Wild kittens that will number in the tens of millions this year are starting to be born, and overtaxed shelters will be forced to euthanize most of the millions they receive. It is a grim reality a leading advocate calls "one of the last major problems" plaguing the animal welfare movement.

Scads of good-intentioned people who discover wild litters of baby cats will take them to shelters, which are overrun with the animals. The facilities turn to euthanasia when their limited resources are stretched even thinner by the massive influx of kittens and the babies' required round-the-clock care. But groups that trap, neuter and release feral cats and shelters that are able to open 24-hour kitten nurseries are doing their part to stem the deaths.

"The problem of community cats dying in shelters is one of the last major problems we in the animal welfare movement are tackling," said Gregory Castle, CEO of Best Friends Animal Society, a leader in the no-kill movement that runs the largest animal sanctuary in the country.

He says "there's a ways to go" but has seen a dramatic drop in deaths whenever his group connects a shelter with a local organization that traps, neuters and releases free-roaming cats. Wild kittens socialized early enough can be great pets, his group says.

A staggering 40 million feral kittens will be born throughout the country this year, but 20 million of them will die at birth, said Becky Robinson, president of Bethesda, Md.-based Alley Cat Allies, which promotes trap, neuter and release and is the country's only cat advocacy group. Of those who survive, millions will be taken to shelters, where the majority will be euthanized. The explosive reproduction isn't tied to domestic cats because studies show 80 percent are sterilized, she said.

Kittens, which can't see, hear or do much else on their own for the first week, are difficult to care for because they need to be bottle-fed every two hours, are susceptible to disease until they can be vaccinated and need a place to stay until they're old enough to be spayed or neutered and put up for adoption.

That's why some shelters have opened volunteer- and donation-run nurseries. Thousands of feral kittens are saved at 24-hour facilities in California, Indiana, New Jersey, Texas, Illinois and other states. But the trend, started just a few years ago, needs time to expand to make a dent.

Nurseries need to work with neuter-and-release groups, animal control workers and shelters with aggressive adoption programs to reverse the massive numbers of feral cats, said Robinson and Janice Dankert, community cat program supervisor at Best Friends' headquarters in Kanab, Utah.

It's rare that a wild adult cat can be socialized enough to be adopted, but feral kittens trained before 3 months old make great pets, Dankert said.

Best Friends opened a 100-kitten nursery at the no-kill shelter it runs for Los Angeles Animal Services, and the nursery is full, said Marc Peralta, executive director of the group's Los Angeles chapter. People can't take kittens directly to the nursery — Peralta's staff picks them up from the city's six shelters.

Best Friends and 69 other groups joined forces two years ago to help the Los Angeles shelters end euthanasia. Over that time frame, the number of healthy, adoptable pets being euthanized has dropped from 17,400 to 9,075.

Feral kittens are the biggest obstacles left. Of 9,075 healthy dogs and cats killed in 2013 in Los Angeles, 5,200, or 57 percent, were unweaned kittens. The same is true in varying numbers at shelters that euthanize across the country.

About 72 percent of all cats — neonatal, feral or pets — are killed in shelters, Robinson said.

Last year, the LA nursery was able to take in 1,800 feral kittens, but it had to leave 6,200 behind at the city's shelters, Peralta said.

At the nursery, kittens range from a day old to 4 weeks old. Shelter staff and volunteers try to find foster homes quickly for the youngest because they aren't old enough to vaccinate, so are more susceptible to diseases, Peralta said.

Once a kitten weighs 2 pounds — around 2 months of age — it can be spayed or neutered. That's the most adoptable time for a kitten and when many of the shelter's partners will launch aggressive adoption campaigns.

The LA nursery is always accepting volunteers, Peralta said, because it has 100 mouths to feed every two hours.

Sarita Carden, 59, of Los Angeles, volunteers two days a week, feeding kittens and socializing shy adult cats.

"It's a great feeling, making a difference and knowing the kittens would have had zero chance if it weren't for the nursery," Carden said. "It takes time, it's hard work, it can be really messy, and it can be heartbreaking."

But the joy is overwhelming, she said.