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.
Our tagging efforts are focused exclusively on the eastern population. However, there are some programs that are tagging in California. One difference between the two programs is that we tag the butterflies as they head off to the overwintering sites and the other program tags at the overwintering sites. The reason for this difference is that the two populations vary in distribution, making the prime time to tag them differ.