Last Saturday evening my wife and I took our terminally ill cat to an animal hospital, where a veterinarian put him peacefully to sleep as he sat on my lap. I wasn’t really a cat lover when we adopted him seven years ago, but this unusually affectionate and communicative kitty cat converted me. I’m surprised how much I’m grieving for the loss of him. Years before Toby came into my life I wrote a story for newspapers about domestic felines as deadly hunters of migratory songbirds. Several bird species, such as the Cerulean Warbler, are becoming vulnerable to extinction as their tropical-forest habitat disappears. A comparable threat on the other end of their migration is the predatory nature of pet cats, which by scientists’ estimates kill hundreds of millions of small animals every year. So how do I square my concern about animal-killing cats with the affection I feel for one late individual of the species? Shouldn’t I be blaming cats for killing songbirds and threatening the survival of species not only of birds but of small mammals, amphibians, and reptiles? Of course not. Cats do what evolution has programmed them to do. What has gone awry is not cats’ wants, which are natural, but their numbers, which are not. Nature is usually balanced in ways that make extinction a rare event—unless mortality levels reach levels that tilt the balance dangerously. That’s what has happened with pet cats. The United States alone is home to some 90 million. Most of them spend some time outdoors, and many of them kill. There never could be anywhere near this many domestic cats, obviously, if there weren’t even more human beings to care for them, just as my family did ours. It’s a pint-sized example of a point I make in More: Population, Nature, and What Women Want. Environmental unsustainability tends to have much more to do with scale than with any essential aspect of our behavior. Cats aren’t bad because they kill birds; they’re just cats. But there are so many of them, and with cats, just as with humans, numbers matter. (In an endnote to Chapter 10, I note geographer Vaclav Smil’s estimate that livestock weigh 20 times as much as all the planet’s wild animals. Unfortunately, I couldn’t find a similar comparison between companion and wild animals.) Since no one would seek a sudden reduction in the population of people—or of their pets—we often focus on modifying individual behavior to reduce environmental risks. In this case, the most important step the world’s hundreds of millions of cat owners can take to protect small animals is to keep their cats indoors. Fortunately for me, Toby had no interest in wandering outside, so he never killed anything bigger than the occasional bug that crawled past him on the floor. Our house was his whole world, which is why it feels so empty and sad as I write this post at home.