October 2016

An overwintering western monarch butterfly (Danaus plexippus) at the Goleta Butterfly Grove and Coronado Butterfly Preserve in Goleta, CA. Monarch butterflies in western North America spend the winter in aggregations along the California coast. These butterflies arrive at their overwintering sites only after developing as caterpillars throughout the western range. In order to evaluate multiple hypotheses about the natal origins of these butterflies, we compared the hydrogen isotope ratios from overwintering monarch wings with a map of expected hydrogen isotope values based on natural, continental-scale variation in precipitation isotopes. We also compare variation in wing size and shape with isotopic values that are indicative of migratory distance. We document considerable individual variation in the natal origins of western monarch butterflies, suggesting both local and long-distance movement to overwintering sites. Photo by Louie H. Yang.

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