Pacific Swell | Southern California environment news and trends

Thanksgiving counts track monarch migration in California

Western Monarch Butterfly in Huntington Beach
Western Monarch Butterfly in Huntington Beach
Courtesy of Huntington Beach Tree Society
Western Monarch Butterfly in Huntington Beach
Western Monarch Butterfly at Gibbs Park in Huntington Beach
Courtesy of Huntington Beach Tree Society

Each year, western monarch butterflies trek from southern Canada, and down the U.S. coast to Mexico to escape the cold winter.

The populations have decreased in recent years, possibly due to the decrease in milkweed, the only plant where Monarchs lay their eggs, the use of pesticides, changes in agriculture, drought.

Monarch overwintering populations in the west have declined from more than 1 million butterflies at 101 sites in 1997 to fewer than 60,000 at 74 sites in 2009.

But they're showing up in higher numbers than expected across California this year, according to the Xerces Society, the San Francisco Chronicle reported. The society, a nonprofit that focuses on invertebrate conservation, monitors around 90 monarch sites during the Western Monarch Thanksgiving Count.

Experts speculate that the increase in butterfly populations may be due to more rain, which produces more abundant and heartier milkweed, and lower temperatures that match more closely to climates the butterflies used to thrive in, the Chronicle reported. 

David Marriott, executive director of the Monarch Program in Encinitas, has conducted counts at 22 sites in Southern California so far this season, as part of an annual Thanksgiving count. The populations are between 40 and 60 percent lower than last year, he said, but it's still too early to tell.  

The count is done by trained volunteers each year and is similar to the Audubon Society's Christmas Bird Count. Most of the butterflies have settled into their habitats by Thanksgiving, he said, but as the winter solstice approaches the butterflies begin moving to other areas.

"A lot of times when the monarchs come in late October, they move north to Ventura and Santa Barbara counties," he said. "But they're insects and each year it fluctuates a lot.

He said the Thanksgiving counts will end in about 10 days or so, and then there will be more precise data about the populations for this year.

Researchers at the University of Massachusetts Medical School recently sequenced the butterfly's DNA to crack the mystery of how monarchs find their way to their annual migration sites – places they have never been before.

They discovered that the insects have visual areas that might help them use the sun differently to find their way, and circadian rhythms that might alter their responses to light. DNA also determines which butterflies migrate and which ones stay put, the researchers found. 

If you're looking to spot a few butterflies before they're on their way, Marriott recommended Ocean Avenue Park and Camino Real Park in Ventura County, where his team counted between 4,000 and 5,000 monarchs.

A bit farther north in Santa Barbara, visitors can see close to 15,000 butterflies at the Ellwood Butterfly Grove, he said. Monarch sites in Los Angeles County tend to be smaller or on private property, but Marriott counted around 50 at Leo Carillo State Park.

Richard James, a naturalist at the El Dorado Nature Center in Long Beach, said small clusters of the butterflies can be seen in the early mornings, and the park has a few resident butterflies that stay year-round. In 1997, the park hosted close to 2500 monarchs.