Southern California is all aflutter thanks to an annual migration of butterflies known as the painted lady. In recent days, the skies of some areas have been filled with the winged creatures.

People took to social media to document the insects fluttering around California. The butterflies travel from the deserts in Mexico and fly as long as their fat reserves last before breeding. Generations of the insects can reach all the way to the Pacific Northwest.

"We've been waiting for them to get up here, but they haven’t shown up yet," said Arthur M. Shapiro, a professor at the University of California, Davis' Department of Evolution and Ecology, College of Biological Sciences, which is near Sacramento.

"Years of tremendous wildflower blooms typically are really big painted lady years," he said, adding that the last really big one was in 2005 with estimated billions of butterflies.

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Shapiro said he’s gotten reports of the butterflies, Vanessa cardui, in Temecula, north of San Diego, and in Anza-Borrego Desert State Park, as well as in Pasadena and in the Coachella Valley.

The entire North American population of painted lady butterflies migrates to west Texas and northern Mexico during the winter. As caterpillars, they feed on desert annual plants — their favorites are the families of mallows, borages, and thistles and their relatives — and then once butterflies, they begin traveling north.

They can live up to six weeks, but most don’t live that long. There will be waves of migration as the first generation makes it to northern California, they breed and then the next generation makes the trip to the Pacific Northwest, Shapiro said.

On the way back, the next generations of butterflies will begin making the trip south. In Calfornia, the largest number of butterflies are expected on the eastern side of the Sierra Nevada, Shapiro said.

"The south side is never as big as a big northern migration," he said.

A similar phenomenon happens in Europe, with some of the butterflies traveling south across the Sahara in Africa and others to Ethiopia and Eritrea, and then the migration begins north across the Mediterranean.

Painted Lady, Vanessa cardui, with Zinnia flower. Jim Mayes / Getty Images

Painted lady butterflies can travel fast. "They can pace cars at 25 miles per hour," Shapiro said.

And they are good at generating body heat through muscle contractions, which allows them to fly at lower temperatures. The painted ladies have the highest observed altitude record among all butterflies at around 22,000 feet, he said.

The size of the northern migrations are often tied to the blooms of wildflowers in the desert, because heavy rains there contribute to the growth of host plants, Shapiro said. Anza-Borrego Desert State Park said this week that many parts of that park were experiencing great displays of wildflowers.

Only a few types of birds actually catch the butterflies on the wing, with most being eaten if they are cold or flying slowly or in mud puddles on the ground, he said. And numbers are on the butterflies' side. "The birds that do eat butterflies will very quickly get satiated," he said.

"Yesterday, March 11th, was the anniversary for when the front reached Davis in 2005," Shapiro said. "I thought it would be really cute if they arrived the same date this time, but it didn’t happen."