Many contend it is impossible to accurately determine feral cat populations. In fact, the inability to determine a feral population would call for any humane effort to reduce that population to rely on guess work. All TNR programs should be required to produce measurable results to ensure continued support and funding; and the measure of success depends on knowing the baseline population.
In a study titled, The Birth and Death Rate Estimates of Cats and Dogs in U.S. Households and Related Factors, published in 2005 in volume 7.4 of the Journal of Applied Animal Welfare Science we find a responsible formula for calculating feral cat populations. This study was published by John C. New Jr. and William Kelch of the University of Tennessee, Jennifer Hutchison of the Australian Department of Agriculture, Fisheries and Forestry, Mo Salman and Mike King of Colorado State University, Janet Scarlett of Cornell University, and Philip Kass of the University of California at Davis.
The formula evolved from a 1996 survey of 7,399 U.S. households. The survey found a crude birth rate of about 11.2 kittens per 100 cats in households and an attrition rate that included a death rate of 8.3 and a disappearance rate of 3%. That is, cat births in households equaled attrition. It was further found that the movement of feral/stray cats into homes and shelters was approximately equal to the net growth in the household population plus the number of cats killed in shelters.
In other words, the number of feral/stray cats can be estimated by adding net cat acquisition to the number of cats killed in the shelter(s) and multiplying by three (to account for the one queen, one tom, and at least one sibling not entering homes or shelters who must exist to produce the known feral/stray cats).
More to the point, the feral cat population equals three times the number of cats killed in the shelters serving that area, plus the net cat acquisition (number of cats added to households) minus pet cat mortality.
Let me give you an example of how this formula would work. In a Los Angeles No-Kill plan that I submitted to Mayor Villaraigosa, I identified an area in South LA where spay/neuter efforts should be targeted. In this area 3,917 cats were impounded and 2,212 cats died or were euthanized in the SLA shelter in 2008. This targeted area has an estimated 1.25 million people living in 397,433 households according to the Los Angeles Planning and Demographic Research Unit. According to an AVMA formula this area has 128,768 cat-keeping households, with a total of 283,290 cats among them.
The combined mortality (8% or 22,663 cats) and disappearance (3% or 8,500 cats) rate of 11% per year is equal to the estimated number of births annually. This means there is a net self-replacement of an estimated 32,000 cats per year.
According to the U.S. norm for pet cat population increase over the past 20 years, the Los Angeles pet cat population is increasing at about 1% per year. Thus net acquisitions in this South Los Angeles area exceed attrition by about 2,850 additional cats per year, beyond births.
Of these 2,850 acquired cats, 1,705 come from LA Animal Services (3,917 impounds minus 2,212 killed). Another 1,114 (2,850 minus 1,705) come from other sources. Based on national averages, no more than 290 come from breeders, leaving 824 acquired from other sources like pet stores.
LA cat acquisitions include LA shelter adoptions including feral-born kittens and impounded stray cats, both kittens and tamed strays. The annual adjustment to the feral/stray population is 2,529 (1705 placed by shelters + 824 placed by other sources + the 2,103 who were killed). This totals 4,632 cats. Assuming that each cat had a mother, a father, and at least one surviving sibling, a crude estimate for the feral/stray cat population in the targeted area is 13,896.
Leonardo Fibonacci is considered the greatest European mathematician of the middle ages, born in Pisa, Italy about 1175 AD. Fibonacci developed a formula relating to agriculture productivity. His formula was later used by Pasteur to predict 70% of a susceptible population has to be vaccinated to prevent an epidemic. Fibonacci’s 70% Rule is recognized by World Health Organization and Center for Disease Control.
If you think of spay/neuter as inoculating feral cats to prevent pregnancy, then according to the Fibonacci Rule, 70% of all feral cats must be sterilized before the successful breeding encounters of the remaining 30% are reduced to a rate sufficient only to replace normal attrition. This means 9,927 (or rounding up for good measure, 10,000) feral/stray cats must be spayed or neutered just to stabilize the feral/stray cat population in the targeted area. Meaningful and sustained reductions will occur only when this rate is exceeded.
It is only with this knowledge that local animal control and/or local foundations can make a meaningful and measurable impact on local feral cat populations.