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Some days, you need a bit of imagination to view a house cat as a predator. Sitting in a sunny window, curled up on a bed or stretched out in front of a fireplace, most tabbies hardly seem to have the energy to attack their food dish. But cats are definitely hunters, and there are plenty of them in town, in barns and free-ranging in fields to put the bite on a variety of birds and other wildlife.
Cat populations are tough to gauge accurately. U.S. Census data track those cats that people claim to own as pets, and the numbers are impressive. From 1970 to 1990, the number of urban and rural cats tabulated in the census rose from 30 million to 60 million. Nationwide, approximately 30 percent of households "own" cats. In rural areas, where free-roaming cats often are not regarded as pets, and not recorded by the census, as many as 60 percent of households keep cats on their property. In Wisconsin alone, with 550,000 rural households, we estimate the number of barn cats and outside cats may be as high as two million. Nationwide, there must be at least 100 million cats, mainly concentrated in the same places we find people.
Domestic cats (Felis catus) descend from the European and African Wild Cat (Felis silvestris). Although domestic cats make affectionate pets, their skills and behaviors as predators remain essentially unchanged from those of their ancestors, and they hunt as effectively as their wild forebears.
However, house cats differ from wild predators in several important ways: First, people protect cats from disease, predation and competition. Modern veterinary practices, from vaccination to bone setting, substantially extend a pet cat's lifespan. Second, domestic cats adapt well to human domiciles. Unlike native predators, cat densities are not limited by space or the availability of prey. Even barn cats have significantly better shelter, food and water supplies than bobcats, foxes and coyotes. The cat's range extends every time people build new homes and outside shelters. Third, although most people supply their cats with a dependable supply of food, research shows feeding does not suppress the cat's instinct to hunt and kill.
These factors combine to make free-ranging cats a potent predatory force, especially in rural areas. We've estimated that in some parts of rural Wisconsin, cat densities reach 114 animals per square mile – much higher than all mid-sized native predators. Given ample food supplies and high reproductive rates, cat densities can exceed nine animals per acre.
Rural cats have access to many animals and undoubtedly take the greatest toll. Small mammals like mice and voles make up about 70 percent of their diets, birds constitute about 20 percent of their kills and a mix of other animals constitute the remaining 10 percent. Our research in Wisconsin suggests that free-ranging rural cats may be killing up to 219 million birds in the state; many are native songbirds whose populations are already stressed by a host of factors including development, habitat destruction and pesticide pollution.
Many of these birds are ground-nesting grassland birds, like meadowlarks and sparrows, or birds that often feed on the ground, like robins. Especially in rural areas, these birds inhabit pastures and hayfields around farms where high densities of cats seriously threaten them.
The cat predation affects wildlife dynamics in other ways. Domestic cats eat many of the same animals that native predators eat. Studies show that large numbers of cats reduce available prey for predators such as hawks and weasels.
Free-ranging cats may also transmit diseases to wild animals. Domestic cats have spread feline leukemia virus to mountain lions and may recently have infected the endangered Florida Panther with feline distemper (feline panleucopenia). Unvaccinated domestic cats can also transmit rabies and toxoplasmosis to people. Pregnant women are now routinely advised to avoid contact with cats and litter boxes to minimize the risk of infection.
Given these concerns, here are seven sensible suggestions to prevent growing cat populations from becoming an even greater threat to wild animals:
No one has collected enough data to definitively predict the number of birds killed by rural free-ranging cats. Our four-year study of cat predation in Wisconsin, coupled with data from other studies, predicts a range of values based on the following assumptions.
We estimate 1.4-2 million free-ranging cats in rural Wisconsin. We further estimate 23 percent of their diet consists of birds. This figure is consistent with other studies indicating roughly 20-30 percent of free-ranging cat" kills are birds.
The number of animals killed by an individual cat varies greatly from zero to much more prey than a cat can consume. One rural cat was recorded to have killed 1,690 animals in an 18-month period. On an annual basis, studies record low estimates of 14 animals per free-ranging urban cat to at least one animal per day for rural cats. Other studies reported 28 kills per year for urban cats and 91 kills per year for rural cats.
Here are our best guesses at low, intermediate and high estimates of the number of birds killed annually by rural cats in Wisconsin based on the formula:
(number of rural cats) x (number of kills/cat/year) x (% of kills that are birds).
Low value:(lowest population estimate) x (twice kill rate by urban cats) x (low percentage of kills that are birds) 1.4 million cats x 28 x 20% = 7.8 million birds killed by rural cats
Intermediate value: (mean population estimate) x (intermediate kill rate) x (higher percentage of kills that are birds)1.7 million cats x 91 x 25% = 38.7 million birds killed by rural cats
High value: (highest population estimate) x (highest kill rate) x (highest percentage of kills that are birds) 2 million cats x 365 x 30% = 219 million birds killed by rural cats
Note, these estimates do not include predation by urban cats.
Also note, in northern states such as Wisconsin, most kills occur in spring and summer, though predation at winter feeders is substantial. Many of the spring kills would include nestlings and fledglings.
The densities of free-roaming cats in rural Wisconsin are several times higher than the typical combined densities of other mid-sized predators like foxes, skunks, opposums and raccoons. Clearly, free-ranging cats are a major predator of birds in rural Wisconsin.
John S. Coleman is a wildlife ecologist with the Great Lakes Indian Fish and Wildlife Commission stationed in Madison, Wis. Professor Stanley A. Temple teaches and oversees research projects in the Department of Wildlife Ecology at the University of Wisconsin-Madison.