The news that free-roaming cats are responsible for killing up to 3.7 billion birds and 20.7 billion mammals every year — a death toll that makes cats a greater threat to wildlife than nearly any other human-linked cause — is enough to make any cat lover cringe.

You might think the solution lies in simply keeping Mittens inside, and that is certainly an important step toward protecting not only wildlife but also cats.

But a new Smithsonian study indicates that “owned” cats account for the deaths of only about 29 percent of the birds and 11 percent of the mammals killed by cats.

The rest are killed by the estimated 80 million stray and feral cats eking out an existence in our nation’s alleys and barns.

Eighty million — let that number sink in for a moment. That’s nearly 6 million more than the estimated 74.1 million cats living in homes — and that’s not even counting the unwanted cats turned over to animal shelters every year, who also number in the millions.

How did we end up with so many homeless cats?

A female cat can come into heat at as young as 4 months and can have as many as three litters of six kittens every year.

In seven years, one female cat and her offspring can produce a staggering 370,000 cats.

We have a bizarre double standard when it comes to cats and dogs. Someone who would never dream of letting a dog roam the neighborhood thinks nothing of allowing a cat — altered or otherwise — to do so.

Similarly, people often move away and, rather than taking their unwanted cat to an animal shelter, simply leave her sitting on the doorstep. “It’s OK,” they assure themselves. “She’ll catch mice.”

Even animal shelters have a double standard.

Some backwater shelters make no accommodations for cats at all.

One shelter in rural North Carolina was simply turning cats loose in the nearby woods until PETA built a structure to house the cats.

Some “no-kill” shelters refuse to accept cats because they are harder to place than dogs.

In Easton, Pa., the homeless cat population exploded after the local shelter became “no-kill” and was perpetually too full to accept strays.

One shelter in Ohio has the following message posted on its Petfinder.com page: “Due to the overpopulation of cats currently at our shelter, we are not taking any cats or kittens at this time.”

A shelter in California enacted a new policy last year to refuse to accept “healthy” feral cats and even justifies this by saying that such cats “help control the rodent population,” although it makes no mention of the billions of songbirds that feral cats also “control.”

Can you imagine a reputable shelter having a similar policy for stray dogs?

Those who promote feral-cat trap-neuter-return programs have a similar double standard.

If we wouldn’t encourage people to abandon their own cats in a parking lot or at the end of a country road, how can we say that this approach is acceptable for any cat?

Feral cats do not die of old age. They are attacked by other animals, are hit by cars or succumb to exposure, starvation, parasite infestations or deadly infectious diseases.

PETA receives calls every single day about free-roaming cats who are shot, drowned, poisoned, beaten, set on fire or subjected to other horrors.

If we want to get serious about protecting wildlife — and cats — we need to change the way we view cats. We need to start thinking of them as our best friends, our beloved companions, our lifelong responsibility.

Like dogs, they should be licensed, included in “leash laws” (i.e., required to be kept indoors unless accompanied) and, most importantly, spayed or neutered.

This is the only way we will ever start to put a dent in the staggering homeless cat problem — and allow America’s wildlife to breathe a sigh of relief.

ALISA MULLINS 

People for the Ethical Treatment of Animals