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…