But why are they rebounding? That, experts say, remains unclear.
It could simply be that the butterflies had an especially good breeding season (insects can reproduce rapidly, and their populations do tend to fluctuate), or that especially warm fall weather last year changed the butterflies’ breeding and migration behavior, throwing off the count.
The current numbers, however, are still a far cry from previous population totals: In the 1980s, millions of monarchs flocked to California for the winter. In 2017, an annual count found about 200,000 butterflies. Last year, the same count found fewer than 2,000.
“I was really saddened,” Oberhauser told me, adding that she had worried “we might be seeing the end of an incredible migratory phenomenon.”
But the rebound, she and others say, is cause for cautious optimism. This year, volunteers have already counted more than 100,000 butterflies, according to the Xerces Society for Invertebrate Conservation.
“It’s close to miraculous,” said Paul Meredith, 77, a volunteer with the butterfly sanctuary, who that Sunday was seated — binoculars around his neck, insect pin in his cap — among the trees.
But, he added, “there’s a lot of things we don’t understand.”
To see the magnificent butterflies, visit these groves:
Monarch Butterfly Grove in Pismo Beach (20,000 butterflies, estimated by the Xerces Society)
Monarch Butterfly Sanctuary in Pacific Grove (14,000)
Camino Real Park in Ventura (3,000)
Natural Bridges State Beach in Santa Cruz (2,000)
Fiscalini Ranch Preserve in Cambria (1,700)
Livia Albeck-Ripka is a reporter for The New York Times, based in California.