Spay/neuter success makes old projection methods obsolete––& makes doing new surveys essential
Counting feral cats and street dogs in the early 21st century is not what it was in your grandparents’ time, or your own time any more, either, if you have lived much longer than the oldest cat or dog of your acquaintance.
For the first 100 years after American Museum of Natural History ornithologist Frank M. Chapman in 1908 made the first known systematic effort to estimate the numbers of cats and dogs in the U.S., the science of the matter did not change much.
Study methods and the quality of the input data improved enormously, of course, especially through the contributions of National Family Opinion research company founders Howard and Clara Trumbull, who did landmark surveys in 1927, 1937, and 1947-1950.
But the basic approach was still just survey-and-multiply, with no need to take into consideration any compensatory factors or anything else more complex than the basic numbers.
All anyone wanted or needed to know, for the most part, was simply how many cats and dogs were out there.
This did not change, whether the goal was to eradicate them all, as it was for Chapman, or to estimate the potential market for cat and dog food, as it was for the Trumbulls, or to vaccinate them all against rabies, assess the effects of cat and dog predation on wildlife, or even to project the number of sterilization surgeries that would be necessary to eliminate lethal animal control methods.
Wealth of studies, but garbage in, garbage out
Enthusiasm for achieving no-kill animal control, in particular, either in individual cities or in whole nations as big and geographically diverse as the U.S. and as populous as India, produced a plethora of cat and dog population studies during the past several decades.
These studies simultaneously yielded an unprecedented wealth of cat and dog population data, and a great deal of inapplicable nonsense.
The underlying problem is that even as the many studies were underway, successful efforts to suppress cat and dog population growth moved the baseline for knowing what the numbers mean.
Can no longer project births & survival just from just counting
The increasing ubiquity of spay/neuter, in particular, means that it is no longer possible, almost anywhere in the world, to project the numbers of litters who will be born, and the numbers of cats or dogs who may survive to reproduce, just from knowing approximately how many cats or dogs there are.
A shortcut approach that worked very well in most of the world as recently as the first decade of the 21st century was to guesstimate that the number of feral cats and street dogs at large was, on average, about three times the number as were impounded, or otherwise killed by animal control.
This ended with the advent of techniques such as neuter/return and return-to-field as routine animal control practice.
Can you bake a cherry pie?
Estimating feral cat and street dog populations accurately enough to project forward what needs to be done to keep the numbers in check, and therefore to budget resources for doing the job, has become relatively complicated.
But not so complicated that anyone who knows how to stretch a recipe cannot do it, if starting out with the right tools and concepts.
The very first concept to understand is that animal populations are still, as always, governed chiefly by food availability. The maximum number of animals who can survive on the available food for that type of animal is the “carrying capacity” of the habitat.
The “carrying capacity” of any given habitat may all be used by one species, or divided among many. But “carrying capacity” will not be left vacant. If food sources are available, some animal will take advantage of whatever they are.
Because cats and dogs tend to breed much more rapidly than most wildlife, and are less wary about living close to humans, historically cats and dogs consumed most of the edible refuse and rodents in human communities.
Cats & dogs no longer breed up to carrying capacity
Historically cats and dogs were almost always able to breed up to the carrying capacity of the habitat, with just a small amount of food competition around the margins from other bold and highly adaptable species such as crows, foxes, jackals, coyotes, raccoons, and monkeys.
But high-volume sterilization means that cats and dogs are no longer able to breed up to the carrying capacity of the habitat.
Some of the species that have moved into habitat no longer occupied by cats and dogs, eating refuse and rodents, also eat cats and dogs.
What share of carrying capacity do cats & dogs still have?
Thus, a feral cat and/or street dog population assessment can no longer presume, as people doing estimates long did, that the number of cats or dogs was somewhere close to the carrying capacity of the habitat, and would remain there, more or less, for decades.
What we are now trying to estimate is what share of carrying capacity is still claimed by feral cats and street dogs, and whether those cats and dogs are likely to continue to decline in number because of the combination of sterilization with food competition, or are likely to reclaim habitat if sterilization efforts are relaxed––or if, for example, a misguided purge of foxes, coyotes, raccoons, or monkeys leaves vacant carrying capacity that feral cats and/or street dogs can fill.
“Herd immunity” to reproduction
The second essential concept to understand is that cat and dog population control works in exactly the same manner as vaccination.
The underlying idea behind vaccination is to establish “herd immunity,” so that viruses cannot easily pass from an infected host to a host who is not protected.
“Herd immunity” is achieved when at least 70% to 80% of a potentially vulnerable population is vaccinated, which reduces the odds of a virus passing successfully from one host to another to the point that most viral diseases quickly burn out.
Sterilizing cats and dogs is, in effect, immunizing them against either becoming pregnant or impregnating others. At a sterilization ratio of 70% to 80%, a feral cat or street dog population will stabilize; as the sterilization rate increases, the population will drop.
Why 70% to 80%?
Why this occurs is a matter of simple math. Of each successfully birthed average-sized litter of four feral kittens or street dog puppies, half will be female; half will be male.
On average, because feral cats, street dogs, and indeed all wildlife have high juvenile mortality, only half will survive past weaning, and only half of those kittens or puppies will survive long enough to reproduce.
In other words, 75% of the kittens or puppies born will, on average, not become part of the breeding population.
To give birth to a litter, or sire a litter, the one survivor of the litter of four must find a mate from among the survivors of other litters.
Then, on average, these survivors will produce one-and-a half to two-and-a-half litters in their lifetimes, depending on food availability and predation pressure, including animal control killing.
If they produce only one to one-and-a-half litters of four, the feral cat or street dog population they represent will drop; if they produce two litters of four, it will remain stable; if they produce two-and-a-half litters of four, it will increase.
How sterilization simulates predation
The third essential concept to understand is that sterilizing feral cats and street dogs has essentially the same effect on their population as nest predation.
Sterilization amounts to removing kittens or puppies from the habitat, while preventing the already existing population of adult cats or dogs from producing more. At a 75% sterilization ratio, the existing population of adult cats or dogs will barely be able to replace itself, let alone grow.
This is why sterilizing cats or dogs will lastingly reduce their numbers, while simply killing them does not.
Why killing 75% doesn’t reduce the population
Proponents of population control killing, the chief approach to feral cat and street dog population management since civilization began, tend to argue that if at least 75% of the feral cats or street dogs in a community are killed, the net effect is the same as if 75% were sterilized––but it isn’t.
If one starts with a population of eight feral cats or street dogs, and then kills the first six who can be caught, what remains are two warier and still fertile cats or dogs who can breed back up to eight in just two breeding cycles, or perhaps even one.
Their reproductive capacity is uninhibited, and they are likely to be able to raise somewhat more survivors than average from each litter because they will have less food competition.
Get 75% or flunk
This brings us to the last and in some ways the most essential concept to keep in mind before doing a feral cat or street dog population estimate that is meant to be projected forward into a public policy recommendation: the difference between compensatory and additive predation.
“Compensatory predation” means that if you are the animal under discussion, if one thing doesn’t get you, something else will. Feral cats and street dogs already have about 75% mortality before raising their first litter. Therefore, until 75% are sterilized, or killed by animal control, either sterilization or population control killing is only simulating compensatory predation.
If your test is whether the feral cat or street dog population declines, you must get 75% or flunk.
Only after 75% are sterilized or killed is the population reduction “additive,” meaning that it subtracts from the number of feral cats or street dogs who contribute to sustaining their numbers.
Current feral cat & street dog populations cannot be projected from an ivory tower
With all of that in mind, it is still necessary––and actually more necessary than ever––to do some site-specific surveying to put together input numbers that tell you anything at all about your current feral cat and/or street dog population.
You cannot just sit in an ivory tower and input numbers and ratios taken out of academic studies done in years past, because the circumstances that produced those numbers and ratios have changed––and not just a little, and have changed to different degree and in different ways from place to place, depending on how much neuter/return has been done and what urban wildlife has moved into the newly opened habitat.
20th century feral cat and street dog population projections typically began with the presumption, no longer true (and never true in the developing world), that these cats and dogs were fecund outcasts––escaped or abandoned––from the owned pet population.
There was also a typical presumption that feral cats and street dogs were frequently breeding with owned cats and dogs who were allowed to roam at large.
Many feral cats and street dogs, and in some areas perhaps most, may still be escaped or abandoned former pets, but they are not necessarily able to reproduce.
Neither are they necessarily likely to find mates among owned but wandering pets, now that keeping pets indoors is the social norm in most of the developed world, while more than 90% of the U.S owned cat population have been sterilized, and well over 70% of the owned dog population (except for pit bulls, among whom the sterilization ratio remains below 25%.)
Producing an accurate projection of a feral cat or street dog population today requires discovering, somehow, not only how many cats and dogs are at large, but also what their approximate mortality and reproductive rates are today, not what they were in 2000 and 1990.
Fortunately, gathering the needed baseline data about feral cats and street dogs is not rocket science. Anyone who can make tally marks on a clipboard can do a good job of it, if motivated to do the necessary legwork.
Be aware that much more can be learned from just a small amount of data, if it is well-collected, than most people realize.
“Well-collected” is the key. Formal studies often spend tens of thousands of dollars to find out little or nothing that people of good sense do not already know, but well-designed studies done on a budget of nothing frequently prove invaluable.
Be aware, too, that four often used sources of information on cat and dog populations are inherently inclined to exaggerate: feeders, rescuers, people with a grievance, and the animal advocacy sector.
A 1989 study of cat feeders in Brooklyn, done by Robert Calhoun and Carol Haspel, found that the feeders in any given apartment block collectively overestimated the number of feral cats they were feeding by about a third, partly because several different people were feeding each cat.
Exhausted and financially struggling rescuers commonly over-estimate the numbers of cats and dogs at large by a factor of 10 or more, simply because what they see tends to look like endless, overwhelming suffering and need.
Birders, and the organizations representing them, often claim there are at least four times as many cats at large as any actual population study has ever found, and more than 12 times as many as any actual population studies done since 1990 suggest.
The same is true of organizations militantly opposed to the presence of street dogs in India, and probably in most of the nations that have street dogs.
But feral cat and street dog advocacy organizations often use the same inflated numbers in the mistaken belief that they will inspire more donors.
In summary, do not be surprised to find far fewer feral cats and street dogs than the loudest advocates both for and against their existence insist are there, and to be attacked from both directions.
Where to begin?
Start your feral cat and/or street dog population survey by finding out what data is already available to you, including historical data that can be used to develop a longterm trend line.
Among the questions to ask:
a) How many cats and/or dogs have already been sterilized? Who is fixing how many, from within what radius? How does the number of sterilization surgeries compare with the owned dog and cat population?
Data useful in answering these questions, within the U.S., is available from the U.S. Pet Ownership Demographics Sourcebook, published every five years by the American Veterinary Medical Association, probably accessible at your nearest university library.
Poll the veterinary clinics within the area under study. Do they distinguish feral cats from owned cats in their record-keeping. (These days, many if not most do.)
How do their numbers compare from year-to-year in recent years?
b) How many feral cats and/or street dogs are people feeding in the survey area? Poll feeders. How many animals are they feeding at each location? How many were they feeding at each location last year?
Beware of the sort of double counting that Calhoun and Haspel found, but be aware also that surveys done since 1996 have consistently discovered that feeders who practice neuter/return are feeding ever fewer cats over time.
A house to house survey of representative postal codes can find out how many people feed dogs and cats other than their own indoor pets.
Who feeds the animals is the question to ask, not who owns them, because feeders usually prefer not to accept further responsibility for the animals they serve.
c) Is anyone counting roadkilled cats and dogs? This is an extremely important benchmark, if the data exists. Road maintenance departments may be the first place to check. Because feral cats and street dogs spend their lives at large, they are the cats and dogs most likely to be roadkilled. Therefore, nothing else is as likely to tell you accurately what the feral cat and street dog population trends have been in any given community as the roadkill count, if it is available.
If you don’t have historical roadkills to go by, count roadkills anyway, when you do your own censusing, and be sure to distinguish between kittens and adult cats, and puppies and adult dogs. This may tell you more about the age structure of the population at large than any other kind of surveying.
During the late spring and summer, the count should tilt very heavily toward young animals. If it does not, you may already have fecundity just about under control.
Urban wildlife counts
d) What local wildlife counts are available?
Breeding bird survey counts, done in spring, and National Audubon Society bird counts, done at Christmas, can tell you what competition feral cats may have from hawks, owls, eagles, and to a lesser extent, from crows, ravens, and gulls.
In general, the more mouse-eating birds there are, the fewer feral cats you will find.
Nature centers and university wildlife departments have often done surveys of other predator species that might either compete with feral cats and street dogs, or prey upon them.
When you do your own feral cat and street dog populations, you will probably not also be able to do a comprehensive census of the wildlife populations that interact with them. However, having some idea of the abundance of those wildlife species is invaluable to putting your counts into context.
Filling in the gaps
Once you have whatever data is already available, you can begin to develop it by doing your own surveying to fill in the gaps.
This can be really simple. Suppose you find out that you have seven feral cats and five street dogs living at a strip mall or marketplace. Canvas other strip malls or marketplaces to see how many cats and dogs they have. (This is called doing “area counts.”)
Once you have a reasonably good sampling of similar sites in different areas, you can find out the number of strip malls or marketplaces within the city or state you are studying and multiply.
How many is a good sample? That depends on what you find. If you find that every marketplace you visit has 5-8 cats and 5-10 dogs, you don’t need to survey as many as if the range is 0-20 for either species.
The more variation there is, the larger your sample base needs to be.
Night & day difference
The simplest, yet most useful method of gathering feral cat, street dog, and wildlife population data consists of simply walking through representative neighborhoods by both day and night, counting cats, dogs, and any wild animals who might either be cat or dog prey, might compete with cats or dogs to catch prey, or might eat cats or dogs.
Doing separate counts by day and night is necessary because most mammals are mostly nocturnal, including feral cats, street dogs, rodents, and other wildlife of significance to feral cat and street dog populations.
The daytime population cannot be estimated from a night count; neither can the night population be estimated from a daytime count.
Catch them “changing the guard”
This is because the conditions that influence cats and dogs to be mostly nocturnal or mostly diurnal are highly variable, and often not obvious to humans, for example the presence of coyotes or jackals. You may never see a coyote or jackal, but cats and dogs, with much more sensitive ears and noses than yours, will be acutely aware if either one is nearby.
With that much said, one often can count both the day and night feral cat and street populations on a single out-and-back line transect, if careful to work only at dawn and dusk, when the populations are “changing the guard,” and tend to socialize a bit.
A walking survey can take the form of an “area count,” or can be done as a “line transect,” commonly used to count wildlife.
To do a thorough line transect of a neighborhood, town, or even a whole city, walk straight through it, from one side to the other, by several different routes that will take you through every part of it: residential areas, industrial areas, open habitat, dumps, shops, etc.
The more transects you do, the better your study, but each transect must travel as straight a line as is practicable.
If you start wandering around to count the entirety of a particular neighborhood, you are doing an “area count” instead. This is not a problem, so long as your data logging shows what you counted, where. Typically feral cats, street dogs, and urban wildlife tend to be concentrated in particular places, with relatively sparsely inhabited blocks in between.
Your “line transects” will show you where the concentrations are, and where they are not; “area counts” will help you to estimate how many animals are in the concentrated locations.
Why work on foot?
Line transects can be done from a car, as well as on foot, but the car must be moving very slowly, with multiple observers, covering every direction, and the count will still be less accurate than if done on foot.
Counting animals from a car, the observers will not be able to see nearly as far ahead, or see cats or dogs lounging in gutters and doorways, behind parked cars or food stands or other obstacles.
Usually observers in a car will not be able to hear dogs barking from behind walls and closed doors, or be able to smell scent mounds and urine markings that will testify to the presence of tomcats, in particular, even if they are not out and about.
Neither will observers in a car be able to see kittens or puppies in boxes behind trash cans down alleys.
Combine counting methods
The best feral cat and street dog surveys combine “area counts” with “line transects.”
For example, if you have done several “area counts” finding 5-8 cats and 5-10 dogs at each marketplace, you can use “line transects” to discover the number of comparable marketplaces there are in your city.
Or, if you start with “line transects,” you will soon see which locations warrant doing an “area count,” in order to get a more accurate basis from which to multiply what you see by the total area of the community.
A common problem in counting street dogs is that they tend to move around quite a lot, and tend to follow and investigate anything different. This can lead to counting the same dogs multiple times. To get an accurate dog count, try to avoid attracting the dogs’ attention. Walk as if going somewhere, not as if hunting rats, carrying treats, or looking for a place to throw trash.
Counting “Cheshire cats” & “roof rabbits”
Counting feral cats is more difficult than counting street dogs, because of the furtive habits of feral cats. In order to do a good cat count, it is necessary to walk at dawn or dusk, and may be necessary to climb to the highest rooftop on each block to see how many cats may be seen on other rooftops.
Fortunately, it is not necessary to see cats in order to count them. Scat counts can also be highly reliable, figuring about one cat per identifiable scat, because scats don’t last long in outdoor environments.
There is value in doing multiple line transects along the same routes, if you have the time or the volunteers to do it, and then averaging the findings.
This is especially true in places with significant seasonal climate variation. Because most mammal species are born in spring and early summer, and reach low ebb in winter, winter counts will usually find only half as many animals as midsummer counts.
Sometimes weather can cause a count to vary even from day to day, sometimes the hour of the day makes a difference, and most significantly, the population of feral cats or street dogs in a particular area may increase or decrease abnormally if someone is doing something that actively attracts them or drives them away — e.g., dumping meat scraps, keeping a bitch in heat, or putting out poison.
Over a whole city, these factors tend to even out. On any given block, though, it is possible to see something abnormal, and multiple counts reduce that chance.
There are neighborhoods where either walking or driving at night, especially, might not seem appealing. Inexpensive wildlife cameras may be used instead of doing long night walks––but you will probably have to set up your camera (or cameras) in several different locations, protected from theft, to cover as much territory.
On the other hand, because wildlife cameras operate all night, if you can use them you are more assured of seeing the whole nocturnal animal population than if you just walk through once.
Covering more habitat is less valuable than being thorough
The most important point to keep in mind, in doing any feral cat or street dog survey, is that covering twice as much territory with 50% accuracy is not even half as valuable in trying to do an accurate projection of the data, either over more territory or forward in time, as covering half as much, with 90% accuracy.
And, of course, trying to project feral cat and street dog numbers without doing population surveying first is putting the cart before the horse.
(The ANIMALS 24-7 team have counted feral cats and street dogs in more than 40 nations, on every inhabited continent, beginning in 1962.)
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