biodiversity Environment

Keep your Cat Indoors

Two months ago I wrote about finding a family of cats along the trail in the Galápagos Islands. The beautiful kittens would grow up to be murderers of vulnerable animals such as the rare species of hawk unique to the Galápagos. It was with mixed feelings that I learned that a ranger would be dispatched with dogs to obliterate this invasive family.
Can these lovable pets be a hazard here in the USA? Do they cause any problems with wildlife in our backyards?
The answer, unfortunately, is that cats allowed outside can be a serious problem, causing the deaths of billions of small animals.
Once, eating dinner as a kid, Cookie (our family’s cat) proudly joined us with her own meal in her mouth—a robin that she had just caught and killed. Until then we had no idea that she had been hunting in our backyard.
Cookie was not unique. Recent reviews suggest that cats kill huge numbers of small animals. In some cases we benefit from cats’ lethal effect, such as barn cats killing mice and rats. Unfortunately they are also responsible for the slaughter of many birds.
Fortunately not all cats are killers. Indoor cats aren’t given the opportunity to commit mayhem. And some pet cats that go outside leave little critters alone. Feral cats, who survive by tooth and claw, are the most destructive. However, free ranging domestic cats are estimated to kill over 2 billion birds every year in the USA.
Islands are where cats have caused the most trouble. Many unique species have developed in the isolation of islands; Darwin’s finches are a famous example. They are all unique to the Galápagos since they evolved in that remote cluster of islands.
Pirates and other sailors brought cats to the Galápagos over 300 years ago. These four-legged intruders caused a rapid decline in the numbers of birds—and of the unique marine iguanas. A concerted effort to eradicate cats has been successful on some of the islands, with subsequent increases in members of endemic species that live nowhere else. Fortunately, the Galápagos hawk is one of those that is bouncing back due to cat control.
Things didn’t work out so well in the Hawaiian Islands. The black-faced honeycreeper (Po’o-uli) is critically endangered. There are many reasons some birds aren’t doing well in Hawaii; avian malaria, destruction of habitat and killing by predators (including cats) are all serious threats. Of the 33 species of the small, stunning honeycreepers that have existed recently, 12 are now extinct and 9 are critically endangered or probably extinct. Only two honeycreeper species seem safe from extermination.
A recent study of killing by cats divided Felis catus into three categories: those that always stay inside, those pets that have homes but spend time outside, and those that live outside. The latter category includes barn cats whose purpose and livelihood are controlling rodents. It also includes feral cats that have no attachment to humans.
The undomesticated populations are responsible for most, but not all, of the killing. It is estimated that they do away with two to three billion birds each year in the USA, and up to 20 billion small mammals, reptiles and amphibians. They certainly have earned their place on the list of the world’s most destructive exotic species! We know that birds also die from collisions with windows and windmills, but cats destroy a much larger number.
Spying on domestic cats’ habits with cameras attached to their necks suggests that innocent-appearing Fluffy may actually be a serial killer. Indeed, indoor pet cats with outdoor access may kill a half billion birds a year in the USA! The story of one pet is shown at: For more evidence that pet cats might be a closet killers check out “Crittercam” videos at
Animal rights folks focus on protecting cats, but seem blind to the mayhem that cats cause. Feral cats number perhaps 50 million in the USA. Animal rights people have developed two types of programs in an attempt to control them. The Trap-Neuter-Return policy seems most humane because it doesn’t kill these wild creatures. Unfortunately TNR programs allow cats to return to their accustomed lethal pursuits. Euthanizing wild cats is better than TNR since it prevents massacre as well as reproduction.
As loveable as cats are, they can be killers. Devices to make it more difficult for cats to kill wildlife are available; locally, For the Birds carries a couple. It is best to keep cats inside for wildlife’s sake!
© Richard Grossman MD, 2013

By Richard

I am a retired obstetrician-gynecologist who has been fortunate to live and work in the wonderful community of Durango, Colorado for 40 years.

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