In August while climbing Mount Lassen (Lassen Volcanic National Park CA) Leonard and I were surrounded by literally thousands of California tortoiseshells (Nymphalis californica). The butterflies were hitting us, landing on us, covering the snow and talus and filling the sky. What a magical experience!! For a couple weeks in late summer millions of California tortoiseshells migrate over Mount Lassen. Leonard and I were fortunate to accidentally have chosen a day to summit when the spectacle was occurring.
I could not find any definitive explanation for this phenomenon. However, California tortoiseshells that overwinter as adults in California’s Central Valley and Inner Coast Range have an early generation in place. This early generation seasonally moves north and east upslope to the higher elevations of the Klamaths, Sierra Nevada and Cascades. There they have another generation whose larvae (caterpillars) feast on the abundant Ceanothus species. In the late summer and early fall this generation returns downslope to their winter home. Depending on weather conditions and food supply, the returning tortoiseshells are often part of a population explosion. On their return the butterflies ride warm currents up and over the young volcanic crater. I like this reason best.
Common ravens (Corvus corax) are the most frequent predators on the California tortoiseshells migrating over Lassen Peak. Leonard and I saw many of these corvids noisily cawing while feasting on the butterflies that landed on the snow and rocks.
Many of the migrating butterflies displayed tattered wings and looked worse for wear – such as this specimen. I am not posting any other photographs because I just could not adequately capture the numbers of tortoiseshells surrounding us. The pictures are black blurs against the background.
More information on California tortoiseshell butterflies can be found in my previous post: “California Tortoiseshell” on 07-29-16.
Leonard and I were so happy to see this spectacle.