For the first time in decades, unusually large numbers of California tortoiseshell butterflies are appearing in the Sierra Nevada, and that could mean that millions of the windshield-smearing insects will slow motor traffic through mountain passes this summer. Art Shapiro, a UC Davis ecologist and butterfly expert, has been studying seasonal movements of butterflies in Northern California for 28 years. His research has shown that population peaks and valleys are normal -- and the region is overdue for a tortoiseshell outbreak. The California tortoiseshell (Nymphalis californica) is about half the size of a monarch butterfly. Like the monarch, it is orange and black on its upper surface; unlike the monarch, it is wood-brown below. Most tortoiseshell butterflies migrate in spring from California's Coast Range and Sierra foothills, where they overwinter, to breeding grounds in the Sierra. By mid- to late July, grandchildren of the winter adults are emerging from pupae at around 7,000 feet and heading for the very high country, above the tree line. "In an outbreak year, millions of these butterflies will head for the mountaintops at once," Shapiro said. "The last great outbreak in the northern Sierra and southern Cascades was in the early 1970s, and it made travel difficult across the Sierra passes and on Interstate 5 near Mount Shasta City and Weed. "Motorists had to stop to scrape the smears of yellow butterfly fat off their windshields and birds hopped along the roadsides, feasting on piles of dead butterflies several inches deep. Based on the high number of butterflies we saw over the Memorial Day weekend, that might happen again this summer." After such an outbreak, the butterfly numbers typically dwindle in a week or so, Shapiro said. Ironically, Shapiro reported in December that 1999 was the worst in 28 years for many urban, suburban and agricultural butterfly populations that spend their lives in the Sacramento Valley and nearby foothills. It's not known why it's turning out to be such a good year for tortoiseshell butterflies in particular, Shapiro said. "We really don't know where these butterflies came from," he said. Editor's note: Two color photographs of a California tortoiseshell butterfly -- one with open wings, one with closed wings -- are available by e-mail; write to aerafferty@ucdavis.edu.