Monarch Butterflies

Challenges of Tracking Fall Migration

Clearest Pattern The map of overnight roosts reveals migration pathways and timing most clearly. Roosts show large concentrations of monarchs and serve as a proxy for peak migration.

• We can't always tell if monarchs are migrating: People don't know for certain that the monarchs they see are migrating unless they see large numbers of monarchs moving, roosting, or flying in clear "directional flight."

• We can't track the "first" migrating monarchs. It's impossible to tell when the first monarchs leave and begin to migrate. People in the northern U.S. and Canada see monarchs all summer and can't determine when migration begins.

• We can't tell the origin of "first" monarchs in southern states. In many southern states, monarchs are not usually seen during June or July but re-appear in August. Where do they come from? Are they early migrants moving down from the north? Or are they the offspring of a small, local population whose numbers are building with each new generation? It's imposible to know.

• We can't track the "last" monarch: People would not know immediately that they were seeing the "last" monarch, so we can't track the last monarch easily either. (What's more, late monarchs may be too late to migrate to Mexico due to cold temperatures. Thus, such a map would have little meaning as a migration map.)

• The nature of fall monarch migration: The migration does not occur at a predictable time along a specific pathway; all monarchs don't travel together in a clear, single wave. Monarchs can travel high overhead during good migration weather and avoid being seen at all, even during peak migration. A person may see a spectacular migration one day but not a single monarch the next day in the very same place!

• Spring migration is different: A clear wave of monarchs moves up the map in the spring because we map sightings of the first monarchs seen. This wave represents the "leading edge" of the spring monarch migration.

Because of these challenges, we collect sightings in two categories: "adult sightings" and "peak migration." Try to identify peak migration, but don't be concerned if you can't. Repor to either category. When we review sightings we can edit categories and/or contaact you for more information.