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.
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."
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.
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.
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.)
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!
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.
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.