Monarch Watch's focus is Eastern population; the group of monarchs that breed in states east of the Rocky Mountains in summer and migrate to central Mexico annually in the fall. The group of monarchs, cleverly dubbed the Western population, breed west of the Rocky Mountains and migrate to southern California (one of the major overwintering sites is Pacific Grove, CA) annually.

The consensus among Monarch biologists (although the topic is still debated in certain circles) is that the two populations are genetically distinct. That is, they aren't known to interbreed.

For information on tagging and rearing monarchs west of the Rockies please contact The Xerces Society or Monarch Alert.