Monarch Butterfly Natural History
There are two populations of the monarch butterfly in North America. Although there is almost no difference between the two populations (eastern monarchs have ever so slightly larger wings), we talk about two different populations of butterflies because the eastern monarchs overwinter in Mexico and the western monarchs overwinter in California.
The Western Monarchs
In the fall, monarchs west of the Rocky Mountains migrate through the western states and the southern portions of western Canada to overwintering sites along the California sea coast. The wintering colonies in wetern North America extend from Mendocino County south along the coast to the Ensendada region of Baja California Sur. Roost have also been recorded on Santa Cruz Island, Bakersfield, and three sites in the Saline Valley. Roosts are reported on Santa Catalina Island.
Clustering of butterflies begins in the fall during September and October. Monarch numbers begin to build at various sites near the ocean. These are called Autumnal Sites. The butterflies persist there for a few days to a few months. The butterflies are attracted by a source of drinking water and nectar. These sites may hold monarchs through the entire winter, if conditions are mild. During November and December, the monarchs leave "poorer" Autumnal Sites for "better" Autumnal Sites, which persist through the winter and are now called Permanent Sites. The monarchs begin mating in late January and by March they leave the colony on their spring migration.
These overwintering monarchs will lay eggs inland on milkweed in areas like the foothills of the Sierra Nevada and die. The next generation will cross the Sierra Nevada into Nevada. Others will fan out into Oregon or Arizona. Successive generations will continue to fan out across the West. The last generation of the season, about the 4th generation, will make the return journey of their great great grandparents back to California. This generation returns to California because the interiors of the country will soon be covered with snow which can kill the cold sensitive monarchs and which will bury the milkweed, the only foodplant of the monarch caterpillars.
The Eastern Monarchs
In the eastern United States (which monarch biologists have dogmatically demarcated as east of the Rocky Mountains or Continental Divide), monarchs head to Mexico. In the fall with the approach of the first winter storms from Canada, the monarchs head south. Upon reaching the Gulf states, the monarchs swing west through the southern tip of Texas into Mexico. Recently researchers have found that some actually cross the Gulf of Mexico and spend evenings roosting on themany oil platforms out at sea. They end up in the Transvolcanic Mountain Range in central Mexico at about the same latitude as Mexico City. About a dozen plus sites are located in the Oyamel fir forest at elevations of 9-11,000'. Most are in the state of Michoacan. A small number (1000's) of monarchs will make their way into Florida where they will spend the winter.
The Mexican colonies are extremely large. One starts counting in millions. The one site that is open to the public is called the El Rosario site, located near the town of Angangueo, will often have 40 million monarchs.
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