If you think you’ve noticed a recent upsurge in orange- and black-patterned insects around Palo Alto, it’s all right, you’re not just seeing things. A large migration of Painted Lady butterflies, scientifically known as Vanessa cardui hit parts of Northern California in March and April.

The Painted Lady butterfly is about one to two inches wide, and its wings are orange with a black lace-like pattern. The species resembles a smaller version of the Monarch butterfly, whose annual migration from west of the Rockies to Southern California is much more commonly known.

The migration occurs every year; butterfly eggs hatch in Southern California and northern Mexico and travel up the California coastline throughout March into early April. The butterflies move through Oregon, Washington and even up to British Columbia, Canada, sometimes flying for three days straight. Millions of painted lady butterflies search for food on this journey, in addition to mating and laying eggs.

The largest migration in recent years occurred in 2005, when dense throngs of the insect were reported to have been impeding traffic in the Californian deserts. In that massive movement, people in Sacramento counted three butterfly sightings per second.

According to a Web site dedicated specifically to Painted Ladies by Art Shapiro, a University of California, Davis professor of evolution and ecology, winter rains in Southern California have led to the increase in migration population this year. In waves, these butterflies are known to travel over buildings, forests and even thousand-feet mountain ranges, instead of around them. Local butterfly projects have counted rates from one butterfly per 10 seconds up to one butterfly per second.

“To witness a mass migration of this level it is truly spectacular,” Ryan Phillips, a biologist at De Anza College, said in the San Jose Mercury News. He counted over 1000 butterflies near Mount Hamilton in San Jose. “Butterflies were literally stacked on top of each other.”

The Bay Area is a mere stop along the way in the border-crossing journey, but residents across the Peninsula have reported sightings of flurries of butterflies in parks and backyards. The butterflies travel in a straight-line route, not stopping until they’ve burned through the “yellow fat” reserve, according to Shapiro. Shapiro documented the first wave through Northern California on March 15, with a more intense increase around April 3. Individual butterflies then began dropping out of the migration in order to to feed on flowers and mate.

Senior Kelsey Teramoto got up close and personal with the of butterflies during a recent trip to Los Angeles over spring break. “As we were driving down Highway 5, they just splattered on our car,” she said. “It sounds like a rock is hitting you, but it’s really a butterfly and you see these big yellow splotches on your window.”

To take a look for yourself, visit denser locations in Palo Alto, including the area between Highway 101 and El Camino Real, Edgewood Plaza Shopping Center and Foothill Park. The northbound migration is set to end by mid-April, but the new painted lady butterfly population will continue the cycle of life as it returns south this August.