SHERMAN OAKS—An unusually large butterfly migration is headed to Sherman Oaks, Woodland Hills, and the Santa Monica Mountains from the deserts of Mexico.

According to the Director of Butterfly Farms, Tom Merriman, roughly 1 billion common butterflies called Painted Ladies are flying at 20 mph in a northerly direction from Mexico to reach their breeding grounds in Oregon. Butterfly Farms is a non-profit organization whose mission is to restore the health of butterflies as pollinators.

“This kind of large migration is unusual,” Merriman said in an interview with Pasadena Star News. “They’ve laid tons of eggs in the desert, and so there may be over a billion butterflies.” Merriman believe the massive butterfly migration may continue for a month or until the butterflies exhaust themselves. “Let them go. They are determined to go where they want to go. They want to go north, and they are moving pretty quick,” Merriman told the Pasadena Star News.

Experts like Arthur M. Shapiro, who is a professor at the University of California Davis said in an interview with NBC that years with an abundance of wildflower blooms yields many butterflies. “Years of tremendous wildflower blooms typically are really big Painted Lady years,” Shapiro told NBC.

There was a similar massive butterfly migration back in 2005. Painted Ladies are often mistaken for their closely related cousin, the Monarch butterfly. There is a distinct difference between the two species of butterfly. Pink Lady Butterflies are described as having a wingspan of 2 to 2 x 7/8 inches by the time they reach adulthood. This would make them smaller than the Monarch who has a wingspan of 3 to 4 inches.

They can be detected by their orange-brown, black and white patches. While the two butterflies have the same colors, the wings on a Painted Lady are not clearly veined like that of the Monarch. Painted Ladies reside and thrive in most environments, but prefer open undisturbed areas.