The society has preliminary counts from 97 sites, most of them along California’s coast, representing an area that traditionally accounts for nearly 77 percent of the state’s winter monarch population. In 2017, the sites hosted about 148,000 monarchs. But in 2018, that dropped to an estimated 20,456 monarchs, with large numbers of them counted in Pismo Beach, Big Sur and Pacific Grove.

In November volunteers fan out across California’s coastal cities to find and count the monarch population. Ms. Pelton said the total count could be higher once final numbers from the census arrive next week.

Monarchs in the western part of the United States migrate for the winter to California, where they gather mostly among fragrant eucalyptus trees, which provide hospitable living conditions.

Monarchs from the eastern part of the United States, by contrast, winter in Mexico. Ms. Pelton said the count of eastern monarchs had not been released.

Ms. Pelton warns that if nothing is done to preserve the western monarchs and their habitat, the butterflies could face extinction. In a 2017 study, scientists estimated that the monarch butterfly population in western North America had a 72 percent chance of becoming near extinct in 20 years if the monarch population trend was not reversed.