With four million to six million unwanted animals-mostly cats-put to sleep each year in the United States, some people might consider it more humane to free the felines into the wild. But as these cats forage for food and establish their territories, they kill more than a billion small mammals and birds each year, many of which are threatened or endangered, a University of Florida study shows.

Feral, or free-roaming, untamed cats pose a serious threat to endangered species nationwide as colonies of the wild cats have grown, largely because local groups provide funding and resources to sustain them, according to the University of Florida study commissioned by the U.S. Fish and Wildlife Service.

In addition, although the study found those who release cats into the wild or support feral cat colonies are violating numerous federal and state wildlife protection laws, enforcement of the law in these cases has largely been ignored, according to the study that was published in the spring 2004 volume of the "Journal of Land Use & Environmental Law."

"The domestic cat species is not indigenous to Florida or anywhere else in North America," said Pamela Hatley, a law student at University of Florida's Levin College of Law who conducted the study. "They impact native wildlife in three primary ways: predation, competition, and disease."

Feral cats spread diseases-rabies in particular-that can also kill wildlife, according to John Cely, wildlife biologist with the S.C. Department of Natural Resources (DNR) Wildlife Section. The Centers for Disease Control and Prevention report that rabies is more than twice as common in cats as it is in dogs or cattle, and cats have the highest incidence of rabies among domestic species.

The number of feral cats in the United States is estimated to be 40 million to 60 million, according to the study, which was commissioned by the U.S. Fish and Wildlife Service to determine the applicability of federal, state and local wildlife laws to the practice of releasing cats into the wild and maintaining feral cat colonies. Compounding the problem is that another 40 million domestic cats nationwide also roam outside, hunting and killing small animals.

For example, the Lower Florida Keys marsh rabbit is a federal endangered species with a remaining population of about 100 to 300. A 1999 study found free-roaming cats were responsible for 53 percent of the deaths of these rabbits in one year, and a 2002 study indicated the species could be extinct within two or three decades.

Cats also have been recognized as instinctive predators and a serious threat to the Key Largo cotton mouse, Key Largo woodrat, Choctawhatchee beach mouse, Perdido Key beach mouse, green sea turtle, roseate tern, least tern and Florida scrub jay.

Cat predation also is a serious problem in California and Hawaii, where, like Florida, the climate is ideal for cats to survive outside and breed year-round. As a result, endangered animals, such as the Hawaiian goose, California brown pelican and blunt nosed leopard lizard also face additional threats.

There are some 15 million cats in Florida that spend all or part of their time outside preying on wildlife. It is estimated that cats kill as many as 271 million small mammals and 68 million birds each year in Florida, many of these members of threatened and endangered species.

As an alternative to euthanasia, many cat advocates believe in trap-neuter-release, or TNR, programs, in which feral cats are spayed or neutered and returned to colonies where caretakers look after them. While the programs aim to reduce wild cat populations, however, irresponsible pet owners continue to release unwanted cats that often join feral cat colonies.

In Florida, such colonies are known to exist in 17 counties. The largest, in Key Largo, may include as many as 1,000 cats and operates on an annual budget of $100,000. TNR programs and managing large numbers of cats in colonies do not effectively control cat overpopulation or the predation of endangered animals, the study found.

And although the study determined that releasing cats into the wild and supporting feral cat colonies is a violation of federal laws, such as the Migratory Bird Treaty Act and the Endangered Species Act, enforcement of these and other state and local regulations with the same goals is rare against those who release cats or support feral colonies.

The Florida Fish & Wildlife Conservation Commission, for example, has left enforcement of state laws up to local governments, whose ordinances vary greatly as to what they prohibit and the fines involved. "It is essential that our state and local governments take steps to educate the public about the destructive impact of free-roaming cats on native wildlife, and strictly enforce against the release of cats into the wild," Hatley said.

Michael Wooten, an associate professor of biology at Auburn University, has done extensive research on endangered beach mice nationwide. His studies have found limited direct evidence that feral cats hunt the endangered mice, but he said he has observed immense indirect evidence, including cat paw prints in the dunes where mice live and mouse-tracking devices in the bellies of cats.

"Predators in general have taken quite a toll on the beach mouse population," Wooten said. "Where you have cat colonies, there is a decrease on the mouse population."

Feral cat colonies are a well-intentioned but misguided idea, according to Wooten "If people really loved animals, they wouldn't release large groups of predators into the wild," he said.