Alley Cat Allies responded to a study conducted by the Smithsonian that branded feral cats as an "invasive species" since they were brought over to North America by European settlers, and have no natural predators here to keep their populations in check. There are more than 80 million feral cats estimated in the U.S., and among the policies they recommend for bringing populations under control is widespread extermination.
According to the abstract of the study, "free-ranging domestic cats…have contributed to multiple extinctions on islands," and discusses that, up until now, no real studies based on science have actually explored the issue. This study estimates that feral cat populations kill 1.4 billion to 3.7 billion birds, and anywhere from over 6 to 20 billion mammals, annually. This is 3 to 4 times more than previously thought.
However, according to Alley Cat Allies, the study was extremely small, looking at only 69 birds of one species within one Maryland town. Furthermore, their analysis of the study indicates that only nine deaths of all the birds killed in this small population could be attributed directly to cats.
The problem with trapping and exterminating is that trapping these cats can be very difficult, and even if a large percentage of a colony is captured and put down, those that remain still have breeding capability, and cats are able to birth multiple litters per year. Furthermore, the areas that do get cleared of one colony may become home to another due to a newly growing food supply and no competition.
An article published on the Feral Cat Coalition's website discusses the case of Marion Island, a small island off of South Africa, where people tried to rid the place of 2,500 cats through extermination. Deliberate infection with a virus and hunting combined took 16 years to succeed. Marion Island is an isolated area though; without people bringing cats to the island, there's no way for cats to get there. Such is not true in the U.S., and cats from outside who find a new food source will create a new colony, even if that area was once cleared of feral cats.
Trapping and euthanizing is tedious, difficult, and costly for local governments to implement. TNR programs are generally run by non-profit organizations at their own expense, and is a much cheaper, safer, and more effective method at controlling and reducing feral cat populations.
The cats of the Atlantic City Boardwalk are among the most famous of feral cat colonies in the country, and Alley Cat Allies' most successful TNR project. The population is cared for and stable, helping stop the spread of disease and allowing the colonies to shrink through natural attrition. Almost all of these cats have been fixed, and therefore, can no longer breed.
Widespread TNR programs nationwide will reduce the feral cat population nationwide over time humanely, and at a far smaller cost to taxpayers and society than extermination. Alley Cat Allies has a petition on their website calling for the Smithsonian to stop spreading these myths.