Monarch Watch Blog

Fall Monarch Population and Migration

Friday, August 17th, 2001 at 3:09 pm by Jim
Filed under Monarch Migration, Monarch Population Status | No Comments »

Although the number of monarchs at the overwintering sites in Mexico last winter was the lowest yet to be recorded (28.3 million) and the number of females that survived to reproduce this spring in the southern states was quite low, the monarch population appears to have made a remarkable recovery over the last 5 months. Reports from throughout the breeding range indicate the migratory population this fall will be much larger than that of 2000 and it may exceed the population in 1999 and perhaps even the population in 1997. In recent days large numbers of clustered monarchs have been reported from Winnipeg, northern Michigan and west central Wisconsin. The butterflies in Winnipeg should have already begun moving southward and the migration will begin in areas north of 45 degrees in the next week.

If you want to determine the approximate time for the migration to begin in your area, visit

Follow the instructions on the site and enter your latitude, date, time (12:01PM), and time zone (for the central time zone use “F” (GMT+6:00)) and time basis (solar). If you don’t know your latitude, go to and type in the name of the nearest city. For example, for Winnipeg (49:54 N) the altitude angle for today (17 August) is 53.76 degrees.

Generally, the migration starts close to when the AA reaches 56 degrees and peaks when the AA reaches 52 degrees. Most monarchs leave a particular latitude by the time the AA of the receding fall sun has reached 47 degrees. The interval, or migration window, defined by 56-47 is about 3.5 weeks. For example, in Lawrence, KS the first wave of the migration usually arrives on the 10-11th of September and the peak is near the 20th (AA = 52.39) with all but the last few migrants gone by the 4th of October (AA = 46.95).

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