Biology and Behavior of the Cat
Cats have lived outdoors for thousands of years—they are part of the natural landscape, and there are many biological and behavioral reasons why they are not a threat to other species. But opponents of Trap-Neuter-Return (TNR) still accuse outdoor cats of threatening birds or wildlife. Science clears cats of blame—studies prove that cats are not a significant threat (especially when compared to the very real dangers from human activities) and that they actually play important roles in balancing the local ecosystem.
Dietary Biology: What Cats Really Eat
Studying cats’ diet and behaviors exonerates them as a culprit behind wildlife decline
Finders Keepers—Cats Are Scavengers
Feral cats are opportunistic feeders—they will eat whatever food is easiest to find that will also satisfy their nutritional needs. Today, feral cats’ main source of food is almost always people’s garbage.
Just as cats 10,000 years ago were attracted to the easy, consistent food source that the first human settlements provided (see The Natural History of the Cat), feral cats today scavenge on the scraps that all human habitats inevitably produce.1 A study of a feral cat colony in Brooklyn found that the cats depended more on local garbage for food than on either prey or feeding by caregivers, and that the neighborhood produced enough garbage to feed three times more cats than actually lived in that area.2
If They Hunt, Cats Specialize in Rodents
Opponents of TNR often claim that cats belong indoors because they impact songbird populations. However, decades of studies prove that when cats do hunt—which is not nearly as often as they scavenge—they much prefer a diet of rodents. Studies have shown cats to be far more efficient hunters when they sit and wait for prey—outside a rodent burrow, for example—than when they stalk and pounce, the way they approach birds.3 As opportunistic feeders, cats are more likely to go for your garbage or sit and wait to catch rodents than to take their chances chasing birds who can easily spot them and fly away.
In fact, studies of samples from the diets of outdoor cats confirm that mammals appear three times more often than birds. The same studies also show that when cats do hunt for food, they mostly feed regularly on a particular species of mammal. Birds are consumed only incidentally and not according to a regular feeding pattern.
Endangered Species Not on the Menu
As noted above, cats primarily scavenge for food and eat scraps and refuse—whatever food is easiest and most abundant to find—and only hunt when other, easier food sources are scarce. When they do hunt for prey, cat diet studies show that instead of catching a large number of a specific species of bird, the samples contain individual birds of several different species—usually the most common bird species in the area.4 A study of cat predation on rabbits similarly observed that cats’ prey is the most numerous prey species in that area for that time of year. Endangered species were rarely on the menu, and even then not as the primary prey.5
Understanding Ecosystems: Why Removing Cats Is Worse for Everyone
Opponents of TNR often call for cats to be trapped and killed or relocated to protect wildlife. Since cats are not a threat, however, removing them is not a solution. Cats play a complex role in local ecosystems and cannot simply be removed from any environment without consequences. The scientific phenomena discussed in this section—compensatory predation, the mesopredator release effect, and the vacuum effect—illustrate why removing cats is harmful to the entire habitat, and why Trap-Neuter-Return is truly the best approach.
Although cats do occasionally prey on other animals, taking a bird here or there doesn’t necessarily impact the survival of the species. Evidence suggests that cat predation is often “compensatory predation”—preying on animals that would likely have died anyway from disease or hunger. Studies show that the animals caught by predators are generally weaker and more diseased than those killed by manmade sources.6,7 One study found that “birds killed by cats had significantly lower mass, fat scores, and pectoral muscle mass scores” than birds of the same species killed by cars or windows.8 These studies indicate that cats are catching what some biologists refer to as the “doomed surplus”9—animals who would not have lived, and so whose death does not affect overall population levels.
Mesopredator Release Effect
Maintaining ecological balance is far more complicated than cats versus birds, predator versus prey. While cats sometimes might be the top predators in their environments, some of the animals they prey on are also predators, like rats. The predators who fall lower on the food chain are called mesopredators. They prey on certain species—in the case of rats, small or fledgling birds and bird eggs—while being prey to larger predators themselves.
Removing cats from the ecosystem can destabilize the relationships between the different predator and prey species, with dire consequences.10 On Amsterdam Island in the Indian Ocean, an attempt to eradicate feral cats to protect endangered birds caused a spike in the rat and mouse population. The rats and mice then preyed on the birds—making the cat eradication ineffective at conserving bird populations.11
Mathematical models in scientific studies project that cats, rats, and birds can find a balance where all three species co-exist. But when the cats are removed in these simulations, the rat population surges out of control, wiping out the birds completely. Although the cats in this scenario may occasionally prey on a bird, removing the top predator completely is far worse for the prey species’ survival.12,13
When people misguidedly remove cats to protect wildlife, they risk seriously harming both the environment and the species they aim to protect. A real-life study of a coordinated cat eradication effort on an island—intended to protect endangered species from predation—saw the rabbit population on the island spike wildly. Without the cats to keep prey species in check, the rabbits devastated local vegetation, which harmed other animal species, and a wave of more than 130,000 rodents entered the ecosystem. In their report of the eradication effort, the researchers directly linked this damage to the removal of the cats, concluding: “the unintended consequences have been dire.”14
The Vacuum Effect
Cats choose to live in an area for two reasons: because there is a food and water source—like the garbage in Brooklyn—and because there is shelter. The availability of these resources determines the number of cats who can live off of these resources. If the cats are removed, other cats will take advantage of these same resources, whether they move in from neighboring territories or are born from survivors. This phenomenon, known as the vacuum effect, is scientifically documented across a variety of species—and corroborated by decades of failed animal control policy.
Learn more about the vacuum effect.
Trap-Neuter-Return Benefits Cats and Wildlife
Trap-Neuter-Return is the approach that stabilizes the cat population—no more kittens—while allowing these top predators to remain in the habitat, preempting problems like the destabilization of the ecosystem or attracting new, intact cats. Choosing the survival of either cats or wildlife over the other is a false choice because the best approach for all animals is the same: TNR.
Seeing the Whole Picture
Predation and conservation are not about one cat and one bird. These complex issues affect entire species, whole ecosystems, and millions of animals. As animal advocates, we want what’s best for all of them. A cat will eat whatever is easiest to obtain—be it garbage, cat food, or sometimes prey. But when we look at the big picture, scientific evidence consistently exonerates the domestic cat species of threatening wildlife.
 Yamane, A., J. Emoto and N. Ota. Factors affecting feeding order and social tolerance to kittens in the group-living feral cat (Felis catus). Applied Animal Behaviour Science 52 (1997): 119-127.
 Calhoon, Robert E. and Carol Haspel. Urban cat populations compared by season, subhabitat and supplemental feeding. Journal of Animal Ecology 58 (1989): 321-328.
 Fitzgerald, B. Mike and Dennis Turner. Hunting behaviour of domestic cats and their impact on prey populations. In The Domestic Cat: The Biology of Its Behavior, 2nd Ed., Turner, Dennis C. and Patrick Bateson eds. (Cambridge University Press: New York, 2000) 153-154.
 Ibid., 164.
 Ibid., 160.
 Møller, Anders P., Johannes Erritzøe and Jan T. Nielsen. Frequency of fault bars in feathers of birds and susceptibility to predation. Biological Journal of the Linnean Society 97 (2009): 334-345.
 Leyhausen, Paul. Cat Behavior: The Predatory and Social Behavior of Domestic and Wild Cats, (New York: Garland STPM Press, 1979), 78.
 Baker, Philip J., et. al. Cats about town: is predation by free-ranging pet cats Felis catus likely to affect urban bird populations?. Ibis 150 (Suppl. 1) (2008): 86-99.
 Lilith, Maggie. Do pet cats (Felis catus) have an impact on species richness and abundance of native mammals in low-density Western Australian Suburbia? Ph.D. thesis for Murdoch University, Western Australia. 2007.
 Soulé, M. E., D. T. Bolger, A. C. Alberts, J. Wright, M. Sorice, and S. Hill. 1988. Reconstructed dynamics of rapid extinctions of chaparral-requiring birds in urban habitat islands. Conservation Biology 2:75–92
 Isabelle M. Cote and William J. Sutherland, “The Effectiveness of Removing Predators to Protect Bird Population,” Conservation Biology 11 (1997): 402.
 Fan, Meng, Yang Kuang, and Zhilan Feng. Cats Protecting Birds Revisited. Bulletin of Mathematical Biology 67 (2005): 1081-1106.
 Courchamp, Frank, Michel Langlais, and George Sugihara. Cats Protecting Birds: Modeling the Mesopredator Release Effect. Journal of Animal Ecology 68 (1999): 262-292.
 Bergstrom, Dana M., A. Lucieer, K. Kiefer, J. Wasley, L. Belbin, T.K. Pedersen and S.L. Chown. Indirect effects of invasive species removal devastate World Heritage Island. Journal of Applied Ecology 46 (2009): 73-81.