You're likely to hear them before you see them.

They're alone and afraid. No mom in sight.

The sound of kittens meowing.

These kittens were placed in two boxes and left on the side of a service road. Tracy Parisi

With springtime just around the corner comes the warmer weather, the flowers, the showers — and, unfortunately, litters of unwanted kittens.

It's the time of year when unspayed cats give birth. Rescue groups and shelters nationwide become overrun with litters of unwanted kittens who are often euthanized due to lack of space and not nearly enough people to foster and adopt them.

"We are embarking on kitten season," Eric Brown, cofounder and vice president of Arizona's Homeless Animals Rescue Team (H.A.R.T.), told The Dodo. "The biological clocks of cats have realized that it has started warming up, and cats are now in heat or already pregnant. There is a 64-day gestation period. And we are almost there. There will be throngs of kittens any day."

These 13 kittens were placed in two boxes and left on the side of a service road.Tracy Parisi

So what should you do if you find a litter of kittens? For starters, it depends on how and where you found them, whether their mother is around, and how old they are.

Approximately 3.4 million cats enter shelters nationwide every year and 1.4 million cats are euthanized yearly.

The 13 kittens found on the side of a service road were rescued.Tracy Parisi

H.A.R.T.'s mission is a method of population control called trap, neuter and return (TNR). This means feral cats are humanely trapped, sent to a veterinary clinic, and spayed or neutered. Each cat then has a tiny part of his ear removed while under anesthesia. Known as "tipping," this helps identify the cat as part of a managed colony. The cats are then returned to the community where they were found.

Stray or feral?

"When you think of your pet cat who is inside and the cat you see outside, you might think they are a different species, but they are still part of the domestic cat species," Kayla Christiano, campaigns manager for Alley Cat Allies in Bethesda, Maryland, told The Dodo.

Humane traps are used to capture cats for TNR.H.A.R.T.

Feral cats are not socialized and won't make eye contact, and strays could be abandoned or lost but may make eye contact.

Kittens in a box

It can be very dangerous for a litter of kittens to be on their own, unless their mom is out for a couple hours with the intent to return.

If you find kittens in a box, someone most likely dumped them there. If the kittens are found behind a bush, it's best to closely monitor them, and see if the mother returns.

"Every kitten needs to be fed every three hours at the max," Brown said. "Mom must return every three hours to feed her babies. Like clockwork, her body tells her to do so. If momma doesn't return within that time frame something is wrong, and humans must intervene. She could be dead, injured or trapped."

There's also the chance she abandoned them.

The safest place for kittens is with their mothers. "If you don't see the mom, there's a couple different ways to tell if mom is around," said Christiano. If the kittens are clean and quiet, the mother is most likely coming back. However, if they are dirty and crying, the mother may not return. At that point, Christiano recommends taking them.

These kittens were humanely trapped.H.A.R.T.

"Alley Cat Allies does not recommend taking neonatal kittens [under four weeks] to animal shelters," she said. "Most shelters and shelter employees are not equipped to provide round-the-clock care for these babies. They need care every two hours.

"More than 70 percent of cats entering shelters are killed there," she said. "And that number rises to 100 percent when dealing with feral cats."

Although public shelters aren't the ideal place to bring kittens, they can be a resource for support. Christiano suggested reaching out to ask if they know of any fosters who can help. You can also ask small, local no-kill rescues.

Young kittens are very vulnerable and need round-the-clock care.H.A.R.T.

When dealing with kittens, especially neonatal kittens, remember that they are starting their lives, so they can go either way: feral or socialized. "If with mom and they are outside, they can be part of a TNR program. If not with mom, bring them in, and they can be adopted."

Taking in the young kittens

If the kittens were left in a box, take them. If there is no box and you need to step in because the mother doesn't return, find a box, gather up the kittens, and place them inside the box.

The one thing you should not do is nothing. If left to fend for themselves, the kittens will die.

First determine approximate age. One way to tell if kittens are under three weeks old is by eye color. According to Brown, all babies are born with blue eyes. If the eyes are another color, the cats are older than three weeks. "Older kittens are much more mobile, and have all their teeth and claws," he said.

You must be committed to the task if you bottle-feed kittens.H.A.R.T.

You must keep them safe, even in a bathroom if it's for a short duration, according to Brown. Make sure to keep a litter box nearby.

Next, assess whether they need to be bottle-fed, which can be a big undertaking. Reach out to rescues or your local humane society if you need guidance. If you take on the task, be committed. "You have to use goat's milk or kitten replacement milk, and bottle-feed every three hours," Brown said. "You have to stimulate them, and take a wet washcloth and stimulate their bottoms and their genitals in order for them to eliminate."

According to Christiano, at four to five weeks you can wean them onto a little wet food, and mix that in with kitten formula. "They will need a little bottle or syringe, some heat sources, Snuggle Safe [a microwavable heating pad for pets], a larger carrier to retain heat, some bedding, and we recommend having a food scale on hand. It's important to weigh kittens daily."

Whatever you do, keep the kittens away from cow's milk. "Many cats, nine of 10, are lactose intolerant," Brown said. "Cow's milk causes indigestion and potentially death because it can cause diarrhea, which causes dehydration, which equals death in a cat." Brown also suggested goat's milk as it is the closest thing to cat's milk. "It's totally safe and nutritional."

"The important thing to highlight is that most kittens found outside come from community cats," Christiano said. "If you notice kittens outside make sure you or someone you know is doing TNR with cats to prevent unwanted litters."

Don't turn a blind eye

If you've stepped in, don't turn away. "You have now become part of this rescue as a result of stumbling upon the situation," Brown said. "If you turn a blind eye you cause more of a problem. If you can't do something, alert somebody.

"Keep them safe until you can find a solution," Brown said. "Just make some phone calls."

A kitten who was humanely capturedH.A.R.T.

If you find cats you suspect are in a feral community, Alley Cat Allies has resources to help. Click here to watch webinars on kitten care. From information on bottle-feeding and general care, click here.