Butterflies are returning to Northern California after what a UC Davis entomologist calls "the worst spring on record." It has taken three months for butterfly populations at low elevations to bounce back to normal levels, says Arthur Shapiro, a professor of evolution and ecology who has studied butterflies for decades. Early spring butterfly counts of common Sacramento and San Joaquin Valley species dropped to as low as 5 percent of average. Flooding and cloudy, humid weather caused mass mortality among butterflies, Shapiro says, but probably also hit their predators and parasites even harder. Most of the species breed rapidly, producing a generation in about a month in warm weather, allowing their numbers to build up rapidly, he says. As for higher-elevation butterflies that typically thrive in wet years, the near-record snowpack has made many areas of Sierra Nevada and northern mountains inaccessible to Shapiro. In a recent visit to Sierra Valley north of Truckee, however, he found an "explosion" of butterflies. Evidence from 24 years of tracking butterfly populations suggests that, in the case of long-term climate changes, butterflies will follow their preferred habitats rather than staying in the same place and adapting, he says.