As they do every Fall, monarch butterflies (Danaus plexippus) continue to arrive on the coastline of California to spend the winter. They have come from throughout western North America, but the majority come from the foothills and lower elevations of the Sierra Nevada. This year, as in years past, I start to see them leisurely migrating as singles through the Central Valley beginning in August. Enroute and especially once they get to the coast, they feed greedily on the nectar from the flowers of Pride of Madeira (Echium candicum) and mule fat (Baccharis salicifolia). I have seen adults whose abdomens are so fat that you wonder how they are able to fly, but the internal food supply will help them survive the winter.
With the shortening day length, the monarchs form Temporary Bivuoacs, sites where the monarchs form clusters of individuals on the branches of the trees. The butterflies continuously stream in and out of the Temporary Bivuoacs and these sites persist because of the constant immigration of animals; Temporary Bivuoacs usually have a good source of nectar. When the air temperature drops, the movement of the monarchs stops, and the sites where they spend the cold months of the year are known as Wintering Colonies. Interestingly, during mild winters or in southern California, Temporary Bivuoacs may persist through the entire winter. The only way to accurately determine if a site is a Temporary Bivuoac or a Wintering Colony is to mark individual butterflies to see if there is any movement of the animals.
In late January into February, the monarchs leave the sites; some stay around the general area, but the majority head into the coast range, especially the areas located in western Kern and San Luis Obispo counties, or into the foothills of the Sierra Nevada.
Another observation - efforts to conserve California monarchs that focus solely on protecting the Temporary Bivuoacs and Wintering Colonies are doomed to failure. The conservation of the spring and summering grounds in the foothills and lower elevation of the Sierra Nevada is critical. In the past twenty years, there has been a burst of ranchettes and development in these areas. Some of the most favored areas for the residential development, and agriculture, especially vineyards, may be best places for milkweed, the foodplant of the monarch.
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