Thousands of painted lady butterflies are passing through Palo Alto and surrounding communities this month.
The mass of butterflies, which can be spotted at a rate of one every 10 seconds or more, typically migrate north in waves from desert areas along the U.S.-Mexico border. They can be seen in the early spring weeks through March, according to a website by scientists at University of California, Davis.
The butterflies, known by the scientific name Vanessa cardui, are about 1 1/2 inches to 2 1/4 inches wide. They are orange with a black, lacy pattern on the wings and a series of "eye spots" on the lower-lobe wings.
Northward-migrating painted ladies can travel the distance from Bishop to Davis in three days, surviving on yellow-fat reserves in their bodies, according to the Davis painted lady webpage.
The butterflies do not stop to feed or mate until they have burned up their reserves, carried over from the caterpillar stage. They fly in a straight line from southeast to northwet, like "bats out of Hell," and go over obstacles rather than trying to go around them, according to Davis researchers.
In 2005, the population was so dense that their numbers impeded traffic in desert regions, researchers said.
Recent sightings in Palo Alto have included a corridor between U.S. Highway 101 and El Camino Real and concentrations were seen around Edgewood Plaza Shopping Center.
Beginning in August, the movement reverses and butterflies head south toward the desert wintering grounds, according to the U.C. Davis website.