ARDENWOOD FARM SEE MONARCH BUTTERFLY BOOM

By Jonathan Bloom, KGO

FREMONT, Calif. (KGO) -- Monarch butterflies flock to California's coastline each winter and right now, they're here in record numbers. There's a big surge among the butterfly that is one of nature's great mysteries.

At first glance, they look like clumps of leaves, hanging high in a eucalyptus grove. However, if you look closely and you'll see what may be the biggest population of Monarch butterflies that Ardenwood Historic Farm has ever seen.

"We had 4,188 butterflies," said Ira Bletz, a supervising naturalist. That count is about 10 times the amount from the year before. "And more than we've had in the past 10 years."

Naturalists say it's possible last year's rainfall caused a spike in the growth of the milkweed plant where monarchs lay their eggs, which looks like a little white pinprick that will hatch into a caterpillar. Those caterpillars will eat the milkweed nonstop for 15 days, before starting its transformation.

Most Monarchs live only a few weeks, but each year, one generation will live for six months -- migrating across the country to where the climate is just right. These butterflies have come from as far east as the Rocky Mountains and as far north as British Columbia.

To survive the winter, Monarch butterflies need a ring of trees that's shielded from the wind, but lets in just enough sunlight to keep them warm. And the thing is they don't go looking for it, they just seem to know where it is.

"They've never been here before, they've never seen any other butterflies coming here, but somehow they find their way to specific spots, and they come there to spend the winter," said Bletz.

Because they come from all over, the population surge tells a story far bigger than this grove in Fremont.

"Monarch butterflies are really an indicator of a healthy environment. A good population indicates that things are going well all across the western us," said Bletz.

And in February, they'll head back out across the country to lay their eggs and let the cycle begin again.

These butterflies will not return here, but their great-great-grandchildren will come back next fall to spend the winter here in our eucalyptus grove," said Bletz.