Tweety Bird landed on his feet more often than the proverbial cat, and always managed to escape the salivating Sylvester slinking after him in the Warner Bros. cartoon.

In real life, the feathery critters aren't so lucky when they meet felines face to face. Ornithologists say America's birds, some of them endangered, are the casualties of a brutal assault waged by hard-nosed criminals like Fifi and Mittens — who need to be stopped in their tracks before it's too late.

But birdie snacks are part of the natural order for Sylvesters in the wild. After all, shouldn't we just let cats be cats?

"I think for someone to ask you to keep your pet inside is unrealistic," said outdoor cat owner Celerina Bernal of California.

The American Bird Conservancy doesn't agree. Pointing to figures showing the common housecat is the No. 2 cause of death for endangered birds in this country, after human developers, the conservancy is cracking down on killer kitties. It's launched its "Cats Indoor Campaign," which urges owners to put Fluffy under house arrest.

"Outdoor cats harm wildlife and are at risk of injury and disease," warns the preservation group in its initiative. "But cats can find plenty of entertainment indoors."

Kitty lovers say they're not about to keep their feline friends inside — where they often literally climb the walls out of boredom — or take their finicky pets out on leashes. Left to roam free outside, cat fanciers say, Socks and Tiger are happier. Plus they get exercise and fresh air.

"Cats are not leash animals. They're not dogs," Bernal said. "Cats are very independent. That's always been their nature."

But it seems the purring pussycats lead Mr. Hyde existences when they're skulking around the neighborhood. Conservancy ornithologists say the popular pets are the second leading cause of death for endangered birds — responsible for 5 million feathered victims a day and 1.8 billion a year. Currently Americans own a total of more than 60 million housecats, as opposed to about 50 million pet dogs, according to veterinarians.

"There are an awful lot of cats out there, and they are killing a lot of birds and wildlife," said Linda Winter of the conservancy.

The problem is compounded, preservationists say, because of all the birds that die thanks to suburban sprawl and habitat destruction. And the expansion of suburbia also means more housecats.

Alan Hopkins of the Golden Gate Audubon Society works to save endangered birds in San Francisco's Golden Gate Park, where marauding cats threaten the California quail, the white-crowned sparrow, the spotted towhee and the dark-eyed junco.

"The birds just couldn't adapt," said Hopkins. He likened cats' hunter tendencies to the human sport of poaching.

"We throw people in jail for poaching, and they get serious fines," he said.

Chances are slim that such fines would do much to deter Sylvester from relentlessly pursuing a Tweety feast — no matter how much the pudgy yellow bird pleaded with him and managed to escape at Sylvester's expense. Cats are animals of instinct, after all.

"Now wait a minute ... don't lose your temper, Mr. Cat. Please be reasonable," Tweety would say to his furry, black-and-white predator, just before Sylvester pounced.

One thing is certain in this battle between species: If feline owners are hit with new leashing laws and fines for Whiskers' outdoor escapades, the fur — and feathers — are sure to fly.

Fox News' Elka Worner, Jon Du Pre and Jon Brady contributed to this report.