It’s almost that magical time of year that the Humane Society of America likens to a “natural disaster.” Kitten season.

“The level of emotions for months on end is so draining,” said Ann Dunn, director of Oakland Animal Services, a city-run shelter in the San Francisco Bay Area. “And every year we just know it’s going to get harder.”

Across the United States, summer is the height of “kitten season,” typically defined as the warm-weather months between spring and fall during which a cat becomes most fertile. For over a decade, animal shelters across the country have noted kitten season starting earlier and lasting longer. Some experts say the effects of climate change, such as milder winters and an earlier start to spring, may be to blame for the uptick in feline birth rates.

This past February, Dunn’s shelter held a clinic for spaying and neutering outdoor cats. Although kitten season in Northern California doesn’t typically kick off until May, organizers found that over half of the female cats were already pregnant. “It’s terrifying,” Dunn said. “It just keeps getting earlier and going later.”

Cats reproduce when females begin estrus, more commonly known as “going into heat,” during which hormones and behavior changes signal she’s ready to mate. Cats can go into heat several times a year, with each cycle lasting up to two weeks. But births typically go up between the months of April and October. While it’s well established that lengthening daylight triggers a cat’s estrus, the effect of rising temperatures on kitten season isn’t yet understood. 

One theory is that milder winters may mean cats have the resources to begin mating sooner. “No animal is going to breed unless they can survive,” said Christopher Lepczyk, an ecologist at Auburn University and prominent researcher of free-ranging cats. Outdoor cats’ food supply may also be increasing, as some prey, such as small rodents, may have population booms in warmer weather themselves. Kittens may also be more likely to survive as winters become less harsh. “I would argue that temperature really matters,” he said.

Others, like Peter J. Wolf, a senior strategist at the Best Friends Animal Society, think the increase comes down to visibility rather than anything biological. As the weather warms, Wolf said people may be getting out more and noticing kittens earlier in the year than before. Then they bring them into shelters, resulting in rescue groups feeling like kitten season is starting earlier.

Regardless of the exact mechanism, having a large number of feral cats around means trouble for more than just animal shelters. Cats are apex predators that can wreak havoc on local biodiversity. Research shows that outdoor cats on islands have already caused or contributed to the extinction of an estimated 33 species. Wild cats pose an outsized threat to birds, which make up half their diet. On Hawaiʻi, known as a bird extinction capital of the world, cats are the most devastating predators of wildlife. “We know that cats are an invasive, environmental threat,” said Lepczyk, who has published papers proposing management policies for outdoor cats.

Scientists, conservationists, and cat advocates all agree unchecked outdoor cat populations are a problem, but they remain deeply divided on solutions. While some conservationists propose the targeted killing of cats, known as culling, cat populations have been observed to bounce back quickly, and a single female cat and her offspring can produce at least 100 descendants, if not thousands, in just seven years. 

Although sterilization protocols such as “trap, neuter, and release” are favored by many cat rescue organizations, Lepczyk said it’s almost impossible to do it effectively, in part because of how freely the animals roam and how quickly they procreate. Without homes or sanctuaries after sterilization, returning cats outside means they may have a low quality of life, spread disease, and continue to harm wildlife. “No matter what technique you use, if you don’t stop the flow of new cats into the landscape, it’s not gonna matter,” said Lepczyk. 

Rescue shelters, already under strain from resource and veterinary shortages, are scrambling to confront their new reality. While some release materials to help the community identify when outdoor kittens need intervention, others focus on recruiting for foster volunteer programs, which become essential caring for kittens who need around-the-clock-care.

“As the population continues to explode, how do we address all these little lives that need our help?” Dunn said. “We’re giving this everything we have.”