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More About Our Fall Migration Maps

Tracking Fall Migration is Tricky
It is easy to spot monarchs in the fall, but it can be hard to tell if they are actually migrating.

Our fall monarch migration maps do not show a precise wave of migration. Rather, it reflects the reality of overlapping
Here are some challenges:
  • We can't track the "first" monarch: People see monarchs all summer in the northern U.S. states and in Canada. This makes it impossible to tell when the first monarchs leave and begin migration.
  • We can't always tell if monarchs are migrating: Unless people see large numbers of monarchs moving—or roosting—they don't know for certain that the monarchs are migrating. (People can watch for "directional flight" and be confident that a single monarch is migrating.)
  • 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.
  • We can't tell the origin of "first" monarchs in Southern states. In many southern states, monarchs are not usually seen during June and July. They re-appear in August. Where did 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.
  • The nature of monarch migration is not a clear, single wave — nor do the monarchs follow a specific pathway. A person may see a spectacular migration one day and not a single butterfly in the same place a day later. Monarchs can travel high overhead and, with good weather, avoid being seen.
  • Can you think of other challenges?
  • Spring migration is different: A clear wave of monarchs moves up the map in the spring and we map sightings of the first monarchs seen. This wave represents the "leading edge" of the spring monarch migration.
Don't Hesitate to Report: All Monarch Sightings Are Important!
  • Because of these challenges, we collect migration sightings in three categories, then make sense of the patterns. They categories are: "migration sightings," "overnight roosts," and "peak migration." Don't be concerned about selecting the right category when you report your sightings. We review all sightings carefully and can edit categories, if needed.
How do we identify "Peak Migration"?
  • With Difficulty! Sometimes it is only clear when the peak occurred after the migration wanes. We often move sightings to the "Peak Migration" map later in the season, when it becomes easier to identify "peak." Our Peak Migration map gives you the best snapshot of the wave of migrating fall monarchs because we edit it so regularly. (You will be able to identify peak migration in your area if you monitor the migration on a regular basis.)
Try This! Map "Migration Highlights"

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