Maybe you were walking along Huntington to grab a quick swig of boba at the Santa Anita mall. Or perhaps you might’ve been sitting in the front seat of your car, boredly staring through the glass of the passenger window. But no matter where you were, what you probably saw while gazing up into the sky were butterflies— hundreds of them furiously flapping their delicate wings as if they were about to miss their next train. Those orange-and-black speckled insects are called painted ladies, and they seem to be migrating by the millions across California. But why is this happening in the first place, and why so suddenly?

It’s common to find painted ladies setting off from their wintering ground in the Mojave and Colorado deserts in southeastern California up north as spring rolls around the corner. They typically travel along the route every year, flying northwest to Sacramento, then to Oregon, Washington, and even as far as Alaska. But what’s unusual about the migration this year is the sheer number of painted ladies. Scientists say that the number of these butterflies hasn’t climbed this high since 2005 when the population of painted ladies was at about one billion. What makes this explosion in butterfly population even stranger is how California has recently been in a decline of butterfly species for decades. According to a monarch count conducted by Xerces Society for Invertebrate Conservation found that in November, only 28,429 painted ladies were making their journey up the California coast. Compared to the previous year, this figure represents an 85% decline in butterfly population and a staggering 99.4% plunge compared to the numbers from 40 years ago. Art Shapiro, an ecologist at UC Davis who has been tracking butterflies in the state for nearly 50 years, said that “It was a terrible—perhaps even catastrophic—butterfly year at all elevations and no, we don’t know why.” At least 20 species, including the large marble butterfly, the field crescent, the west coast lady, the common sooty wing, and the California ringlet, have been disappearing faster than the monarch. There isn’t a clear reason for their disappearance, but researchers have speculated that it is due to a general loss of open space, climate change, as well as changing agricultural practices. As a result, there are fewer flowers and leaves for food for butterflies to eat.

However, scientists say that the reason for this year’s spike in population can be summed up in a single word: rain. More specifically, rain in deserts. Shapiro explained, “The more plants, the more butterflies… so any year you have a real big bloom in the desert is potentially a big year for painted ladies.” Once these butterflies retreat to California deserts for the winter, they wait for rain in order to make plants germinate, causing them to “breed like crazy”. Butterflies will lay their eggs on plants so once they hatch, their offspring will have something to eat. The abundance in rain this year allowed for more vegetation to sprout up in deserts, giving painted lady caterpillars more food to munch on. As a result, more caterpillars than usual were able to survive into adulthood.

According to Shapiro, we won’t be seeing these butterflies for long—only for another week or two— as they depart for new adventures up north. But this may not be the last we see of these butterflies this year thanks to this winter’s extreme abundance of rainfall.

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