“Neutering of feral cats as an alternative to eradication programs,” published in the Journal of the American Veterinary Medical Association, 1993.
This study  presents the results of a three-year trap-neuter-return (TNR) program undertaken on the campus of the Gillis W. Long Hansen’s Disease Center, a research hospital near Baton Rouge, Louisiana.
The TNR program was initiated following the failure of previous removal efforts (presumed to result in the killing of any cats caught).
Researchers documented a 12% population reduction — from 41 to 36 cats (including 6 new arrivals) — over three years, and no evidence of kittens born on campus during that period. Results of this study are comparable to those reported elsewhere by researchers investigating the effectiveness of targeted TNR efforts [2–7].
A census of the cats on the hospital campus was undertaken 18 months and 36 months after all known cats had been sterilized. Of the 41 cats included in the program, 30 (73%) were observed at campus feeding stations 36 months later, along with 6 cats new to the campus.
The researchers involved suspected that these new cats, all of whom arrived in the final 18 months of the observation period, were likely abandoned on or near the campus.
"Several cats were successfully removed from the colony every year, but a noticeable reduction in overall numbers was never achieved [via lethal removal].”
“Several cats were successfully removed from the colony every year,” explain the authors of the study, “but a noticeable reduction in overall numbers was never achieved [via lethal removal].” This decision — choosing to implement a sterilization program after recognizing the ineffectiveness of lethal removal efforts [8,9], or in anticipation of a subsequent “vacuum effect” typically associated with such efforts  — has been reported by other institutions.
Study authors point out that the facility’s patients generally appreciated the presence of these cats, regularly feeding cats (in violation of posted rules) and even releasing cats that had been trapped for removal. The TNR program was likewise well received by patients, echoing the results of public opinion surveys showing broad support for TNR over other feral cat management methods .
Zaunbrecher, K.I.; Smith, R.E. Neutering of feral cats as an alternative to eradication programs. Journal of the American Veterinary Medical Association 1993, 203, 449–452.
Levy, J.K.; Gale, D.W.; Gale, L.A. Evaluation of the effect of a long-term trap-neuter-return and adoption program on a free-roaming cat population. Journal of the American Veterinary Medical Association 2003, 222, 42–46.
Nutter, F.B. Evaluation of a Trap-Neuter-Return Management Program for Feral Cat Colonies: Population Dynamics, Home Ranges, and Potentially Zoonotic Diseases, North Carolina State University: Raleigh, NC, 2005.
Spehar, D.D.; Wolf, P.J. An Examination of an Iconic Trap-Neuter-Return Program: The Newburyport, Massachusetts Case Study. Animals 2017, 7.
Spehar, D.D.; Wolf, P.J. A Case Study in Citizen Science: The Effectiveness of a Trap-Neuter-Return Program in a Chicago Neighborhood. Animals 2018, 7.
Swarbrick, H.; Rand, J. Application of a Protocol Based on Trap-Neuter-Return (TNR) to Manage Unowned Urban Cats on an Australian University Campus. Animals 2018, 8.
Spehar, D.D.; Wolf, P.J. Back to School: An Updated Evaluation of the Effectiveness of a Long-Term Trap-Neuter-Return Program on a University’s Free-Roaming Cat Population. Animals 2019, 9.
Mendes-de-Almeida, F.; Remy, G.L.; Gershony, L.C.; Rodrigues, D.P.; Chame, M.; Labarthe, N.V. Reduction of feral cat (Felis catus Linnaeus 1758) colony size following hysterectomy of adult female cats. Journal of Feline Medicine & Surgery 2011.
Mendes-de-Almeida, F.; Faria, M.C.F.; Landau-Remy, G.; Branco, A.S.; Barata, P.; Chame, M.; Pereira, M.J.S.; Labarthe, N. The Impact of Hysterectomy in an Urban Colony of Domestic Cats (Felis catus Linnaeus, 1758). International Journal of Applied Research in Veterinary Medicine 2006, 4, 134–141.
Tennent, J.; Downs, C.T.; Bodasing, M. Management Recommendations for Feral Cat (Felis catus) Populations Within an Urban Conservancy in KwaZulu-Natal, South Africa. South African Journal of Wildlife Research 2009, 39, 137–142.
Wolf, P.J.; Schaffner, J.E. The Road to TNR: Examining Trap-Neuter-Return Through the Lens of Our Evolving Ethics. Frontiers in Veterinary Science 2019, 5, 341.