The tortoiseshell butterfly migration passing through West Marin over the last three weeks has been hard to miss. Butterflies ended up in bedrooms, trees and lunchboxes, and splattered on windows. “Although these butterflies migrate seasonally every year, their course is extremely variable and hard to predict,” said Arthur Shapiro, a University of California, Davis butterfly expert. “Their numbers can be in the hundreds of millions, but vary by five to six orders of magnitude. Every few years they’ll come plowing into cars on the highway, leaving a pad of yellow fat on the windshield.” The adults of the first generation of the year emerge in late May and generally migrate north-northeast and upslope. They lay eggs on the young sprouts of their host plants—wild lilacs—and this new generation continues to higher elevations, where they find new wild lilac growth to hold their eggs. The next adults emerge in late July and continue to migrate north by way of an internal sun compass that allows them to maintain a continuous direction throughout the day. The same individuals never migrate both north and south, and their exact orientation mechanism remains a mystery. The butterflies are generally found at Mendocino Pass, mid-elevation areas of the Sierra Nevada or Idaho, although there is no known reason for this pattern. The individuals seen migrating through West Marin in recent weeks are thought to be headed for the Trinity Alps, and their volume is unprecedented, according to Point Reyes National Seashore wildlife ecologist Dave Press, who has worked in the field for 21 years. “I can’t say that I’ve ever seen these numbers before. It’s a pretty unique event,” he said. Beginning in August, the new generation of butterflies begin to migrate south toward the highest elevations. They assemble on rocky summits above the tree line and estivate—a summer version of hibernation—to conserve energy, living off the fat from their caterpillar phase. As the days get shorter and nights get colder, they will begin to weave between mountains and migrate downslope to where they hibernate. They undertake this phase of travel individually, unlike the spring and summer migrations. As long as they remain, residents have an added incentive to drive cautiously. “It makes me feel guilty just to drive,” Inverness resident Sam Hinckley said at the peak of their presence. “I don’t want to run them over.”