In an insect version of the swallows returning to San Juan Capistrano, monarch butterflies by the hundreds gather at this time of year in the eucalyptus groves at Ardenwood Historic Farm in Fremont.
The butterflies overwinter here as part of their complicated, migratory life cycle, which takes them as far north as Canada and as far south as Mexico. Monarchs migrate south in the fall, then north again in the spring.
But no single butterfly makes the entire trip. Females lay eggs along the way for the next generation to continue the cycle. It's been described as an intergenerational relay race.
Ardenwood is one of only a few sites on San Francisco Bay where the monarchs gather. There's a spot in San Leandro, and some have been seen in the past at Point Pinole Regional Shoreline in Richmond. But Ardenwood seems to be the main stopping point other than Pacific Grove on Monterey Bay.
The monarchs rest high in the eucalyptus trees, overlapping shingle-fashion to keep warm. Periodically, they may break away to fly around in a colorful, fluttering cloud. It's called a cascade.
Taking advantage of this spectacular natural phenomenon, Ardenwood naturalists have scheduled a series of monarch butterfly walks and talks between now and New Year's Day.
First out the gate is "Butterfly B & B," a program from 2 to 3 p.m. Saturday. Naturalist Chris Garcia will meet visitors at the Ardenwood Granary for a short walk to the eucalyptus grove to see how many monarchs have arrived.
Chris will host three more monarch programs: from 1 to 2:30 p.m. on Friday, Nov. 25 and at 11 a.m. and 1 p.m. on Saturday, Nov. 26.
Ardenwood is located at 34600 Ardenwood Blvd., just north of Highway 84 in Fremont. It's a restored 19th century farming estate, with the beautiful Patterson House as centerpiece. For more information, phone 510-544-2797 or visit the East Bay Regional Park District website, www.ebparks.org.
The chaparral at Black Diamond Mines Regional Preserve in Antioch is putting on its winter coat. Naturalist Bob Kanagaki will lead a walk through the chaparral community from 10 a.m. to noon Saturday. The trail is a bit rough and steep, best for ages 8 and older.
Meet Bob at the upper parking lot at the south end of Somersville Road. The hike is free; Black Diamond Mines has a parking fee of $5 per vehicle when the kiosk is attended. For more information, phone 1-888-327-2757, ext. 2750.
This is a good time of year to visit Vasco Caves Regional Preserve south of Brentwood, a parkland accessible only through guided tours.
Vasco Caves' attractions include spectacular rock outcrops, eagles and other birds of prey, vernal pools containing endangered fairy shrimp, and Indian rock art from a culture that dates back thousands of years.
Tours are restricted to ages 10 and older. Registration is required. The fee is $30 per person ($34 for nondistrict residents). Tours are scheduled from 9 a.m. to 2 p.m. Nov. 20, and Dec. 3, 4, 17 and 31.
For information and reservations, phone 888-327-2757.
Ned MacKay writes a regular column about East Bay Regional Park District sites and activities. E-mail him at email@example.com.