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

biodiversity conservation biology

Galápagos Hawk

Galápagos Hawk


“… the introduction of any new beast of prey must cause [extinction] in a country, before the instincts of the indigenous inhabitants have become adopted to the stranger’s craft or power.”

Charles Darwin


We had just seen a Galápagos hawk soaring overhead when we heard an incongruous sound; here were cats mewing beside the path. I caught a glimpse of a tabby kitten with the bluest eyes.

“I like cats” the guide said “but I’ll have to report these to the rangers. They will come here with dogs.”

We were returning from hiking on the tortured volcanic surface of one of the world’s largest active calderas, Cerro Negro, on Isla Isabela. We walked carefully along the border of the volcano’s crater; the path was narrow and muddy with a fatal drop-off. Fortunately there was a slender border of green between the sheer wall and us. This is where these feline invaders lived.

Cats are an exotic species in the Galápagos Islands, maybe introduced to catch the rats that were also introduced accidentally by man. Sometimes biocontrol (using one species to control another) works out well, but other times it is a disaster. Cats in the Galápagos are a disaster. Darwin recognized this danger almost two centuries ago.

Galápagos hawks only live on that isolated group of islands; they are endemic, meaning they are found nowhere else in the world. They nest in low trees and in the hollows of walls of the volcanic rock of which the islands are made. Formerly the hawks had no natural enemies but now their eggs, nestlings and fledglings are easy prey for cats.

A census of Galápagos hawks taken late in the 20th century estimated fewer than 1000 individuals. These magnificent birds have been eradicated from much of their former habitat so now this species is found on only four of the islands. The Red List of Threatened Species states that these hawks are vulnerable to extinction because of introduced predators and their small population.

Of the six ways that mankind sends other species to extinction, perhaps introduction of exotic plants and animals is the second most common.

Exotics may not seem so bad since our homes and gardens are filled with beautiful plants from other lands. Unfortunately they can cause problems in at least three ways. Exotics may outcompete natives. They often don’t have local pests to help control them, as is the case with cats in the Galápagos. Pollinators often find exotic plants unattractive so pollinators become scarce, but they are essential to native species.

Ecuadorians are doing a fine job of dealing with exotic species. We paid $100 for a permit to visit the Galápagos; that money helps fund the rangers and other efforts to protect the unique environment. Our small group of “adventure tourists” also was given an orientation urging us to respect this amazing place with its many endemic plants and animals.

Destruction of habitat and climate change are two other ways that humans are causing the mass extinction of species. As the numbers of humans has grown, and as we “improve” the landscape with more agriculture and construction, there is simply less space for other species. Not only do we use more and more of the land, but also we break up what there is so it is of less value to critters.

Climate change is also wreaking havoc with many plants and animals. Species that need cold, or whose pests are controlled by cold, are susceptible to our climate’s warming. A local example is the little pikas that live high in the mountains. They are happiest with cold winters and lots of snow. They can move up in altitude as the world warms, but our mountains only go so high. If the world gets too hot they will all “go to heaven”. Another example is the spruce bark beetle, which has killed so many of our spruce trees. Sub-zero temperatures and adequate moisture have controlled this pest in the past, but the beetles are thriving with climate change and drought.

There is hope for endangered species, at least in the Galápagos. Efforts to protect their endemic hawk have apparently been successful because I have heard that there are now almost 2000 members. There are even plans to reintroduce them to islands where they had been driven to extinction. I feel good that our small contributions paid for by the permits to enter the islands have been effective in helping to preserve one of nature’s many wonders.

© Richard Grossman MD, 2013