02/16/2013 12:07 pm ET Updated Apr 18, 2013

Domestic Cats and Billions of Birds

The old adage about taking the cat out of the wild, but not the wild out of the cat now has a number associated with it. That number is 2.4 billion.

That staggering number is the median estimate of birds in the United States killed each year by domestic cats. Fluffy, Ginger, Tabitha, etc. Many thousands of millions of birds because we put feral cats in our woodlands (the majority of the kills) and allow our house cats out of the house. These numbers are revealed in a new study by Smithsonian and Fish and Wildlife Researchers, which in turn is built upon more than a score of close studies.

Their results modeled a range of 1.4 to 3.7 billion kills of birds by cats, and, perhaps more incredibly, kills of 6.9 to 20.7 billion kills of small mammals by cats in our country each year. Billions! This study makes clear that cats are, by far and away, the most single important cause of wildlife loss by an anthropogenic (human-linked) cause in the nation. Birds killed by cars, by windows, by lighted towers, by wind energy? Merely millions. Cats? Billions.

I am a conservation biologist and have been part of diverse studies of other human-linked effects on wildlife through the years. While the concerns in those studies are usually about specific species and regional threats, those numbers (if estimated) usually estimate dozens, or scores, or hundreds, and maybe sometimes thousands of birds lost to an issue. Our and other recent success in working with the Department of Interior in protecting key Arctic Alaska habitats from development meant the protection of millions of nesting birds and hundreds of thousands of caribou.

Only millions and thousands. Not billions. Passenger pigeons were considered the most abundant bird on earth, and were thought to number between 3 and 5 billion birds at the time of European settlement in North America. The extinction of the Passenger Pigeon is considered one of the most dramatic extinctions due to humans ever. Our cats, particularly the feral cats we have directly or indirectly put out to roam, kill the equivalent of passenger pigeon peak abundance on an annual basis.

Most everyone would agree that keeping house cats (estimated to be some 29 percent of the kills -- say "only" some 700 million birds worth) in the house and confronting the bigger problem of feral cats is needed. If you love cats and care about nature, there are billions of reasons to keep your cats out of the wild.