Cover photo © Elise Wormuth.
Natural Bridges State Beach is almost as famous for its monarch butterflies as it is for its natural rock arch. For decades tens of thousands of the west-coast monarch butterflies (Danaus plexippus) have overwintered here. They start arriving in October and generally are gone by March. In the shelter of a protected grove of eucalyptus trees they cluster by the thousands to keep warm. They actually intertwine their legs to avoid being blown off the branches.
If the temperature gets above 55°F they can leave their clusters and search for flower nectar and water. Adult monarchs feed on the nectar of many flowers, but they breed only where there is milkweed (Genus Asclepias), which is eaten by the larvae. Near the end of winter they mate and fly inland to look for milkweed plants where the females lay their eggs. The round-trip migration journey consists of 4-5 generations of monarchs. It takes only one generation to make the trip south to the overwintering site, but 3-4 to make the journey back north.
The Monarch Walk
At Natural Bridges State Beach, there is a wooden walkway across from the visitor center that takes you to Monarch Grove. There are docent-led tours of the grove from mid-October through January at 11:00 am and 2:00 pm. on Saturdays and Sundays. Meet at the visitor center. Monarch migration is variable. To find out the current population or other information, call (831) 423-4609.
The monarchs are so beloved, the park has an annual Welcome Back Monarchs Day on the second Sunday in October and a Migration Festival on the second Saturday in February.
More on Monarch Migration
Monarchs are the only butterfly species that, like birds, migrate to escape the cold. Other butterfly species can overwinter as larvae, pupae or even as adults, but monarchs cannot survive the northern winters. Environmental cues in the fall tell them to head south on air currents and thermals. They can travel 50-100 miles a day and take up to two months to go as far as 3,000 miles. Monarchs from eastern North America go to the Sierra Madre of Mexico. Monarchs in western North America overwinter along the California coast.
Numbers in Decline
In recent years, monarch numbers have been generally on the decline in North America. Because they overwinter in only a few spots, the populations are vulnerable to harsh winter conditions or human impacts on their habitat. Thus their numbers can be greatly impacted if one or more sites are disturbed. Also, breeding populations are vulnerable to harsh weather and destruction of milkweed habitat. Natural Bridges State Beach has, in particular, experienced a decline in monarch numbers. In the 1970s and 80s it was the largest overwintering site in Santa Cruz and one of the largest in California with numbers ranging from a low of 40,000 to a high of 150,000.
Since the 1990s, numbers at Natural Bridges have been severely declining with overwintering numbers ranging only in the few thousands. The winter of 2013 had a slight uptick with about 6,000. Additionally, those that do come in October are not staying to over-winter but fly away around Christmas time.
Natural Bridges in particular has lost some of its overwintering habitat. Several of the large eucalyptus trees within the Monarch Grove have fallen over. The eucalyptus trees began to fall after the Monterey pines, which sheltered the grove, weakened and died after being infected with pitch canker in the 1990s. The good news is that other locations such as Lighthouse Field have had large increases in overwintering numbers. However, the overall numbers are still not close to what they were two decades ago.
Take the Self-Guided Mobile Tour
This piece is part of the West Cliff Drive Tour. Download the free app with many tours of the Santa Cruz area and beyond.
- California State Parks Website. Natural Bridges State Beach Pamphlet. 2012.
- Flight of the butterfly: Winged visitors suddenly scarce in Santa Cruz. Lily Dayton. Santa Cruz Sentinel. November 24, 2009.
- Monarch Butterfly Migration and Overwintering. US Forest Service Website.
- Monarch Butterfly Migration Update: Monarch Butterfly Journey North Website