 58.5% of landowners that responded, one quarter of them owned outdoor cats. On average a cat depredated between 0.7 and 1.4 birds per week. A total of 23+ species (12.5% of breeding species) were on the list of being killed, including two species of conservation concern (Eastern Bluebirds and Ruby-throated Hummingbirds). Across the three landscapes there were ~800 to ~3100 cats, which kill between ~16,000 and ~47,000 birds during the breeding season, resulting in a minimum of ~1 bird killed/km/day. While the number and density (no./ha) of free-ranging cats per landowner differed across the rural to urban landscapes, depredation rates were similar. Landowner participation in bird feeding showed no relationship with the number of free-ranging cats owned. Similarly, selected demographic characteristics of landowners were not significantly related to the number of free-ranging cats owned. Our results, even taken conservatively, indicate that cat predation most likely plays an important role in fluctuations of bird populations and should receive more attention in wildlife conservation and landscape studies. 

(Reprinted from Biological Conservation 155:191-201, Lepczyk et al. "Landowners and cat predation across rural-to-urban landscapes," 2003, with permission from Elsevier.)

As The Wildlife Society in its position statement notes:

"Extensive popular debate over absolute numbers or types of prey taken is not productive. The number of cats is undeniably large. Even if conservative estimates of prey taken are considered, the number of prey animals killed is immense."

Sandy Beck, a journalist with the Tallahassee Democrat, asked an Internet wildlife rehabilitation chat group: "What percentage of your wildlife patients are victims of outdoor cats?" Responses from professional rehabbers around the country ranged from 14 to 30 percent.

The American Bird Conservancy has launched a national campaign to educate cat owners and encourage them to keep their cats indoors.  No only does wildlife benefit, so do the cats:  While outdoor cats commonly have a life expectancy of fewer than 5 years, indoor cats can live for up to 17 years. With less exposure to disease, other cats and animals, and fewer opportunities to have accidents (1.5 million cats are killed by autos annually), vet bills are less and cats live healthier lives!

[1] 60 million based on 1990 U.S. Census (people responding that they own cats); over 68 million according to the American Veterinary Medical Association (AVMA) 2001 U.S. Pet Ownership & Demographics Sourcebook; 90 million based on the American Pet Products Manufacturer's Association's 2005/2006 National Pet Owners Survey.

[2] National People and Pets Survey, 1995; 1997 survey cited by Humane Society of the United States; see also Lepczyk et al. paper above.

[3] Figures and citations within Impacts of feral and free-ranging domestic cats on wildlife in Florida; see also literature above.