Natural Bridges State Beach is a 65-acre California state park in Santa Cruz, California, in the United States. The park features a natural bridge across a section of the beach and a eucalyptus grove provides habitat for monarch butterflies. During monarch butterfly migration, Monarch Butterfly Natural Preserve has up to 150,000 monarch butterflies from Octobe…Natural Bridges State Beach is a 65-acre California state park in Santa Cruz, California, in the United States. The park features a natural bridge across a section of the beach and a eucalyptus grove provides habitat for monarch butterflies. During monarch butterfly migration, Monarch Butterfly Natural Preserve has up to 150,000 monarch butterflies from October through early February.