Trap-neuter-release has not been proven to reduce the number of feral cats through natural attrition. Colonies can grow since not every cat can be trapped and those that remain are fed and better able to breed. Neighborhood cats find the food source and irresponsible owners dump their cats into colonies. TNR is based on perpetual colony maintenance. Reduced rates of euthanasia at shelters are not proof that the actual number of feral cats is decreasing.
Feral cats are not wildlife; their home is not outdoors. Releasing domestic companion animals to live and die in the wild is neither ethical nor compassionate. Outdoor cats are exposed to many hazards including fatal feline diseases. Many adult feral cats can be socialized and adopted. Euthanasia, as a last resort, is a more humane outcome for these cats. Trap-and-remove has been proven to work when the food source is also removed. Wild animals do move into an area to fill an ecological niche, but the only reason domestic cats congregate is due to an artificial food source.
These colonies pose a public health risk. The National Association of State Public Health Veterinarians has stated that there is no evidence that colony management programs will reduce diseases. Since not every cat can be trapped, not every cat is vaccinated. Re-vaccination seldom occurs due to the difficulty of trapping a feral cat a second time. Rabies-vector species including skunks and raccoons often dine alongside colony cats.
TNR is environmentally irresponsible. Any free-roaming cat poses a serious risk for native wildlife, already struggling to survive. Well-fed cats still hunt and can further compromise threatened and endangered species.
TNR may be a violation of the Endangered Species Act and the Migratory Bird Treaty Act. Releasing these non-native predators is just one more way humans degrade habitat. Colonies should never be managed on public or municipal land.